Python Cheat Sheet

Essential Python syntax for everyday development. Covers data structures, functions, comprehensions, and common patterns.

Variables and Types

x = 10              # int
y = 3.14            # float
name = "Alice"      # str
flag = True         # bool
items = [1, 2, 3]   # list
data = {"a": 1}     # dict
coords = (1, 2)     # tuple
unique = {1, 2, 3}  # set

Strings

s = "Hello, World!"
s.lower()           # "hello, world!"
s.upper()           # "HELLO, WORLD!"
s.split(", ")       # ["Hello", "World!"]
s.replace("H", "J") # "Jello, World!"
s.strip()           # remove whitespace
f"Name: {name}"     # f-string formatting

List Comprehensions

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)]

Dictionary Comprehensions

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

Functions

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

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

def config(**kwargs):
    return kwargs

Lambda Functions

square = lambda x: x ** 2
add = lambda a, b: a + b
sorted(items, key=lambda x: x["name"])

Classes

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def greet(self):
        return f"Hi, I'm {self.name}"

class Employee(Person):
    def __init__(self, name, age, salary):
        super().__init__(name, age)
        self.salary = salary

Error Handling

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")
except Exception as e:
    print(f"Unexpected: {e}")
finally:
    print("Always runs")

File I/O

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

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

with open("data.json", "r") as f:
    data = json.load(f)

Common Built-ins

len([1,2,3])        # 3
range(5)            # 0,1,2,3,4
enumerate(items)    # (0,a), (1,b)...
zip(list1, list2)   # paired items
map(func, items)    # apply func
filter(func, items) # filter items
sorted(items)       # new sorted list
any([False, True])  # True
all([True, True])   # True

Useful Modules

import json          # JSON parsing
import os            # OS operations
import sys           # System params
import re            # Regular expressions
import datetime      # Date/time
import collections   # defaultdict, Counter
import pathlib       # Path handling
import requests      # HTTP requests