Python 速查表

# This is a single-line comment
"""
This is a
multi-line comment (docstring)
"""
print("Hello, World!")        # print to stdout
print("A", "B", "C", sep="-") # A-B-C
print("No newline", end="")   # suppress newline

# Python uses indentation (4 spaces) to define blocks
if True:
    print("inside if")
    if True:
        print("nested block")
print("outside block")

# No braces! Indentation IS the syntax
def func():
    x = 1
    return x + 1

# input() always returns a string
name = input("Enter your name: ")
age = int(input("Enter your age: "))  # convert to int

print(f"Hello {name}, you are {age} years old")

# formatted output
print("Pi is approximately {:.2f}".format(3.14159))
print(f"{1000000:,}")  # 1,000,000 with thousands separator

# multiple statements on one line (discouraged)
a = 1; b = 2; c = 3

# explicit line continuation
total = 1 + 2 + 3 + \
        4 + 5 + 6

# implicit continuation inside brackets
nums = [
    1, 2, 3,
    4, 5, 6
]

result = (1 + 2
          + 3 + 4)

# Run a script
# $ python script.py

# Run interactively (REPL)
# $ python
# >>> 2 + 2
# 4

# Shebang line for Unix scripts
#!/usr/bin/env python3

# Check Python version
import sys
print(sys.version)
print(sys.version_info.major)  # 3

# Python is dynamically typed - no declaration needed
name = "Alice"          # str
age = 30                # int
height = 5.7            # float
is_active = True        # bool
items = [1, 2, 3]       # list

# Type checking
print(type(name))       # <class 'str'>
print(isinstance(age, int))  # True

# Multiple assignment
x, y, z = 1, 2, 3
a = b = 0  # chain assignment

# Variable annotations
name: str = "Alice"
age: int = 30
scores: list[float] = [90.5, 85.0]

# Function annotations
def greet(name: str, times: int = 1) -> str:
    return (f"Hi {name}! " * times).strip()

# Optional and Union
from typing import Optional, Union
def find(id: int) -> Optional[str]:
    return "Alice" if id == 1 else None

# mypy for static type checking
# $ mypy script.py

# String to number
num_str = str(42)          # "42"
num = int("42")            # 42
float_num = float("3.14")  # 3.14

# Number conversions
print(int(3.99))           # 3 (truncates toward zero)
print(int(-3.99))          # -3
print(round(3.14159, 2))   # 3.14

# Boolean conversion
print(bool(0))             # False
print(bool(""))            # False
print(bool([]))            # False
print(bool("anything"))    # True

# Collection conversions
print(list("abc"))         # ['a', 'b', 'c']
print(tuple([1, 2, 3]))    # (1, 2, 3)
print(set([1, 1, 2]))      # {1, 2}

# Integers (arbitrary precision)
big = 10 ** 100  # no overflow
print(type(big))  # <class 'int'>

# Floats (IEEE 754 double)
pi = 3.14159
print(0.1 + 0.2)  # 0.30000000000000004

# Complex numbers
z = 3 + 4j
print(z.real, z.imag)  # 3.0 4.0
print(abs(z))          # 5.0

# Boolean is subclass of int
print(isinstance(True, int))  # True
print(True + True)            # 2

# Underscores in numbers (3.6+)
million = 1_000_000
binary = 0b_1010_1010

# Python has no true constants - convention only
MAX_SIZE = 100          # ALL_CAPS for constants
PI = 3.14159

# Naming conventions (PEP 8)
variable_name = "snake_case"      # variables, functions
ClassName = "PascalCase"          # classes
CONSTANT_VALUE = 100              # constants
_private_var = "underscore prefix" # private (convention)
__name_mangled = "double underscore" # name mangling

# dunder names (reserved)
__name__, __main__, __init__

s = "Hello, World"

# Case operations
print(s.upper())          # HELLO, WORLD
print(s.lower())          # hello, world
print(s.title())          # Hello, World
print(s.capitalize())     # Hello, world
print(s.swapcase())       # hELLO, wORLD

# Search & replace
print(s.find("World"))    # 7 (index, -1 if not found)
print(s.index("World"))   # 7 (raises ValueError if not found)
print(s.replace("o", "0"))  # Hell0, W0rld
print(s.count("l"))       # 3

# Validation
print("abc".isalpha())    # True
print("123".isdigit())    # True
print("  ".isspace())     # True

name = "Alice"
age = 30

# f-strings (Python 3.6+) - PREFERRED
print(f"Hello, {name}! You are {age}.")
print(f"{name.upper()} is {age * 365} days old")
print(f"{3.14159:.2f}")      # 3.14
print(f"{42:>10}")           # right-align
print(f"{42:<10}")           # left-align
print(f"{42:^10}")           # center
print(f"{1000000:,}")        # 1,000,000

# str.format() method
print("Hello, {}!".format(name))
print("{name} is {age}".format(name="Bob", age=25))

# Old style (avoid in new code)
print("Hello, %s!" % name)

s = "Hello, World"

# Indexing (0-based, negative from end)
print(s[0])        # H
print(s[-1])       # d
print(s[7])        # W

# Slicing [start:stop:step]
print(s[0:5])      # Hello
print(s[7:])       # World
print(s[:5])       # Hello
print(s[::2])      # HloWrd (every 2nd char)
print(s[::-1])     # dlroW ,olleH (reverse!)

# Length
print(len(s))      # 12

# Slicing never raises IndexError
print(s[100:200])  # '' (empty string)

# Split
csv = "a,b,c,d"
print(csv.split(","))          # ['a', 'b', 'c', 'd']
print(csv.split(",", 2))       # ['a', 'b', 'c,d'] (max 2 splits)

# Splitlines
text = "line1\nline2\nline3"
print(text.splitlines())       # ['line1', 'line2', 'line3']

# Partition (splits on first occurrence)
print("[email protected]".partition("@"))
# ('user', '@', 'domain.com')

# Join
words = ["Hello", "World"]
print(" ".join(words))         # Hello World
print("-".join(["2024", "01", "15"]))  # 2024-01-15
print("".join(["a", "b", "c"])) # abc

# String concatenation
s = "Hello" + " " + "World"
parts = ["a"]
parts += "b"  # NOT string concat - adds chars to list!

# Strip whitespace (or specified chars)
s = "  hello  "
print(s.strip())    # "hello"
print(s.lstrip())   # "hello  "
print(s.rstrip())   # "  hello"

# Strip specific characters
print("xxxhelloxxx".strip("x"))  # hello

# Padding / centering
print("42".zfill(5))      # 00042
print("hi".rjust(10))     # "        hi"
print("hi".ljust(10, "-")) # "hi--------"
print("hi".center(10, "*")) # "****hi****"

# expandtabs
print("a\tb".expandtabs(4))  # "a   b"

# Escape sequences
print("Line1\nLine2")     # newline
print("Tab\there")        # tab
print("Quote: \"hi\"")   # escaped quotes
print("Backslash: \\")   # literal backslash

# Raw strings (ignore escapes) - great for regex
path = r"C:\Users\name\file.txt"
regex = r"\d{3}-\d{4}"
print(path)  # C:\Users\name\file.txt

# Triple-quoted strings
multi = """
Multiple
lines
"""

# String multiplication
print("ab" * 3)  # ababab

# Basic operators
print(7 + 3)   # 10  addition
print(7 - 3)   # 4   subtraction
print(7 * 3)   # 21  multiplication
print(7 / 3)   # 2.333...  true division (always float)
print(7 // 3)  # 2   floor division
print(7 % 3)   # 1   modulo (remainder)
print(7 ** 3)  # 343 exponentiation

# Floor division with negatives
print(-7 // 3)  # -3 (rounds toward negative infinity)
print(-7 % 3)   # 2 (result has same sign as divisor)

# Augmented assignment
x = 10
x += 5   # x = x + 5
x **= 2  # x = x ** 2

import math

# Constants
print(math.pi)       # 3.141592653589793
print(math.e)        # 2.718281828459045
print(math.inf)      # inf
print(math.nan)      # nan

# Functions
print(math.sqrt(16))     # 4.0
print(math.pow(2, 10))   # 1024.0
print(math.log(100, 10)) # 2.0 (log base 10)
print(math.log(math.e))  # 1.0 (natural log)
print(math.factorial(5)) # 120
print(math.gcd(12, 8))   # 4

# Rounding
print(math.floor(3.7))   # 3
print(math.ceil(3.2))    # 4
print(math.trunc(-3.7))  # -3 (toward zero)

# Trigonometry (radians)
print(math.sin(math.pi / 2))  # 1.0
print(math.degrees(math.pi))  # 180.0

import random

# Random integers
print(random.randint(1, 100))    # 1 to 100 inclusive
print(random.randrange(0, 10, 2)) # even number 0,2,4,6,8

# Random floats
print(random.random())           # 0.0 to 1.0
print(random.uniform(1.0, 10.0)) # random float in range

# Choice & sampling
colors = ["red", "green", "blue"]
print(random.choice(colors))           # one random item
print(random.sample(colors, 2))        # 2 unique items
print(random.choices(colors, k=5))     # 5 items (with replacement)

# Shuffle (in-place)
nums = [1, 2, 3, 4, 5]
random.shuffle(nums)
print(nums)

# Reproducible randomness
random.seed(42)  # same seed = same sequence

from decimal import Decimal, getcontext
from fractions import Fraction

# Float precision issues
print(0.1 + 0.2)  # 0.30000000000000004

# Decimal for exact decimal arithmetic
a = Decimal("0.1")
b = Decimal("0.2")
print(a + b)  # 0.3 (exact!)

# Set precision
getcontext().prec = 6
print(Decimal(1) / Decimal(7))  # 0.142857

# Fractions for exact rational arithmetic
f1 = Fraction(1, 3)
f2 = Fraction(1, 6)
print(f1 + f2)  # 1/2
print(float(f1))  # 0.3333...

# Fraction from string
print(Fraction("3/4"))  # 3/4

# Bitwise operators work on integers
a = 0b1010  # 10
b = 0b1100  # 12

print(a & b)   # 8  (0b1000) AND
print(a | b)   # 14 (0b1110) OR
print(a ^ b)   # 6  (0b0110) XOR
print(~a)      # -11 (NOT, two's complement)
print(a << 2)  # 40 (left shift, multiply by 4)
print(a >> 1)  # 5  (right shift, divide by 2)

# Binary representation
print(bin(10))   # 0b1010
print(hex(255))  # 0xff
print(oct(8))    # 0o10
print(int("1010", 2))  # 10 (parse binary)

# Common tricks
print(5 & 1)     # 1 (check odd: nonzero = odd)
print(8 & (8-1)) # 0 (check power of 2)

# Lists are ordered, mutable sequences
nums = [1, 2, 3, 4, 5]
mixed = [1, "hello", True, 3.14]

# Adding elements
nums.append(6)           # [1,2,3,4,5,6]
nums.insert(0, 0)        # [0,1,2,3,4,5,6]
nums.extend([7, 8])      # extend with another list

# Removing elements
nums.remove(0)           # remove by value
popped = nums.pop()      # remove & return last
popped = nums.pop(0)     # remove & return by index
del nums[0]              # delete by index
nums.clear()             # remove all

# Slicing (same as strings)
nums = [1, 2, 3, 4, 5]
print(nums[1:3])    # [2, 3]
print(nums[::-1])   # [5, 4, 3, 2, 1] reverse

# Sorting
nums.sort()              # in-place sort
nums.sort(reverse=True)  # descending
sorted_nums = sorted(nums)  # returns new list

# Tuples are ordered, IMMUTABLE sequences
point = (3, 4)
single = (42,)     # note the comma for single-element tuple
empty = ()

# Packing & unpacking
coordinates = 10, 20, 30  # packing
x, y, z = coordinates     # unpacking
x, y = y, x               # swap values!

# Multiple return values
def min_max(nums):
    return min(nums), max(nums)

lo, hi = min_max([3, 1, 4, 1, 5])

# Named tuples (readable)
from collections import namedtuple
Point = namedtuple("Point", ["x", "y"])
p = Point(3, 4)
print(p.x, p.y)       # 3 4
print(p[0], p[1])     # 3 4

# Tuples are immutable but can contain mutable objects
t = (1, [2, 3])
t[1].append(4)  # OK: (1, [2, 3, 4])

# Dicts are key-value mappings (insertion-ordered since 3.7)
user = {"name": "Alice", "age": 30}

# Access
print(user["name"])          # Alice
print(user.get("email"))     # None (no KeyError)
print(user.get("email", "N/A"))  # N/A (default)

# Add/update
user["email"] = "[email protected]"    # add
user["age"] = 31             # update
user.setdefault("role", "user")  # set if missing

# Delete
del user["email"]
val = user.pop("age")        # remove & return
# user.clear()               # remove all

# Iteration
for key in user:             # keys
    print(key)
for k, v in user.items():    # key-value pairs
    print(k, v)
for v in user.values():      # values
    print(v)

# Dict comprehension
squares = {x: x**2 for x in range(5)}

# Merge dicts (3.9+)
merged = {"a": 1} | {"b": 2}

# Sets are unordered collections of unique elements
a = {1, 2, 3, 4}
b = {3, 4, 5, 6}

# Set operations
print(a | b)    # union: {1, 2, 3, 4, 5, 6}
print(a & b)    # intersection: {3, 4}
print(a - b)    # difference: {1, 2}
print(a ^ b)    # symmetric difference: {1, 2, 5, 6}

# Methods
a.add(5)        # add element
a.discard(10)   # remove if present (no error)
a.remove(1)     # remove (KeyError if missing)
a.update([6, 7]) # add multiple

# Membership test (O(1) - faster than list)
print(3 in a)   # True

# Frozen set (immutable)
fs = frozenset([1, 2, 3])

# Common use: deduplicate
unique = list(set([1, 1, 2, 2, 3]))  # [1, 2, 3]

# List comprehension
squares = [x**2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]
pairs = [(x, y) for x in range(3) for y in range(3)]

# Dict comprehension
square_map = {x: x**2 for x in range(5)}
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# Set comprehension
unique_lens = {len(w) for w in ["a", "ab", "abc", "ab"]}

# Generator expression (lazy, memory-efficient)
gen = (x**2 for x in range(1000000))
print(next(gen))  # 0
print(next(gen))  # 1
total = sum(x**2 for x in range(100))  # no extra list

# Nested comprehension (matrix)
matrix = [[i * 3 + j for j in range(3)] for i in range(3)]
# [[0,1,2], [3,4,5], [6,7,8]]

from collections import Counter, defaultdict, deque, OrderedDict

# Counter - counting
words = ["apple", "banana", "apple", "cherry", "banana", "apple"]
cnt = Counter(words)
print(cnt)  # Counter({'apple': 3, 'banana': 2, 'cherry': 1})
print(cnt.most_common(2))  # [('apple', 3), ('banana', 2)]

# defaultdict - no KeyError
dd = defaultdict(list)
dd["fruits"].append("apple")
dd["fruits"].append("banana")
# dd["vegs"] automatically creates empty list

# deque - fast double-ended queue
dq = deque([1, 2, 3])
dq.appendleft(0)   # [0, 1, 2, 3]
dq.append(4)       # [0, 1, 2, 3, 4]
dq.popleft()       # 0, deque is now [1, 2, 3, 4]
dq.rotate(1)       # rotate right

# OrderedDict (less needed since 3.7, dicts are ordered)
od = OrderedDict([("a", 1), ("b", 2)])

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print(f"Grade: {grade}")  # Grade: B

# Conditional expression (ternary)
status = "pass" if score >= 60 else "fail"

# Truthy/falsy values
# Falsy: False, 0, 0.0, "", [], {}, (), None
# Everything else is truthy
if []:    # False
    print("never")
if [0]:   # True (non-empty list)
    print("always")

# range(start, stop, step)
for i in range(5):        # 0, 1, 2, 3, 4
    print(i)
for i in range(2, 10, 2): # 2, 4, 6, 8
    print(i)
for i in range(10, 0, -1): # countdown
    print(i)

# Iterate over collections
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# enumerate for index + value
for idx, fruit in enumerate(fruits):
    print(f"{idx}: {fruit}")

# zip to iterate multiple sequences
names = ["Alice", "Bob"]
ages = [30, 25]
for name, age in zip(names, ages):
    print(f"{name}: {age}")

# Iterate dict
user = {"name": "Alice", "age": 30}
for key, value in user.items():
    print(f"{key} = {value}")

# Basic while
count = 0
while count < 5:
    print(count)
    count += 1

# break - exit loop
while True:
    cmd = input("> ")
    if cmd == "quit":
        break
    print(f"You said: {cmd}")

# continue - skip to next iteration
for i in range(10):
    if i % 2 == 0:
        continue  # skip even numbers
    print(i)  # prints 1, 3, 5, 7, 9

# else clause (runs if no break)
for i in range(5):
    if i == 10:
        break
else:
    print("Loop completed without break")

# pass - do nothing (placeholder)
for i in range(5):
    pass  # TODO: implement

# Structural pattern matching (like switch)
def handle_command(cmd):
    match cmd.split():
        case ["quit"]:
            return "Goodbye"
        case ["hello", name]:
            return f"Hello, {name}!"
        case ["move", direction] if direction in "NSEW":
            return f"Moving {direction}"
        case ["add", x, y]:
            return int(x) + int(y)
        case _:
            return "Unknown command"

print(handle_command("hello Alice"))  # Hello, Alice!

# Matching data structures
match point:
    case (0, 0):
        print("origin")
    case (0, y):
        print(f"on y-axis at {y}")
    case (x, 0):
        print(f"on x-axis at {x}")
    case (x, y):
        print(f"at ({x}, {y})")

# Iterator protocol
nums = [1, 2, 3]
it = iter(nums)
print(next(it))  # 1
print(next(it))  # 2
print(next(it))  # 3
# next(it)  # StopIteration

# Generator function (uses yield)
def count_up_to(n):
    count = 1
    while count <= n:
        yield count
        count += 1

for num in count_up_to(5):
    print(num)  # 1, 2, 3, 4, 5

# Infinite generator
def fibonacci():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

fib = fibonacci()
print(next(fib))  # 0
print(next(fib))  # 1
print(next(fib))  # 1
print(next(fib))  # 2

# Generator expression
squares = (x**2 for x in range(10))

# Basic function
def greet(name):
    return f"Hello, {name}!"

print(greet("Alice"))  # Hello, Alice!

# Default arguments
def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Bob"))              # Hello, Bob!
print(greet("Bob", "Hi"))        # Hi, Bob!

# Keyword arguments
print(greet(name="Carol", greeting="Hey"))

# Return multiple values (tuple)
def stats(nums):
    return min(nums), max(nums), sum(nums) / len(nums)

lo, hi, avg = stats([1, 2, 3, 4, 5])

# Docstrings
def add(a, b):
    """Add two numbers and return the result.
    
    Args:
        a: First number
        b: Second number
    Returns:
        Sum of a and b
    """
    return a + b

# *args - variable positional arguments (tuple)
def sum_all(*args):
    return sum(args)

print(sum_all(1, 2, 3))       # 6
print(sum_all(1, 2, 3, 4, 5)) # 15

# **kwargs - variable keyword arguments (dict)
def print_info(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

print_info(name="Alice", age=30, role="admin")

# Combining all
def func(a, b, *args, **kwargs):
    print(f"a={a}, b={b}")
    print(f"args={args}")
    print(f"kwargs={kwargs}")

func(1, 2, 3, 4, x=5, y=6)
# a=1, b=2, args=(3, 4), kwargs={'x': 5, 'y': 6}

# Unpacking arguments
nums = [1, 2, 3]
print(sum_all(*nums))  # unpack list as args

opts = {"name": "Alice", "age": 30}
print_info(**opts)  # unpack dict as kwargs

# Lambda - anonymous function (single expression)
square = lambda x: x ** 2
print(square(5))  # 25

# Common with sorted, map, filter, reduce
students = [("Alice", 85), ("Bob", 92), ("Carol", 78)]

# Sort by score (key function)
sorted_by_score = sorted(students, key=lambda s: s[1])
# [('Carol', 78), ('Alice', 85), ('Bob', 92)]

# map - apply function to each item
nums = [1, 2, 3, 4, 5]
doubled = list(map(lambda x: x * 2, nums))
# [2, 4, 6, 8, 10]

# filter - keep items where function returns True
evens = list(filter(lambda x: x % 2 == 0, nums))
# [2, 4]

# reduce - accumulate to single value
from functools import reduce
product = reduce(lambda a, b: a * b, nums)
# 120 (1*2*3*4*5)

# Prefer comprehensions over map/filter
doubled = [x * 2 for x in nums]  # more Pythonic
evens = [x for x in nums if x % 2 == 0]

# A decorator modifies a function's behavior
def uppercase_result(func):
    def wrapper(*args, **kwargs):
        result = func(*args, **kwargs)
        return result.upper()
    return wrapper

@uppercase_result
def greet(name):
    return f"hello, {name}"

print(greet("alice"))  # HELLO, ALICE

# Decorator with arguments
def repeat(times):
    def decorator(func):
        def wrapper(*args, **kwargs):
            for _ in range(times):
                result = func(*args, **kwargs)
            return result
        return wrapper
    return decorator

@repeat(3)
def say_hi():
    print("Hi!")

say_hi()  # prints "Hi!" three times

# Practical: timing decorator
import time
def timer(func):
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        print(f"{func.__name__} took {time.time() - start:.4f}s")
        return result
    return wrapper

# Use functools.wraps to preserve metadata
from functools import wraps
def my_decorator(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        return func(*args, **kwargs)
    return wrapper

# LEGB rule: Local, Enclosing, Global, Built-in
x = "global"

def outer():
    x = "enclosing"
    
    def inner():
        x = "local"
        print(x)  # local
    
    inner()
    print(x)  # enclosing

outer()
print(x)  # global

# global keyword - modify global variable
count = 0
def increment():
    global count
    count += 1

# nonlocal keyword - modify enclosing variable
def make_counter():
    count = 0
    def counter():
        nonlocal count
        count += 1
        return count
    return counter

c = make_counter()
print(c())  # 1
print(c())  # 2
print(c())  # 3

class Dog:
    # Class variable (shared by all instances)
    species = "Canis familiaris"
    
    # Constructor
    def __init__(self, name, age):
        # Instance variables
        self.name = name
        self.age = age
    
    # Instance method
    def bark(self):
        return f"{self.name} says Woof!"
    
    # String representation
    def __str__(self):
        return f"Dog({self.name}, {self.age})"
    
    # Official representation (for debugging)
    def __repr__(self):
        return f"Dog(name='{self.name}', age={self.age})"

# Create instances
buddy = Dog("Buddy", 3)
lucy = Dog("Lucy", 5)

print(buddy.bark())       # Buddy says Woof!
print(buddy.name)         # Buddy
print(buddy.species)      # Canis familiaris
print(str(buddy))         # Dog(Buddy, 3)

class Animal:
    def __init__(self, name):
        self.name = name
    
    def speak(self):
        raise NotImplementedError("Subclass must implement")

class Dog(Animal):
    def speak(self):
        return f"{self.name} says Woof!"

class Cat(Animal):
    def speak(self):
        return f"{self.name} says Meow!"

# Polymorphism - same interface, different behavior
def animal_sound(animal):
    print(animal.speak())

animals = [Dog("Buddy"), Cat("Whiskers")]
for a in animals:
    animal_sound(a)
# Buddy says Woof!
# Whiskers says Meow!

# Multiple inheritance
class Swimmer:
    def swim(self):
        return "swimming"

class Flyer:
    def fly(self):
        return "flying"

class Duck(Animal, Swimmer, Flyer):
    pass

duck = Duck("Donald")
print(duck.swim())  # swimming
print(duck.fly())   # flying

# Check inheritance
print(isinstance(duck, Animal))  # True
print(issubclass(Dog, Animal))   # True

class Temperature:
    def __init__(self, celsius=0):
        self.celsius = celsius  # uses setter below
    
    # Getter
    @property
    def celsius(self):
        return self._celsius
    
    # Setter
    @celsius.setter
    def celsius(self, value):
        if value < -273.15:
            raise ValueError("Below absolute zero!")
        self._celsius = value
    
    # Computed property
    @property
    def fahrenheit(self):
        return self._celsius * 9/5 + 32
    
    @fahrenheit.setter
    def fahrenheit(self, value):
        self.celsius = (value - 32) * 5/9

temp = Temperature(25)
print(temp.fahrenheit)  # 77.0
temp.fahrenheit = 100
print(temp.celsius)     # 37.78...

# Name conventions:
# _name   - protected (convention, not enforced)
# __name  - private (name mangling: _ClassName__name)
# __name__ - dunder (reserved by Python)

class MathUtils:
    pi = 3.14159
    
    # Static method - no self/cls, lives in class namespace
    @staticmethod
    def add(a, b):
        return a + b
    
    # Class method - receives the class as first argument
    @classmethod
    def circle_area(cls, radius):
        return cls.pi * radius ** 2
    
    # Alternative constructor (common classmethod use)
    @classmethod
    def from_diameter(cls, diameter):
        return cls()  # would configure instance

# Static: called on class or instance, no special first arg
print(MathUtils.add(2, 3))  # 5

# Class: often used for alternative constructors
print(MathUtils.circle_area(5))  # 78.54...

# Factory pattern with classmethod
class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    @classmethod
    def origin(cls):
        return cls(0, 0)
    
    @classmethod
    def from_tuple(cls, coords):
        return cls(*coords)

p1 = Point.origin()
p2 = Point.from_tuple((3, 4))

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    # String representation
    def __str__(self):
        return f"Vector({self.x}, {self.y})"
    
    # Operator overloading
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    
    def __mul__(self, scalar):
        return Vector(self.x * scalar, self.y * scalar)
    
    # Equality
    def __eq__(self, other):
        return self.x == other.x and self.y == other.y
    
    # Length
    def __len__(self):
        return int((self.x**2 + self.y**2) ** 0.5)
    
    # Make it iterable
    def __iter__(self):
        yield self.x
        yield self.y
    
    # Index access
    def __getitem__(self, index):
        return (self.x, self.y)[index]

v1 = Vector(2, 3)
v2 = Vector(4, 5)
print(v1 + v2)        # Vector(6, 8)
print(v1 * 3)         # Vector(6, 9)
print(v1 == Vector(2, 3))  # True
print(len(v1))        # 3
x, y = v1             # unpacking via __iter__

# Basic exception handling
try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")  # division by zero
finally:
    print("This always runs")

# Multiple exception types
try:
    value = int("abc")
except (ValueError, TypeError) as e:
    print(f"Conversion error: {e}")

# Different handlers for different exceptions
try:
    f = open("nonexistent.txt")
    data = f.read()
except FileNotFoundError:
    print("File not found")
except PermissionError:
    print("No permission")
except Exception as e:
    print(f"Unexpected: {e}")
else:
    print("No exception occurred")
    f.close()
finally:
    print("Cleanup (always runs)")

# Exception hierarchy
# BaseException
#  ├── SystemExit
#  ├── KeyboardInterrupt
#  └── Exception
#       ├── ValueError
#       ├── TypeError
#       ├── KeyError
#       └── ...

# Raise an exception
def divide(a, b):
    if b == 0:
        raise ZeroDivisionError("Cannot divide by zero!")
    return a / b

# Re-raise the current exception
def process(data):
    try:
        return parse(data)
    except ValueError:
        print("Logging parse error...")
        raise  # re-raises the same exception

# Raise with context (from)
try:
    int("abc")
except ValueError as e:
    raise RuntimeError("Failed to process input") from e

# Common built-in exceptions
raise ValueError("invalid value")
raise TypeError("wrong type")
raise KeyError("missing key")
raise IndexError("out of range")
raise RuntimeError("something went wrong")
raise NotImplementedError("override this")
raise FileNotFoundError("no such file")

# Exception with custom args
class ValidationError(Exception):
    pass

raise ValidationError("field is required", "email")

# Custom exception hierarchy
class AppError(Exception):
    """Base exception for the application."""
    pass

class DatabaseError(AppError):
    def __init__(self, message, query=None):
        super().__init__(message)
        self.query = query

class ValidationError(AppError):
    def __init__(self, field, message):
        super().__init__(f"{field}: {message}")
        self.field = field
        self.message = message

class AuthenticationError(AppError):
    pass

# Usage
def login(username, password):
    if not username:
        raise ValidationError("username", "is required")
    if password != "secret":
        raise AuthenticationError("Invalid credentials")

# Catching by hierarchy
try:
    login("", "x")
except ValidationError as e:
    print(f"Validation failed: {e.field}")
except AppError as e:
    print(f"App error: {e}")

# Access exception info
import traceback
try:
    1 / 0
except:
    traceback.print_exc()
    print(repr(sys.exc_info()[1]))

# Context managers handle setup and cleanup
with open("file.txt") as f:
    content = f.read()
# file is automatically closed, even if an error occurs

# Multiple context managers
with open("input.txt") as fin, open("output.txt", "w") as fout:
    fout.write(fin.read())

# Creating a context manager (class-based)
class Timer:
    def __enter__(self):
        import time
        self.start = time.time()
        return self
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        import time
        self.elapsed = time.time() - self.start
        print(f"Elapsed: {self.elapsed:.4f}s")
        return False  # don't suppress exceptions

with Timer() as t:
    # code to time
    sum(range(1000000))

# contextlib for simpler context managers
from contextlib import contextmanager

@contextmanager
def open_db(url):
    db = connect(url)
    try:
        yield db
    finally:
        db.close()

with open_db("localhost") as db:
    db.query("SELECT 1")

# Assertions - for debugging (removed with -O flag)
def divide(a, b):
    assert b != 0, "Divisor cannot be zero"
    return a / b

# Never use assertions for data validation (they can be disabled)
# Use them for invariant checks during development

# Logging (better than print for production)
import logging

# Configure logging
logging.basicConfig(
    level=logging.DEBUG,
    format="%(asctime)s [%(levelname)s] %(message)s"
)

logger = logging.getLogger(__name__)

logger.debug("Detailed info for debugging")
logger.info("Confirmation things are working")
logger.warning("Something unexpected happened")
logger.error("A serious problem")
logger.critical("A fatal error")

# Log exceptions with traceback
try:
    1 / 0
except:
    logger.exception("Failed to divide")  # includes traceback

# Read entire file
with open("file.txt", "r", encoding="utf-8") as f:
    content = f.read()
    print(content)

# Read line by line (memory-efficient for large files)
with open("file.txt", "r") as f:
    for line in f:
        print(line.strip())  # strip removes trailing newline

# Read all lines into a list
with open("file.txt") as f:
    lines = f.readlines()  # ['line1\n', 'line2\n', ...]

# Read specific number of characters
with open("file.txt") as f:
    chunk = f.read(100)  # first 100 chars

# File modes:
# "r"  read (default)
# "w"  write (truncate)
# "a"  append
# "x"  exclusive create (fails if exists)
# "b"  binary mode (e.g., "rb", "wb")
# "+"  read and write (e.g., "r+")

# Write text (overwrites existing)
with open("output.txt", "w") as f:
    f.write("First line\n")
    f.write("Second line\n")
    
    # writelines doesn't add newlines
    f.writelines(["line3\n", "line4\n"])

# Append to a file
with open("log.txt", "a") as f:
    f.write("New log entry\n")

# Write binary data
with open("data.bin", "wb") as f:
    f.write(b"\x00\x01\x02\x03")

# Read and write simultaneously
with open("file.txt", "r+") as f:
    content = f.read()
    f.seek(0)           # move to beginning
    f.write("Updated")  # overwrite
    f.truncate()        # cut off remaining

# Check if file exists
import os
if os.path.exists("file.txt"):
    print("File exists")

from pathlib import Path

# Create Path objects (preferred over os.path)
p = Path("src/main.py")
home = Path.home()           # /home/user or C:\Users\user
cwd = Path.cwd()             # current working directory

# Path components
print(p.name)        # main.py
print(p.stem)        # main
print(p.suffix)      # .py
print(p.parent)      # src
print(p.parts)       # ('src', 'main.py')

# Joining paths (use / operator)
config = home / ".config" / "app" / "config.json"

# Existence and type
print(p.exists())    # True/False
print(p.is_file())
print(p.is_dir())

# Listing directories
for f in Path(".").iterdir():
    print(f)

# Glob patterns
for py_file in Path(".").rglob("*.py"):
    print(py_file)

# Create directories
Path("new/dir").mkdir(parents=True, exist_ok=True)

# Read/write (Path methods)
content = Path("file.txt").read_text()
Path("output.txt").write_text("Hello!")

import json

# Python dict to JSON string
data = {"name": "Alice", "age": 30, "skills": ["Python", "SQL"]}
json_str = json.dumps(data, indent=2)
print(json_str)

# JSON string to Python dict
parsed = json.loads('{"name": "Bob", "active": true}')
print(parsed["name"])  # Bob
print(parsed["active"])  # True (Python bool)

# Write JSON to file
with open("data.json", "w") as f:
    json.dump(data, f, indent=2, ensure_ascii=False)

# Read JSON from file
with open("data.json") as f:
    loaded = json.load(f)

# Custom serialization (e.g., datetime)
from datetime import datetime
def json_default(obj):
    if isinstance(obj, datetime):
        return obj.isoformat()
    raise TypeError

json.dumps({"time": datetime.now()}, default=json_default)

# Type mapping:
# JSON object  <-> Python dict
# JSON array   <-> Python list
# JSON string  <-> Python str
# JSON number  <-> Python int/float
# JSON boolean <-> Python bool
# JSON null    <-> Python None

import csv

# Write CSV
with open("data.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "age", "city"])
    writer.writerows([
        ["Alice", 30, "NYC"],
        ["Bob", 25, "LA"],
    ])

# Read CSV
with open("data.csv") as f:
    reader = csv.reader(f)
    header = next(reader)  # first row
    for row in reader:
        print(row)  # ['Alice', '30', 'NYC']

# DictReader/DictWriter (column access by name)
with open("data.csv") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["age"])

# Pickle (Python-specific, can store any object)
import pickle
with open("data.pkl", "wb") as f:
    pickle.dump({"complex": [1, 2, {"a": 3}]}, f)

with open("data.pkl", "rb") as f:
    obj = pickle.load(f)

# WARNING: pickle is insecure - never unpickle untrusted data!

# Import entire module
import math
print(math.sqrt(16))

# Import specific names
from datetime import datetime, timedelta
now = datetime.now()

# Import with alias
import numpy as np
import pandas as pd

# Import all names (discouraged - pollutes namespace)
# from os import *

# Conditional import (try/except)
try:
    import cjson as json
except ImportError:
    import json

# Check what's in a module
import os
print(dir(os))         # list all attributes
print(os.__file__)     # module location
print(os.__name__)     # module name

# Reload a module (during development)
import importlib
importlib.reload(my_module)

# A module is just a .py file
# mymath.py
def add(a, b):
    return a + b

PI = 3.14159

# A package is a directory with __init__.py
# mypackage/
#   __init__.py
#   module1.py
#   module2.py
#   subpackage/
#     __init__.py
#     module3.py

# __init__.py can be empty or contain package initialization
# mypackage/__init__.py
from .module1 import ClassA
from .module2 import func_b
__version__ = "1.0.0"
__all__ = ["ClassA", "func_b"]

# Using the package
from mypackage import ClassA
from mypackage.subpackage import module3

# __all__ controls 'from package import *'
# Without __all__, * imports only what's in __init__.py

# script.py
def main():
    print("Running main")

def helper():
    print("Helper function")

if __name__ == "__main__":
    # This code only runs when the file is executed directly
    # NOT when imported as a module
    main()

# When you run: python script.py
# __name__ is "__main__" -> main() runs

# When you: import script
# __name__ is "script" -> main() does NOT run
# This lets the module be both a script and an importable library

# Common pattern for CLI tools
def main():
    import argparse
    parser = argparse.ArgumentParser()
    parser.add_argument("--name", required=True)
    args = parser.parse_args()
    print(f"Hello, {args.name}!")

if __name__ == "__main__":
    main()

# os - operating system interface
import os
os.getcwd()           # current directory
os.listdir(".")       # list files
os.environ.get("HOME")  # environment variables

# sys - system-specific
import sys
sys.argv              # command-line arguments
sys.exit(0)           # exit with status code
sys.path              # module search path

# datetime - date and time
from datetime import datetime, timedelta
now = datetime.now()

# collections - specialized containers
from collections import Counter, defaultdict, deque

# itertools - iterator tools
from itertools import chain, cycle, repeat, product

# functools - higher-order functions
from functools import lru_cache, reduce, partial

# typing - type hints
from typing import List, Dict, Optional, Union, Any

# pathlib - path handling
from pathlib import Path

# subprocess - run external commands
import subprocess
result = subprocess.run(["ls", "-l"], capture_output=True, text=True)

# Create a virtual environment
# $ python -m venv venv

# Activate it
# Windows: venv\Scripts\activate
# Unix:    source venv/bin/activate

# Install packages
# $ pip install requests
# $ pip install requests==2.28.0
# $ pip install "requests>=2.25,<3.0"

# Install from requirements file
# $ pip install -r requirements.txt

# requirements.txt example:
# requests==2.31.0
# numpy>=1.21.0
# pandas~=2.0.0  # compatible release

# List installed packages
# $ pip list
# $ pip freeze > requirements.txt

# Uninstall
# $ pip uninstall requests

# Show package info
# $ pip show requests

# Modern alternative: uv (faster)
# $ uv pip install requests

from datetime import datetime, date, time, timedelta

# Current date and time
now = datetime.now()          # local time
utc_now = datetime.utcnow()   # UTC (deprecated in 3.12)
utc = datetime.now(timezone.utc)  # preferred

# Current date
today = date.today()

# Create specific date/time
dt = datetime(2024, 1, 15, 10, 30, 0)
d = date(2024, 1, 15)
t = time(10, 30, 0)

# Access components
print(now.year, now.month, now.day)
print(now.hour, now.minute, now.second)
print(now.weekday())  # 0=Monday, 6=Sunday

# From timestamp
ts = 1705315200
dt = datetime.fromtimestamp(ts)

# To timestamp
print(datetime.now().timestamp())

from datetime import datetime

# Format datetime to string (strftime)
dt = datetime(2024, 1, 15, 10, 30)
print(dt.strftime("%Y-%m-%d"))         # 2024-01-15
print(dt.strftime("%Y/%m/%d %H:%M"))   # 2024/01/15 10:30
print(dt.strftime("%B %d, %Y"))        # January 15, 2024
print(dt.strftime("%A"))               # Monday

# Parse string to datetime (strptime)
dt = datetime.strptime("2024-01-15", "%Y-%m-%d")
dt = datetime.strptime("15/01/2024 10:30", "%d/%m/%Y %H:%M")

# Common format codes:
# %Y  year (2024)     %m  month (01)
# %d  day (15)        %H  hour (14)
# %M  minute (30)     %S  second (00)
# %B  month name      %b  month abbrev
# %A  weekday name    %a  weekday abbrev
# %I  12-hour         %p  AM/PM
# %j  day of year     %U  week number

# ISO format (recommended for storage)
iso = dt.isoformat()  # "2024-01-15T10:30:00"
dt = datetime.fromisoformat("2024-01-15T10:30:00")

from datetime import datetime, timedelta

now = datetime.now()

# Add/subtract time
tomorrow = now + timedelta(days=1)
last_week = now - timedelta(weeks=1)
in_2_hours = now + timedelta(hours=2)
in_90_days = now + timedelta(days=90)

# Difference between dates
date1 = datetime(2024, 1, 15)
date2 = datetime(2024, 6, 18)
diff = date2 - date1
print(diff.days)         # 155
print(diff.total_seconds())

# timedelta components
td = timedelta(days=5, hours=3, minutes=30)
print(td.days)           # 5
print(td.seconds)        # 12600 (3h 30m in seconds)
print(td.total_seconds())

# Comparisons
if now > date1:
    print("now is later")

# Business day calculation (using numpy)
# import numpy as np
# business_days = np.busday_count(date1.date(), date2.date())

from datetime import datetime, timezone, timedelta

# Timezone-aware datetime
utc_time = datetime.now(timezone.utc)
print(utc_time)  # 2024-01-15 10:30:00+00:00

# Create a timezone (offset-based)
tz_ny = timezone(timedelta(hours=-5), "EST")
ny_time = datetime.now(tz_ny)

# Convert between timezones
utc_time = datetime.now(timezone.utc)
ny_time = utc_time.astimezone(timezone(timedelta(hours=-5)))
tokyo_time = utc_time.astimezone(timezone(timedelta(hours=9)))

# Use zoneinfo (Python 3.9+) for IANA timezones
from zoneinfo import ZoneInfo
tz = ZoneInfo("America/New_York")
dt = datetime.now(tz)
print(dt.tzname())  # EST or EDT

# Common IANA timezones:
# "UTC"
# "America/New_York", "America/Los_Angeles"
# "Europe/London", "Europe/Paris"
# "Asia/Tokyo", "Asia/Shanghai"
# "Australia/Sydney"

# Best practice: store UTC, convert for display
utc_stored = datetime.now(timezone.utc)
local_display = utc_stored.astimezone(ZoneInfo("Asia/Shanghai"))

import re

# re.search - find first match anywhere in string
m = re.search(r"\d{4}", "Order #2024 was placed")
if m:
    print(m.group())  # 2024
    print(m.start(), m.end())  # 9 13

# re.match - match at beginning of string
m = re.match(r"Hello", "Hello, World")
print(m.group())  # Hello

# re.fullmatch - entire string must match
m = re.fullmatch(r"\d+", "12345")
print(bool(m))  # True

# re.findall - all matches as list
emails = re.findall(r"\S+@\S+", text)
numbers = re.findall(r"\d+", "a1b22c333")  # ['1', '22', '333']

# re.finditer - all matches as iterator (with positions)
for m in re.finditer(r"\w+", "Hello World"):
    print(m.group(), m.span())

# Match object methods
m = re.search(r"(\w+)@(\w+)", "[email protected]")
print(m.group())   # user@domain (whole match)
print(m.group(1))  # user (first group)
print(m.group(2))  # domain (second group)
print(m.groups())  # ('user', 'domain')

import re

# Character classes
re.findall(r"[aeiou]", "hello")      # vowels
re.findall(r"[^aeiou]", "hello")     # non-vowels
re.findall(r"[a-z]", "Hello123")     # lowercase
re.findall(r"[A-Za-z0-9]", "Hi-1!")  # alphanumeric

# Predefined classes
# .   any char except newline
# \d  digit [0-9]      \D  non-digit
# \w  word char [a-zA-Z0-9_]  \W  non-word
# \s  whitespace       \S  non-whitespace

# Quantifiers
# *     0 or more
# +     1 or more
# ?     0 or 1
# {n}   exactly n
# {n,}  n or more
# {n,m} between n and m
re.findall(r"\d{3}", "1234567")      # ['123', '456']
re.findall(r"\d{2,4}", "12345678")   # ['1234', '5678']

# Anchors
# ^  start of string    $  end of string
# \b word boundary
re.findall(r"^\w+", "Hello World")   # ['Hello']
re.findall(r"\b\w+\b", "hi there") # ['hi', 'there']

# Groups & alternation
re.findall(r"(cat|dog)", "cat and dog")  # ['cat', 'dog']
re.findall(r"(\w+)@(\w+\.\w+)", "[email protected]")

import re

# re.sub - replace matches
result = re.sub(r"\d+", "#", "a1b22c333")
# 'a#b#c#'

# Replace with count limit
result = re.sub(r"\d", "X", "a1b2c3", count=2)
# 'aXbXc3'

# Use backreferences in replacement
result = re.sub(r"(\w+)@(\w+)", r"\2.\1", "user@domain")
# 'domain.user'

# Use function as replacement
def upper(m):
    return m.group().upper()

result = re.sub(r"\b[a-z]", upper, "hello world")
# 'Hello World' (capitalize first letter of each word)

# re.split - split by pattern
parts = re.split(r"[,;\s]+", "a, b; c d")
# ['a', 'b', 'c', 'd']

# Split with capture groups (keeps delimiters)
parts = re.split(r"([,;])", "a,b;c")
# ['a', ',', 'b', ';', 'c']

# Split with maxsplit
parts = re.split(r",", "a,b,c,d", maxsplit=2)
# ['a', 'b', 'c,d']

import re

# Compile pattern for reuse (faster when used many times)
email_re = re.compile(r"^[\w.+-]+@([\w-]+\.)+[\w-]+$")
print(email_re.match("[email protected]"))  # match object
print(email_re.match("invalid"))           # None

# Common flags
re.IGNORECASE  # case-insensitive
re.MULTILINE   # ^ and $ match line boundaries
re.DOTALL      # . matches newline too
re.VERBOSE     # allow whitespace & comments in pattern

# Combine flags with |
pattern = re.compile(r"""
    ^           # start of line
    (\w+)      # capture word
    \s+        # whitespace
    (\d+)      # capture number
""", re.VERBOSE | re.MULTILINE)

# Named groups (more readable)
m = re.match(r"(?P<year>\d{4})-(?P<month>\d{2})", "2024-01")
print(m.group("year"))   # 2024
print(m.group("month"))  # 01
print(m.groupdict())     # {'year': '2024', 'month': '01'}

# Lookahead/lookbehind
re.findall(r"\d+(?= dollars)", "100 dollars, 200 euros")
# ['100']  (positive lookahead)
re.findall(r"(?<=\$)\d+", "$100 and $200")
# ['100', '200']  (positive lookbehind)

import asyncio

# Define a coroutine
async def greet(name, delay):
    await asyncio.sleep(delay)  # non-blocking sleep
    return f"Hello, {name}!"

# Run a coroutine
async def main():
    result = await greet("Alice", 1)
    print(result)

# Run the event loop
asyncio.run(main())

# Concurrent execution with gather
async def main():
    # Run coroutines concurrently
    results = await asyncio.gather(
        greet("Alice", 2),
        greet("Bob", 1),
        greet("Carol", 3)
    )
    print(results)  # all complete after 3 seconds (max delay)

asyncio.run(main())

# asyncio.create_task - schedule without awaiting immediately
async def main():
    task = asyncio.create_task(greet("Alice", 1))
    # do other work here
    result = await task  # await when needed
    print(result)

import asyncio

# Async timeout
async def fetch_with_timeout(url, timeout=5):
    try:
        async with asyncio.timeout(timeout):
            # simulate async operation
            await asyncio.sleep(2)
            return f"Data from {url}"
    except asyncio.TimeoutError:
        return "Request timed out"

# Using aiohttp (third-party) for HTTP
# import aiohttp
# 
# async def fetch(url):
#     async with aiohttp.ClientSession() as session:
#         async with session.get(url) as response:
#             return await response.text()

# Producer-consumer pattern
async def producer(queue):
    for i in range(5):
        await asyncio.sleep(0.1)
        await queue.put(f"item-{i}")
    await queue.put(None)  # sentinel

async def consumer(queue):
    while True:
        item = await queue.get()
        if item is None:
            break
        print(f"Processed: {item}")
        queue.task_done()

async def main():
    queue = asyncio.Queue()
    await asyncio.gather(producer(queue), consumer(queue))

asyncio.run(main())

import threading
import time

# Basic threading
def worker(name, delay):
    print(f"Worker {name} starting")
    time.sleep(delay)
    print(f"Worker {name} done")

# Create and start threads
t1 = threading.Thread(target=worker, args=("A", 2))
t2 = threading.Thread(target=worker, args=("B", 1))
t1.start()
t2.start()

# Wait for threads to complete
t1.join()
t2.join()
print("All done")

# Thread with Lock (for shared state)
counter = 0
lock = threading.Lock()

def increment():
    global counter
    for _ in range(100000):
        with lock:  # acquire/release lock
            counter += 1

threads = [threading.Thread(target=increment) for _ in range(5)]
for t in threads: t.start()
for t in threads: t.join()
print(f"Counter: {counter}")  # 500000 (correct with lock)

from multiprocessing import Process, Pool, Queue
import os

# Basic process
def worker(name):
    print(f"Process {name} PID: {os.getpid()}")

if __name__ == "__main__":
    p = Process(target=worker, args=("A",))
    p.start()
    p.join()

# Process pool for parallel work
def square(x):
    return x * x

if __name__ == "__main__":
    with Pool(4) as pool:  # 4 worker processes
        results = pool.map(square, range(10))
        print(results)  # [0, 1, 4, 9, ..., 81]
    
    # Asynchronous map
    with Pool(4) as pool:
        result = pool.map_async(square, range(10))
        print(result.get())  # blocks until done

# concurrent.futures (higher-level API)
from concurrent.futures import ProcessPoolExecutor, ThreadPoolExecutor

with ProcessPoolExecutor() as executor:
    results = list(executor.map(square, range(10)))

with ThreadPoolExecutor() as executor:
    futures = [executor.submit(square, i) for i in range(10)]
    results = [f.result() for f in futures]

# A decorator is a function that takes a function and returns a new function
def uppercase_result(func):
    def wrapper(*args, **kwargs):
        result = func(*args, **kwargs)
        return result.upper()
    return wrapper

@uppercase_result
def greet(name):
    return f"hello, {name}"

print(greet("world"))  # HELLO, WORLD
# @uppercase_result is sugar for: greet = uppercase_result(greet)

from functools import wraps

def log_calls(func):
    @wraps(func)  # copies __name__, __doc__, __module__
    def wrapper(*args, **kwargs):
        print(f"Calling {func.__name__}({args}, {kwargs})")
        return func(*args, **kwargs)
    return wrapper

@log_calls
def add(a, b):
    """Add two numbers."""
    return a + b

print(add.__name__)  # 'add' (not 'wrapper')
print(add.__doc__)   # 'Add two numbers.'
help(add)            # shows original docstring

# A decorator factory: returns the actual decorator
def repeat(times):
    def decorator(func):
        @wraps(func)
        def wrapper(*args, **kwargs):
            result = None
            for _ in range(times):
                result = func(*args, **kwargs)
            return result
        return wrapper
    return decorator

@repeat(times=3)
def say_hi(name):
    print(f"Hi, {name}!")

say_hi("Alice")
# Hi, Alice! (printed 3 times)

class CountCalls:
    def __init__(self, func):
        self.func = func
        self.count = 0
        wraps(func)(self)  # preserve metadata

    def __call__(self, *args, **kwargs):
        self.count += 1
        print(f"{self.func.__name__} called {self.count} times")
        return self.func(*args, **kwargs)

@CountCalls
def say_hello():
    print("Hello!")

say_hello()  # count=1
say_hello()  # count=2
say_hello()  # count=3
print(say_hello.count)  # 3

class Circle:
    pi = 3.14159

    def __init__(self, radius):
        self._radius = radius

    @property
    def area(self):
        return Circle.pi * self._radius ** 2

    @property
    def radius(self):
        return self._radius

    @radius.setter
    def radius(self, value):
        if value < 0:
            raise ValueError("Radius cannot be negative")
        self._radius = value

    @classmethod
    def from_diameter(cls, diameter):
        return cls(diameter / 2)

    @staticmethod
    def is_valid_radius(r):
        return r >= 0

c = Circle(5)
print(c.area)              # 78.54 (no parentheses!)
c.radius = 10             # uses the setter
c2 = Circle.from_diameter(20)  # alternative constructor

from functools import wraps

def bold(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        return f"<b>{func(*args, **kwargs)}</b>"
    return wrapper

def italic(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        return f"<i>{func(*args, **kwargs)}</i>"
    return wrapper

@bold
@italic
def greet(name):
    return f"Hello, {name}"

print(greet("World"))
# <b><i>Hello, World</i></b>
# Applied bottom-up: italic first, then bold

# A generator uses 'yield' to produce values lazily, one at a time
def count_up_to(n):
    count = 1
    while count <= n:
        yield count
        count += 1

gen = count_up_to(5)
print(next(gen))  # 1
print(next(gen))  # 2
print(list(gen))  # [3, 4, 5] (exhausts the rest)

# Generators are memory-efficient: they don't build the whole list
def fibonacci():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

fib = fibonacci()
print([next(fib) for _ in range(10)])
# [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

# Like list comprehensions, but lazy (uses parentheses)
squares_list = [x ** 2 for x in range(10)]      # builds full list
squares_gen  = (x ** 2 for x in range(10))      # lazy generator

print(squares_gen)        # <generator object>
print(next(squares_gen))  # 0
print(next(squares_gen))  # 1

# Memory comparison
import sys
print(sys.getsizeof([x for x in range(10000)]))  # ~87616 bytes
print(sys.getsizeof((x for x in range(10000))))  # ~200 bytes (constant!)

# Use in sum(), list(), any() etc.
total = sum(x ** 2 for x in range(100))  # no extra list created
print(total)  # 328350

class Range2:
    """A custom iterator that yields even numbers."""
    def __init__(self, start, end):
        self.current = start
        self.end = end

    def __iter__(self):
        return self  # the object is its own iterator

    def __next__(self):
        if self.current >= self.end:
            raise StopIteration
        value = self.current
        self.current += 2
        return value

r = Range2(0, 10)
for num in r:
    print(num)  # 0, 2, 4, 6, 8

# An iterable returns a fresh iterator each time __iter__ is called.
# An iterator returns itself and raises StopIteration when exhausted.

def echo():
    while True:
        received = yield  # yield without a value, receives via send()
        print(f"Echo: {received}")

gen = echo()
next(gen)          # prime the generator (advance to first yield)
gen.send("hello")  # Echo: hello
gen.send("world")  # Echo: world

# throw() injects an exception at the yield point
def safe_gen():
    try:
        while True:
            yield
    except ValueError:
        print("Caught ValueError inside generator")

g = safe_gen()
next(g)
g.throw(ValueError, "boom")  # Caught ValueError inside generator

# close() stops the generator (raises GeneratorExit)
gen.close()

# Chain generators to build data-processing pipelines
def numbers():
    for i in range(1, 11):
        yield i

def squared(seq):
    for n in seq:
        yield n ** 2

def evens(seq):
    for n in seq:
        if n % 2 == 0:
            yield n

# Each stage processes one item at a time — no intermediate lists
pipeline = evens(squared(numbers()))
print(list(pipeline))  # [4, 16, 36, 64, 100]

# Equivalent with generator expressions:
result = (n for n in (x ** 2 for x in range(1, 11)) if n % 2 == 0)
print(list(result))  # [4, 16, 36, 64, 100]

# yield from delegates to a sub-iterator (Python 3.3+)
def flatten(nested):
    for item in nested:
        if isinstance(item, (list, tuple)):
            yield from flatten(item)  # recursive delegation
        else:
            yield item

data = [1, [2, 3, [4, 5]], 6, [7, [8, [9]]]]
print(list(flatten(data)))
# [1, 2, 3, 4, 5, 6, 7, 8, 9]

# yield from also forwards send()/throw() to the sub-generator,
# making it essential for coroutine composition.

# 'with' guarantees cleanup even if an exception occurs
with open("data.txt", "r") as f:
    content = f.read()
# f.close() is called automatically here, even if read() raised

# Without 'with' you must manually close:
f = open("data.txt", "r")
try:
    content = f.read()
finally:
    f.close()  # easy to forget!

# Common built-in context managers:
with open("out.txt", "w") as f, open("in.txt") as g:
    f.write(g.read())  # both files close automatically

class Timer:
    def __init__(self, label="Timer"):
        self.label = label

    def __enter__(self):
        import time
        self.start = time.perf_counter()
        return self  # value bound to 'as' variable

    def __exit__(self, exc_type, exc_val, exc_tb):
        import time
        elapsed = time.perf_counter() - self.start
        print(f"{self.label}: {elapsed:.4f}s")
        # Return False (or None) to propagate exceptions
        # Return True to suppress the exception
        return False

with Timer("Processing"):
    total = sum(i ** 2 for i in range(1_000_000))
# Processing: 0.1234s

from contextlib import contextmanager
import time

@contextmanager
def timer(label="Timer"):
    start = time.perf_counter()
    try:
        yield  # code inside the 'with' block runs here
    finally:
        elapsed = time.perf_counter() - start
        print(f"{label}: {elapsed:.4f}s")

with timer("My task"):
    sum(i ** 2 for i in range(1_000_000))
# My task: 0.1234s

# You can also yield a value to bind with 'as'
@contextmanager
def open_db(path):
    db = connect(path)
    try:
        yield db
    finally:
        db.close()

with open_db("app.db") as db:
    db.query("SELECT 1")

# Python 3.10+ supports parenthesized context managers
with (
    open("input.txt") as fin,
    open("output.txt", "w") as fout,
):
    fout.write(fin.read())

# Pre-3.10: nest them or use contextlib.ExitStack
from contextlib import ExitStack

files = ["a.txt", "b.txt", "c.txt"]
with ExitStack() as stack:
    handles = [stack.enter_context(open(f)) for f in files]
    for h in handles:
        print(h.read())

from contextlib import suppress, redirect_stdout, redirect_stderr
import io, warnings

# suppress: ignore specific exceptions (cleaner than try/except/pass)
with suppress(FileNotFoundError):
    os.remove("temp.txt")  # no error if file doesn't exist

# redirect_stdout: capture print output
buffer = io.StringIO()
with redirect_stdout(buffer):
    print("This goes to the buffer, not console")
captured = buffer.getvalue()

# redirect_stderr: capture error/warning output
err_buf = io.StringIO()
with redirect_stderr(err_buf):
    warnings.warn("a warning")
print(err_buf.getvalue())  # the warning text

# Also: contextlib.chdir (3.11+) to temporarily change directory
# from contextlib import chdir
# with chdir("/tmp"): ...

import asyncio

class AsyncDB:
    async def __aenter__(self):
        print("connecting...")
        await asyncio.sleep(0.1)  # simulate async connect
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        print("closing...")
        await asyncio.sleep(0.1)  # simulate async close
        return False

    async def query(self, sql):
        return f"result of: {sql}"

async def main():
    async with AsyncDB() as db:
        result = await db.query("SELECT 1")
        print(result)

asyncio.run(main())
# connecting...
# result of: SELECT 1
# closing...

# Variable annotations (Python 3.6+)
name: str = "Alice"
age: int = 30
scores: list[float] = [95.5, 88.0, 92.3]
config: dict[str, int] = {"timeout": 30}

# Function annotations
def greet(name: str, excited: bool = False) -> str:
    punctuation = "!" if excited else "."
    return f"Hello, {name}{punctuation}"

print(greet("World", excited=True))  # Hello, World!

# Annotations are optional and NOT enforced at runtime
def add(a: int, b: int) -> int:
    return a + b
add("2", "3")  # runs fine, returns "23" (no error!)

from typing import List, Dict, Tuple, Set, FrozenSet

# Pre-3.9 style (still works, needed for older Python)
names: List[str] = ["Alice", "Bob"]
scores: Dict[str, int] = {"Alice": 95}
point: Tuple[int, int] = (10, 20)
mixed: Tuple[str, int, float] = ("a", 1, 2.0)
variadic: Tuple[int, ...] = (1, 2, 3)  # variable-length

# Python 3.9+ built-in generics (preferred)
names: list[str] = ["Alice", "Bob"]
scores: dict[str, int] = {"Alice": 95}
point: tuple[int, int] = (10, 20)

# Optional means "could be None"
from typing import Optional
def find_user(uid: int) -> Optional[str]:
    if uid == 1:
        return "Alice"
    return None  # could also just 'return'

from typing import Union, Literal, overload

# Union: value can be one of several types
def process(data: Union[str, bytes]) -> str:
    if isinstance(data, bytes):
        return data.decode("utf-8")
    return data

# Python 3.10+ union syntax with | (preferred)
def process2(data: str | bytes) -> str:
    if isinstance(data, bytes):
        return data.decode("utf-8")
    return data

# Literal: restrict to specific constant values
def set_mode(mode: Literal["r", "w", "a"]) -> None:
    print(f"Mode set to {mode}")

set_mode("r")   # OK
# set_mode("x")  # mypy error: not a valid literal

# Literal for boolean-like flags
Direction = Literal["up", "down", "left", "right"]

from typing import TypeVar, Generic, List

T = TypeVar("T")  # a generic type variable

def first(items: List[T]) -> T:
    return items[0]

# Type inference: T is bound to the argument's type
x: int = first([1, 2, 3])        # T = int
y: str = first(["a", "b", "c"])  # T = str

# Bounded TypeVar: T must be a subtype of Number
from typing import TypeVar
from numbers import Number
N = TypeVar("N", bound=Number)

def sum_all(values: list[N]) -> N:
    total = values[0]
    for v in values[1:]:
        total = total + v
    return total

# Generic class
class Stack(Generic[T]):
    def __init__(self) -> None:
        self._items: list[T] = []
    def push(self, item: T) -> None:
        self._items.append(item)
    def pop(self) -> T:
        return self._items.pop()

s: Stack[int] = Stack()
s.push(1)
# s.push("x")  # mypy error

from typing import Callable, Protocol, TypeAlias

# Callable signature: Callable[[ArgTypes], ReturnType]
def apply(func: Callable[[int, int], int], a: int, b: int) -> int:
    return func(a, b)

apply(lambda x, y: x + y, 3, 4)  # 7

# Type aliases (3.12+ uses 'type' statement; older uses assignment)
type Vector = list[float]          # 3.12+
Vector2: TypeAlias = list[float]   # 3.10+

def magnitude(v: Vector) -> float:
    return sum(x ** 2 for x in v) ** 0.5

# Protocol: structural typing (duck typing with static checks)
class Drawable(Protocol):
    def draw(self) -> None: ...

def render(obj: Drawable) -> None:
    obj.draw()  # any object with a draw() method works

class Circle:
    def draw(self) -> None:
        print("drawing circle")

render(Circle())  # OK — Circle has draw()

# Save as example.py, then run:  mypy example.py
from typing import Optional

def divide(a: float, b: float) -> Optional[float]:
    if b == 0:
        return None
    return a / b

result = divide(10, 0)
# Without checking, this would crash at runtime:
# print(result + 1)  # TypeError: NoneType + int

# mypy catches it:
# error: Unsupported operand types for + ("None" and "int")
# fix: check for None first
if result is not None:
    print(result + 1)

# Run strict mode for maximum safety:
#   mypy --strict example.py
# Common strict flags: --disallow-untyped-defs, --no-implicit-optional

from dataclasses import dataclass

@dataclass
class Point:
    x: float
    y: float

p1 = Point(1.0, 2.0)
p2 = Point(1.0, 2.0)

print(p1)         # Point(x=1.0, y=2.0)  — auto __repr__
print(p1 == p2)   # True  — auto __eq__ (compares fields)
print(p1.x)       # 1.0

# Without @dataclass you'd write all this boilerplate:
# class Point:
#     def __init__(self, x, y): self.x = x; self.y = y
#     def __repr__(self): ...
#     def __eq__(self, other): ...

from dataclasses import dataclass, field

@dataclass
class Student:
    name: str
    grade: str = "A"          # simple default
    tags: list[str] = field(default_factory=list)  # mutable default!
    scores: dict[str, int] = field(default_factory=dict)

s = Student("Alice")
print(s)  # Student(name='Alice', grade='A', tags=[], scores={})

s.tags.append("honors")
s2 = Student("Bob")
print(s2.tags)  # [] — each instance gets its own list

# NEVER use [] or {} as a direct default — all instances would share
# the same mutable object (classic Python pitfall).

from dataclasses import dataclass

# frozen=True makes instances immutable (hashable, usable as dict keys)
@dataclass(frozen=True)
class Color:
    r: int
    g: int
    b: int

c = Color(255, 0, 0)
# c.r = 128  # FrozenInstanceError!
print(hash(c))  # works — frozen dataclasses are hashable

# order=True generates __lt__, __le__, __gt__, __ge__ for sorting
@dataclass(order=True)
class Priority:
    level: int

tasks = [Priority(3), Priority(1), Priority(2)]
tasks.sort()
print(tasks)  # [Priority(level=1), Priority(level=2), Priority(level=3)]

# Common combo: frozen + order for immutable comparable values
@dataclass(frozen=True, order=True)
class Version:
    major: int
    minor: int

from dataclasses import dataclass, field

@dataclass
class User:
    email: str
    _email_normalized: str = field(init=False, repr=False)
    id: int = field(default=0)

    def __post_init__(self):
        # runs after __init__; compute derived fields here
        self._email_normalized = self.email.strip().lower()

u = User("  [email protected] ")
print(u.email)             # '  [email protected] '
print(u._email_normalized) # '[email protected]'

# field(init=False) excludes a field from __init__
# field(repr=False) hides it from the repr
# field(compare=False) excludes from __eq__/__hash__
# field(metadata={...}) attaches custom metadata

from dataclasses import dataclass

@dataclass
class Animal:
    name: str
    sound: str = "..."

@dataclass
class Dog(Animal):
    breed: str = "unknown"
    sound: str = "Woof"  # override parent default

d = Dog("Rex", breed="Labrador")
print(d)  # Dog(name='Rex', sound='Woof', breed='Labrador')

# Python 3.10+: slots=True saves memory (no __dict__)
@dataclass(slots=True)
class Pixel:
    r: int
    g: int
    b: int

p = Pixel(0, 128, 255)
# p.new_field = 1  # AttributeError — slots prevent arbitrary attrs
# Saves ~40-50% memory vs regular dataclass for many instances

from collections import namedtuple

# Lightweight immutable class with named fields
Point = namedtuple("Point", ["x", "y"])
p = Point(3, 4)
print(p.x, p.y)       # 3 4  — access by name
print(p[0], p[1])     # 3 4  — also by index
print(p._asdict())    # {'x': 3, 'y': 4}

# More memory-efficient than a full class
# Use _replace to create a modified copy (immutable!)
p2 = p._replace(x=10)
print(p2)  # Point(x=10, y=4)

# Python 3.6+ typing.NamedTuple for type hints:
from typing import NamedTuple
class Point3D(NamedTuple):
    x: float
    y: float
    z: float = 0.0

from collections import Counter

# Count hashable items
words = "the cat sat on the mat the cat".split()
c = Counter(words)
print(c)  # Counter({'the': 3, 'cat': 2, 'sat': 1, 'on': 1, 'mat': 1})

# Most common elements
print(c.most_common(2))  # [('the', 3), ('cat', 2)]

# Arithmetic on counters
c1 = Counter(a=3, b=1)
c2 = Counter(a=1, b=2)
print(c1 + c2)  # Counter({'a': 4, 'b': 3})
print(c1 - c2)  # Counter({'a': 2})  (drops zero/negatives)

# Missing keys return 0 (not KeyError)
print(c["dog"])  # 0

# Update and elements
c.update(["cat", "cat"])
print(sorted(c.elements()))  # ['cat','cat','cat','cat','mat','on','sat','the','the','the']

from collections import defaultdict

# Group items by key without checking if key exists
words = ["apple", "banana", "avocado", "blueberry", "cherry"]
by_first = defaultdict(list)
for w in words:
    by_first[w[0]].append(w)
print(dict(by_first))
# {'a': ['apple', 'avocado'], 'b': ['banana', 'blueberry'], 'c': ['cherry']}

# Counting with int (default 0)
counts = defaultdict(int)
for w in words:
    counts[w[0]] += 1
print(dict(counts))  # {'a': 2, 'b': 2, 'c': 1}

# Nested defaultdicts
tree = defaultdict(lambda: defaultdict(list))
tree["2024"]["Jan"].append("event1")

# vs regular dict: avoids the key-check boilerplate
# d = {}
# for w in words:
#     if w[0] not in d:
#         d[w[0]] = []
#     d[w[0]].append(w)

from collections import OrderedDict, deque

# deque: double-ended queue, O(1) append/pop at both ends
dq = deque([1, 2, 3], maxlen=5)
dq.appendleft(0)   # deque([0, 1, 2, 3])
dq.append(4)       # deque([0, 1, 2, 3, 4])
dq.append(5)       # deque([1, 2, 3, 4, 5]) — oldest dropped (maxlen!)
print(dq.popleft())  # 1
print(dq)            # deque([2, 3, 4, 5])

# deque is ideal for queues, BFS, sliding windows
from collections import deque
queue = deque(["task1", "task2"])
queue.append("task3")
next_task = queue.popleft()  # FIFO — O(1) vs list.pop(0) which is O(n)

# OrderedDict: remembers insertion order (regular dicts do too in 3.7+,
# but OrderedDict has move_to_end and equality is order-sensitive)
od = OrderedDict([("a", 1), ("b", 2)])
od.move_to_end("a")  # move to last
print(list(od))  # ['b', 'a']
od.popitem(last=False)  # pop first item (FIFO)

from itertools import chain, product, combinations, permutations

# chain: flatten multiple iterables
list(chain([1, 2], [3, 4], [5]))  # [1, 2, 3, 4, 5]
list(chain.from_iterable([[1, 2], [3, 4]]))  # [1, 2, 3, 4]

# product: Cartesian product (nested loops)
list(product([1, 2], ["a", "b"]))
# [(1,'a'), (1,'b'), (2,'a'), (2,'b')]
list(product("AB", repeat=2))  # [('A','A'),('A','B'),('B','A'),('B','B')]

# combinations: unordered selections (no repeats)
list(combinations("ABC", 2))  # [('A','B'),('A','C'),('B','C')]
list(combinations("AAA", 2))  # [('A','A'),('A','A'),('A','A')]

# permutations: ordered arrangements
list(permutations("ABC", 2))  # [('A','B'),('A','C'),('B','A'),('B','C'),('C','A'),('C','B')]

# combinations_with_replacement: allow picking same element
from itertools import combinations_with_replacement
list(combinations_with_replacement("AB", 2))  # [('A','A'),('A','B'),('B','B')]

from itertools import groupby, accumulate, starmap, islice, takewhile, dropwhile

# groupby: group consecutive items by a key (sort first!)
data = [("A", 1), ("A", 2), ("B", 3), ("B", 4), ("A", 5)]
data.sort(key=lambda x: x[0])  # MUST sort by key first
for key, group in groupby(data, key=lambda x: x[0]):
    print(key, list(group))
# A [('A',1),('A',2),('A',5)]
# B [('B',3),('B',4)]

# accumulate: running aggregate (sum by default)
list(accumulate([1, 2, 3, 4]))  # [1, 3, 6, 10]
import operator
list(accumulate([1, 2, 3, 4], operator.mul))  # [1, 2, 6, 24]

# starmap: unpack args from tuples before calling
list(starmap(pow, [(2, 3), (3, 2), (10, 3)]))  # [8, 9, 1000]

# islice: slice an iterator (doesn't support negative indices)
list(islice(range(100), 5, 10))  # [5, 6, 7, 8, 9]

# takewhile / dropwhile: filter by predicate
list(takewhile(lambda x: x < 5, [1, 4, 6, 3, 8]))  # [1, 4]
list(dropwhile(lambda x: x < 5, [1, 4, 6, 3, 8]))  # [6, 3, 8]

from functools import lru_cache, partial, reduce
import operator

# lru_cache: memoize function results (Least Recently Used)
@lru_cache(maxsize=128)
def fib(n):
    if n < 2:
        return n
    return fib(n - 1) + fib(n - 2)

print(fib(100))  # instant (without cache: impossibly slow)
print(fib.cache_info())  # CacheInfo(hits=98, misses=101, ...)

# partial: fix some arguments, create a new callable
def power(base, exponent):
    return base ** exponent

square = partial(power, exponent=2)
cube = partial(power, exponent=3)
print(square(5))  # 25
print(cube(3))    # 27

# reduce: cumulatively apply a function, reducing to one value
product = reduce(operator.mul, [1, 2, 3, 4])  # 24
# Equivalent: ((1*2)*3)*4

# Python 3.8+: cached_property for lazy computed attributes
from functools import cached_property
class Data:
    @cached_property
    def expensive(self):
        print("computing...")
        return [i ** 2 for i in range(1000000)]

import json

# Serialize Python object to JSON string
data = {"name": "Alice", "age": 30, "scores": [95, 88, 92]}
json_str = json.dumps(data)
print(json_str)  # {"name": "Alice", "age": 30, "scores": [95, 88, 92]}

# Pretty-print with indent
print(json.dumps(data, indent=2))
# {
#   "name": "Alice",
#   "age": 30,
#   "scores": [95, 88, 92]
# }

# Parse JSON string to Python object
parsed = json.loads(json_str)
print(parsed["name"])  # Alice
print(type(parsed["scores"]))  # <class 'list'>

# Sort keys, handle non-ASCII
print(json.dumps({"name": "Zoë"}, ensure_ascii=False, sort_keys=True))

import json

data = {"users": [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]}

# Write to file
with open("data.json", "w", encoding="utf-8") as f:
    json.dump(data, f, indent=2, ensure_ascii=False)

# Read from file
with open("data.json", "r", encoding="utf-8") as f:
    loaded = json.load(f)

print(loaded["users"][0]["name"])  # Alice

# Type conversions to remember:
# JSON object  <-> Python dict
# JSON array   <-> Python list
# JSON string  <-> Python str
# JSON number  <-> Python int/float
# JSON true/false <-> Python True/False
# JSON null    <-> Python None

import json
from datetime import datetime

# Default behavior: TypeError on non-serializable types
# json.dumps({"now": datetime.now()})  # TypeError!

# Solution 1: default function for unknown types
def default_encoder(obj):
    if isinstance(obj, datetime):
        return obj.isoformat()
    if isinstance(obj, set):
        return sorted(obj)
    raise TypeError(f"Cannot serialize {type(obj)}")

data = {"now": datetime.now(), "tags": {"a", "b"}}
print(json.dumps(data, default=default_encoder))

# Solution 2: custom JSONEncoder subclass
class MyEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, datetime):
            return {"__datetime__": obj.isoformat()}
        return super().default(obj)

print(json.dumps(data, cls=MyEncoder))

# Decoding with object_hook
def decoder(dct):
    if "__datetime__" in dct:
        return datetime.fromisoformat(dct["__datetime__"])
    return dct
json.loads(json.dumps(data, cls=MyEncoder), object_hook=decoder)

import csv

# Basic reader: each row is a list of strings
with open("data.csv", newline="", encoding="utf-8") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)  # ['name', 'age', 'city']

# DictReader: each row is a dict keyed by header
with open("data.csv", newline="") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["age"])  # access by column name

# Handle different delimiters and quoting
with open("data.tsv", newline="") as f:
    reader = csv.reader(f, delimiter="\t", quotechar='"')
    for row in reader:
        print(row)

# Sniffer to auto-detect format
with open("unknown.csv", newline="") as f:
    sample = f.read(1024)
    dialect = csv.Sniffer().sniff(sample)
    f.seek(0)
    reader = csv.reader(f, dialect)

import csv

rows = [
    ["name", "age", "city"],
    ["Alice", 30, "NYC"],
    ["Bob", 25, "LA"],
]

# Basic writer
with open("out.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerows(rows)  # write multiple rows

# DictWriter: write from dicts
with open("out.csv", "w", newline="") as f:
    fieldnames = ["name", "age", "city"]
    writer = csv.DictWriter(f, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerow({"name": "Alice", "age": 30, "city": "NYC"})
    writer.writerow({"name": "Bob", "age": 25, "city": "LA"})

# Control quoting: QUOTE_MINIMAL (default), QUOTE_ALL, QUOTE_NONNUMERIC
writer = csv.writer(f, quoting=csv.QUOTE_ALL)
# QUOTE_ALL wraps every field in quotes: "Alice","30","NYC"

import json

# JSON Lines: one JSON object per line (great for logs, big data)
records = [{"id": 1, "msg": "first"}, {"id": 2, "msg": "second"}]

# Write NDJSON
with open("logs.jsonl", "w") as f:
    for rec in records:
        f.write(json.dumps(rec) + "\n")

# Read NDJSON line by line (memory-efficient for huge files)
with open("logs.jsonl", "r") as f:
    for line in f:
        rec = json.loads(line)
        print(rec["id"], rec["msg"])

# Stream large JSON arrays without loading everything into memory
# Use ijson library for streaming parsing of huge JSON files:
# import ijson
# with open("huge.json", "rb") as f:
#     for item in ijson.items(f, "items.item"):
#         process(item)  # one item at a time

import logging

# Basic configuration (call once at program start)
logging.basicConfig(
    level=logging.DEBUG,
    format="%(asctime)s [%(levelname)s] %(name)s: %(message)s",
    datefmt="%Y-%m-%d %H:%M:%S",
)

# Log levels (severity ascending)
logging.debug("Detailed debug info")     # DEBUG (10)
logging.info("General information")       # INFO (20)
logging.warning("Something unexpected")   # WARNING (30)
logging.error("A real error occurred")    # ERROR (40)
logging.critical("System is down")        # CRITICAL (50)

# Logging exceptions with traceback
try:
    1 / 0
except ZeroDivisionError:
    logging.exception("Division failed")  # includes full traceback

# Get a named logger (best practice per module)
logger = logging.getLogger(__name__)
logger.info("Module-specific log")

import logging
from logging.handlers import RotatingFileHandler

logger = logging.getLogger("myapp")
logger.setLevel(logging.DEBUG)

# Console handler (INFO and above)
console = logging.StreamHandler()
console.setLevel(logging.INFO)
console.setFormatter(logging.Formatter("%(levelname)s: %(message)s"))

# Rotating file handler (DEBUG and above, max 5MB x 3 backups)
file_handler = RotatingFileHandler(
    "app.log", maxBytes=5_000_000, backupCount=3, encoding="utf-8"
)
file_handler.setLevel(logging.DEBUG)
file_handler.setFormatter(
    logging.Formatter("%(asctime)s [%(levelname)s] %(name)s: %(message)s")
)

logger.addHandler(console)
logger.addHandler(file_handler)

logger.debug("debug to file only")
logger.info("info to both console and file")
logger.error("error everywhere")

import unittest

def add(a, b):
    return a + b

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

class TestMath(unittest.TestCase):
    def setUp(self):
        # runs before each test method
        self.data = [1, 2, 3]

    def tearDown(self):
        # runs after each test method
        pass

    def test_add(self):
        self.assertEqual(add(1, 2), 3)
        self.assertEqual(add(-1, 1), 0)

    def test_add_types(self):
        self.assertEqual(add("a", "b"), "ab")

    def test_divide_by_zero(self):
        with self.assertRaises(ValueError):
            divide(1, 0)

    def test_membership(self):
        self.assertIn(2, self.data)
        self.assertTrue(3 in self.data)

if __name__ == "__main__":
    unittest.main()
# Run: python -m unittest test_file.py -v

# test_math.py — pytest is simpler and more powerful than unittest
# Install: pip install pytest
# Run: pytest -v

def add(a, b):
    return a + b

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

# Plain functions, no classes required
def test_add():
    assert add(1, 2) == 3
    assert add(-1, 1) == 0

def test_add_strings():
    assert add("hello", " world") == "hello world"

# Testing exceptions with pytest.raises
import pytest

def test_divide_by_zero():
    with pytest.raises(ValueError, match="Cannot divide by zero"):
        divide(1, 0)

# Approximate float comparison
def test_float():
    assert 0.1 + 0.2 == pytest.approx(0.3)

import pytest

# A fixture provides setup data/resources to tests
@pytest.fixture
def sample_list():
    return [1, 2, 3, 4, 5]

# Use fixtures by passing their name as a parameter
def test_length(sample_list):
    assert len(sample_list) == 5

def test_sum(sample_list):
    assert sum(sample_list) == 15

# Fixture with setup AND teardown (yield)
@pytest.fixture
def db_connection():
    print("\n[setup] connecting to DB")
    conn = {"connected": True}
    yield conn  # test runs here; value passed to test
    print("\n[teardown] closing DB")
    conn["connected"] = False

def test_db(db_connection):
    assert db_connection["connected"] is True

# Fixture scopes: function (default), class, module, session
@pytest.fixture(scope="session")
def expensive_resource():
    return load_large_dataset()  # created once per test session

import pytest
from unittest.mock import patch, MagicMock

# parametrize: run one test with multiple inputs
@pytest.mark.parametrize("a, b, expected", [
    (1, 2, 3),
    (-1, 1, 0),
    (0, 0, 0),
    (100, 200, 300),
])
def test_add_many(a, b, expected):
    assert add(a, b) == expected

# parametrize with IDs for readable output
@pytest.mark.parametrize("x", [1, 2, 3], ids=["one", "two", "three"])
def test_ids(x):
    assert x > 0

# Mocking: replace external dependencies
def fetch_user(uid):
    # imagine this calls a real API
    return {"id": uid, "name": "real_user"}

@patch("__main__.fetch_user")
def test_with_mock(mock_fetch):
    mock_fetch.return_value = {"id": 1, "name": "mocked"}
    result = fetch_user(1)
    assert result["name"] == "mocked"
    mock_fetch.assert_called_once_with(1)

# Skip and expected failure
@pytest.mark.skip(reason="not implemented yet")
def test_future():
    pass

@pytest.mark.xfail(reason="known bug #42")
def test_known_bug():
    assert 1 == 2

import socket
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind(('localhost', 8080))
server.listen(5)
conn, addr = server.accept()
data = conn.recv(1024)
conn.sendall(b'Hello')
conn.close()

import socket
client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client.connect(('localhost', 8080))
client.sendall(b'Hello Server')
response = client.recv(1024)
print(response.decode())
client.close()

import socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(('localhost', 9090))
data, addr = sock.recvfrom(1024)
print(f"From {addr}: {data.decode()}")
sock.sendto(b'Reply', addr)

from http.server import HTTPServer, BaseHTTPRequestHandler
class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.send_header('Content-Type', 'text/html')
        self.end_headers()
        self.wfile.write(b'<h1>Hello</h1>')
HTTPServer(('localhost', 8000), Handler).serve_forever()

import socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.settimeout(5.0)
try:
    sock.connect(('example.com', 80))
    data = sock.recv(1024)
except socket.timeout:
    print('Connection timed out')
finally:
    sock.close()

import sqlite3
conn = sqlite3.connect('example.db')
cursor = conn.cursor()
cursor.execute('''CREATE TABLE IF NOT EXISTS users (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT NOT NULL, email TEXT UNIQUE, age INTEGER)''')
conn.commit()

cursor.execute(
    'INSERT INTO users (name, email, age) VALUES (?, ?, ?)',
    ('Alice', '[email protected]', 30))
conn.commit()
print(f"ID: {cursor.lastrowid}")

cursor.execute('SELECT * FROM users WHERE age > ?', (25,))
rows = cursor.fetchall()
for row in rows:
    print(row)
cursor.execute('SELECT * FROM users WHERE id = ?', (1,))
user = cursor.fetchone()

cursor.execute('UPDATE users SET age = ? WHERE name = ?', (31, 'Alice'))
cursor.execute('DELETE FROM users WHERE age < ?', (18,))
conn.commit()
print(f"Affected: {cursor.rowcount} rows")

conn = sqlite3.connect('example.db')
conn.row_factory = sqlite3.Row
with conn:
    conn.execute('INSERT INTO users (name, email) VALUES (?, ?)',
                 ('Bob', '[email protected]'))
for row in conn.execute('SELECT * FROM users'):
    print(row['name'], row['email'])

import requests
from bs4 import BeautifulSoup
resp = requests.get('https://example.com')
soup = BeautifulSoup(resp.text, 'html.parser')
print(soup.title.string)
print(soup.find('h1').text)

links = soup.find_all('a')
for link in links:
    print(link.get('href'), link.text)
article = soup.find('div', class_='article')
items = soup.select('ul.list > li.item')

table = soup.find('table')
for row in table.find_all('tr'):
    cols = row.find_all(['td', 'th'])
    data = [col.text.strip() for col in cols]
    print(data)

all_items = []
url = 'https://example.com/page/1'
while url:
    resp = requests.get(url)
    soup = BeautifulSoup(resp.text, 'html.parser')
    all_items.extend([i.text for i in soup.select('.item')])
    next_link = soup.select_one('a.next')
    url = next_link.get('href') if next_link else None

import csv
with open('data.csv', 'w', newline='', encoding='utf-8') as f:
    writer = csv.DictWriter(f, fieldnames=['name', 'price'])
    writer.writeheader()
    for item in scraped_data:
        writer.writerow(item)

import aiohttp, asyncio
async def fetch(url):
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as resp:
            return await resp.text()
data = asyncio.run(fetch('https://api.example.com'))

async def fetch_all(urls):
    async with aiohttp.ClientSession() as session:
        tasks = [session.get(url) for url in urls]
        responses = await asyncio.gather(*tasks)
        return [await r.text() for r in responses]

from aiohttp import web
async def handle(request):
    name = request.match_info.get('name', 'World')
    return web.json_response({'message': f'Hello, {name}!'})
app = web.Application()
app.add_routes([web.get('/', handle), web.get('/{name}', handle)])
web.run_app(app, port=8080)

async def ws_handler(request):
    ws = web.WebSocketResponse()
    await ws.prepare(request)
    async for msg in ws:
        if msg.type == aiohttp.WSMsgType.TEXT:
            await ws.send_str(f'Echo: {msg.data}')
    return ws

async def login_and_fetch():
    async with aiohttp.ClientSession() as session:
        await session.post('https://example.com/login',
                          data={'user': 'admin', 'pass': '123'})
        resp = await session.get('https://example.com/dashboard')
        return await resp.text()

from multiprocessing import Pool
def square(x): return x * x
if __name__ == '__main__':
    with Pool(4) as pool:
        results = pool.map(square, range(10))
        result = pool.apply_async(square, (100,))
        print(result.get(timeout=5))

from multiprocessing import Value, Array
counter = Value('i', 0)
arr = Array('d', [0.0, 1.0, 2.0])
with counter.get_lock():
    counter.value += 1

from multiprocessing import Process, Queue
def worker(q):
    q.put('Data from worker')
if __name__ == '__main__':
    q = Queue()
    p = Process(target=worker, args=(q,))
    p.start()
    print(q.get())
    p.join()

from multiprocessing import Process, Pipe
def worker(conn):
    conn.send(['hello', 'world'])
    msg = conn.recv()
    conn.close()
if __name__ == '__main__':
    parent, child = Pipe()
    p = Process(target=worker, args=(child,))
    p.start()
    print(parent.recv())
    parent.send('acknowledged')
    p.join()

from multiprocessing import Process, Lock
def safe_print(lock, msg):
    with lock:
        print(msg)
if __name__ == '__main__':
    lock = Lock()
    procs = [Process(target=safe_print, args=(lock, f'Task {i}'))
             for i in range(5)]
    for p in procs: p.start()
    for p in procs: p.join()

# Create
python -m venv myenv
# Activate (Linux/Mac)
source myenv/bin/activate
# Activate (Windows)
myenv\Scripts\activate
# Deactivate
deactivate

# Generate
pip freeze > requirements.txt
# Install
pip install -r requirements.txt
# Pin versions
flask==2.3.3
requests>=2.28.0,<3.0.0

# Initialize
poetry init
# Add dependency
poetry add flask
poetry add pytest --group dev
# Install all
poetry install
# Run command
poetry run python app.py

# Create environment
pipenv install
# Add package
pipenv install requests
pipenv install pytest --dev
# Activate shell
pipenv shell
# Run command
pipenv run python app.py

# Create
conda create -n myenv python=3.11
# Activate
conda activate myenv
# Export
conda env export > environment.yml
# Recreate
conda env create -f environment.yml

# From GitHub
pip install git+https://github.com/user/repo.git
# Specific branch
pip install git+https://github.com/user/repo.git@branch-name
# Specific commit
pip install git+https://github.com/user/repo.git@abc123

# Install in development mode
pip install -e .
# From a specific path
pip install -e /path/to/package
# With extras
pip install -e ".[dev,test]"

# constraints.txt
flask==2.3.3
pip install -c constraints.txt flask
# Hash checking
pip install --require-hashes -r requirements.txt

# Show cache info
pip cache info
# List cached packages
pip cache list
# Purge entire cache
pip cache purge
# Install with no cache
pip install --no-cache-dir flask

# Use custom index
pip install --index-url https://pypi.custom.com/simple/ flask
# Extra index (fallback)
pip install --extra-index-url https://pypi.custom.com/simple/ flask
# Trusted host (no SSL)
pip install --trusted-host pypi.custom.com flask

def greet(name: str, times: int = 1) -> str:
    return (f"Hello, {name}! " * times).strip()
def process(data: list[int]) -> dict[str, int]:
    return {str(x): x for x in data}

from typing import Optional
def find(items: list[int], target: int) -> int | None:
    for i, v in enumerate(items):
        if v == target: return i
    return None

from typing import TypeVar, Generic
T = TypeVar('T')
class Stack(Generic[T]):
    def __init__(self) -> None:
        self._items: list[T] = []
    def push(self, item: T) -> None:
        self._items.append(item)
    def pop(self) -> T:
        return self._items.pop()

from typing import Protocol
class Closeable(Protocol):
    def close(self) -> None: ...
def cleanup(resource: Closeable) -> None:
    resource.close()
class File:
    def close(self) -> None: print("Closed")
cleanup(File())  # OK - File has close()

# mypy.ini
[mypy]
python_version = 3.11
strict = True
warn_return_any = True
disallow_untyped_defs = True
[mypy-tests.*]
ignore_errors = True

# List: all in memory
squares = [x**2 for x in range(1000000)]
# Generator: lazy evaluation
squares_gen = (x**2 for x in range(1000000))
import sys
print(sys.getsizeof(squares))      # ~8MB
print(sys.getsizeof(squares_gen))  # ~200 bytes

# Slow: creates intermediate strings
result = ""
for s in parts:
    result += s
# Fast: join in one operation
result = "".join(parts)
# Fast: f-strings
msg = f"Hello, {name}!"

import math
# Slow: global lookup
def compute_slow(values):
    return [math.sqrt(v) for v in values]
# Fast: local reference
def compute_fast(values):
    sqrt = math.sqrt
    return [sqrt(v) for v in values]

class Point:
    __slots__ = ('x', 'y')
    def __init__(self, x, y):
        self.x = x
        self.y = y
p = Point(1, 2)
# p.z = 3  # AttributeError

import timeit
t = timeit.timeit('sum(range(100))', number=10000)
print(f"{t:.4f}s")
import cProfile
cProfile.run('sum(x**2 for x in range(10000))')

# BUG: default list is shared
def add_item(item, lst=[]):
    lst.append(item)
    return lst
print(add_item(1))  # [1]
print(add_item(2))  # [1, 2]!
# FIX: use None
def add_item(item, lst=None):
    if lst is None: lst = []
    lst.append(item)
    return lst

# BUG: all print 2
funcs = [lambda: i for i in range(3)]
print([f() for f in funcs])  # [2, 2, 2]
# FIX: default argument
funcs = [lambda i=i: i for i in range(3)]
print([f() for f in funcs])  # [0, 1, 2]

# Small integers cached (-5 to 256)
a = 256; b = 256
print(a is b)  # True (cached)
c = 257; d = 257
print(c is d)  # False (not cached)
print(c == d)  # True

a = [1, 2, 3]; b = [1, 2, 3]
print(a == b)  # True (same values)
print(a is b)  # False (different objects)
# Correct usage of is
if x is None: ...
if x is not None: ...

# GIL prevents true parallelism for CPU-bound tasks
import threading
def cpu_work():
    total = sum(i**2 for i in range(10**7))
# Use multiprocessing for CPU work
from multiprocessing import Pool
with Pool(4) as p:
    p.map(cpu_work, range(4))