Python Cheat Sheet

print("Hello, World!")

# Single line comment
"""Multi-line
comment"""

name = input("Enter your name: ")

int("42")  # str to int
str(42)    # int to str
float("3.14")

type(variable)  # <class 'int'>
isinstance(x, int)  # True/False

name = "John"

age = 25

price = 19.99

is_active = True

fruits = ["apple", "banana", "cherry"]

person = {"name": "John", "age": 30}

coordinates = (10, 20)

unique = {1, 2, 3}

value = None

a, b, c = 1, 2, 3
x = y = z = 0

5 + 3   # 8 (addition)
5 - 3   # 2 (subtraction)
5 * 3   # 15 (multiplication)
5 / 3   # 1.666... (division)
5 // 3  # 1 (floor division)
5 % 3   # 2 (modulus)
5 ** 3  # 125 (exponent)

5 == 5  # True (equal)
5 != 3  # True (not equal)
5 > 3   # True (greater)
5 < 3   # False (less)
5 >= 5  # True (greater or equal)
5 <= 3  # False (less or equal)

True and False  # False
True or False   # True
not True        # False

# Short-circuit evaluation
x and y  # y if x is truthy
x or y   # x if x is truthy

x = 5    # Assign
x += 3   # x = 8 (add)
x -= 2   # x = 6 (subtract)
x *= 2   # x = 12 (multiply)
x /= 3   # x = 4.0 (divide)
x //= 2  # x = 2.0 (floor)

a = [1, 2]
b = a
c = [1, 2]

a is b      # True (same object)
a is c      # False (different object)
a is not c  # True

"a" in "abc"       # True
2 in [1, 2, 3]     # True
"x" not in "abc"   # True
"key" in {"key": 1}  # True

5 & 3   # 1 (AND: 101 & 011)
5 | 3   # 7 (OR: 101 | 011)
5 ^ 3   # 6 (XOR: 101 ^ 011)
~5      # -6 (NOT)
5 << 1  # 10 (left shift)
5 >> 1  # 2 (right shift)

for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

for i in range(2, 8):
    print(i)  # 2, 3, 4, 5, 6, 7

for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

for fruit in fruits:
    print(fruit)

for i, fruit in enumerate(fruits):
    print(i, fruit)

for key, value in person.items():
    print(key, value)

while count < 10:
    count += 1

for i in range(10):
    if i == 5:
        break  # Exit loop

for i in range(10):
    if i == 5:
        continue  # Skip iteration

for i in range(5):
    print(i)
else:
    print("Done!")  # Runs if no break

if x > 0:
    print("positive")
elif x < 0:
    print("negative")
else:
    print("zero")

result = "yes" if condition else "no"

match status:
    case 200:
        print("OK")
    case 404:
        print("Not Found")
    case _:
        print("Unknown")

if x > 0:
    if x > 10:
        print("big positive")
    else:
        print("small positive")

if age >= 18 and age <= 65:
    print("Working age")

squares = [x**2 for x in range(10)]

evens = [x for x in range(10) if x % 2 == 0]

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

unique = {x % 3 for x in range(10)}

gen = (x**2 for x in range(10))

matrix = [[j for j in range(3)] for i in range(3)]

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

def greet(name="World"):
    return f"Hello, {name}!"

square = lambda x: x ** 2

def sum_all(*args):
    return sum(args)

def print_info(**kwargs):
    for k, v in kwargs.items():
        print(f"{k}: {v}")

f"Hello, {name}!"

"a,b,c".split(",")  # ["a", "b", "c"]

",".join(["a", "b", "c"])  # "a,b,c"

"  hello  ".strip()  # "hello"

"hello".replace("l", "x")  # "hexxo"

"hello"[1:4]  # "ell"

with open("file.txt", "r") as f:
    content = f.read()

with open("file.txt", "w") as f:
    f.write("Hello")

with open("file.txt") as f:
    lines = f.readlines()

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")

try:
    risky_operation()
except Exception as e:
    print(e)
finally:
    cleanup()

raise ValueError("Invalid input")

class Dog:
    def __init__(self, name):
        self.name = name

    def bark(self):
        return f"{self.name} says woof!"

class Puppy(Dog):
    def play(self):
        return f"{self.name} is playing!"

import os

from datetime import datetime

import numpy as np

def my_decorator(func):
    def wrapper(*args, **kwargs):
        print("Before")
        result = func(*args, **kwargs)
        print("After")
        return result
    return wrapper

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

def repeat(times):
    def decorator(func):
        def wrapper(*args, **kwargs):
            for _ in range(times):
                func(*args, **kwargs)
        return wrapper
    return decorator

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

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

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

    @radius.setter
    def radius(self, value):
        self._radius = max(0, value)

class Math:
    @staticmethod
    def add(a, b):
        return a + b

Math.add(2, 3)  # No instance needed

class Person:
    count = 0

    @classmethod
    def get_count(cls):
        return cls.count

from functools import wraps

def decorator(func):
    @wraps(func)  # Preserves metadata
    def wrapper(*args, **kwargs):
        return func(*args, **kwargs)
    return wrapper

from dataclasses import dataclass

@dataclass
class Point:
    x: float
    y: float

p = Point(1.0, 2.0)

@dataclass
class User:
    name: str
    age: int = 0
    active: bool = True

@dataclass(frozen=True)
class Config:
    host: str
    port: int

from dataclasses import dataclass, field

@dataclass
class Item:
    name: str
    tags: list = field(default_factory=list)
    _id: int = field(repr=False)

@dataclass
class Rectangle:
    width: float
    height: float
    area: float = field(init=False)

    def __post_init__(self):
        self.area = self.width * self.height

name: str = "John"
age: int = 30
price: float = 19.99
is_active: bool = True

from typing import List, Dict, Set, Tuple

nums: List[int] = [1, 2, 3]
scores: Dict[str, int] = {"a": 1}
unique: Set[str] = {"x", "y"}
point: Tuple[int, int] = (1, 2)

from typing import Optional

def find(id: int) -> Optional[str]:
    return None  # or return "found"

from typing import Union

def process(val: Union[int, str]) -> str:
    return str(val)

from typing import Callable

def apply(func: Callable[[int], int], x: int) -> int:
    return func(x)

from typing import TypeVar, List

T = TypeVar('T')

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

# Simpler syntax in 3.10+
def greet(name: str | None = None) -> str:
    return f"Hello, {name or 'World'}"

list[int]  # Instead of List[int]

class FileManager:
    def __init__(self, filename):
        self.filename = filename

    def __enter__(self):
        self.file = open(self.filename)
        return self.file

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.file.close()

from contextlib import contextmanager

@contextmanager
def timer():
    import time
    start = time.time()
    yield
    print(f"Elapsed: {time.time() - start:.2f}s")

with open("in.txt") as f_in, open("out.txt", "w") as f_out:
    f_out.write(f_in.read())

from contextlib import suppress

with suppress(FileNotFoundError):
    os.remove("temp.txt")

# Assign and use in one expression
if (n := len(items)) > 10:
    print(f"Too many: {n}")

while (line := file.readline()):
    process(line)

# Avoid calling function twice
results = [y for x in data if (y := process(x)) is not None]

import re
if (match := re.search(r'\d+', text)):
    print(match.group())

import json

# Parse JSON
data = json.loads('{"name": "John"}')

# To JSON string
json_str = json.dumps(data, indent=2)

# File operations
with open("data.json") as f:
    data = json.load(f)

import re

re.search(r'\d+', text)      # First match
re.findall(r'\d+', text)     # All matches
re.sub(r'\d+', 'X', text)    # Replace
re.split(r'[,;]', text)      # Split

from collections import defaultdict, Counter, deque

# Auto-initialize missing keys
d = defaultdict(list)
d['key'].append(1)

# Count occurrences
counts = Counter(['a', 'b', 'a'])
counts.most_common(2)

# Double-ended queue
dq = deque([1, 2, 3])
dq.appendleft(0)

from itertools import chain, groupby, islice

# Flatten lists
list(chain([1,2], [3,4]))  # [1,2,3,4]

# Take first n items
list(islice(range(100), 5))  # [0,1,2,3,4]

from pathlib import Path

p = Path("folder/file.txt")
p.exists()       # Check existence
p.read_text()    # Read file
p.parent         # Parent directory
p.suffix         # ".txt"
list(p.parent.glob("*.py"))  # Find files

from datetime import datetime, timedelta

now = datetime.now()
today = datetime.today().date()

# Parse string
dt = datetime.strptime("2024-01-15", "%Y-%m-%d")

# Format
dt.strftime("%B %d, %Y")  # "January 15, 2024"

# Math
tomorrow = now + timedelta(days=1)