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Python Cheat Sheet — Syntax and Built-ins Reference

The Python 3 syntax and built-ins you reach for every day, from slicing and f-strings to comprehensions, classes, and context managers.

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Strings

f"Hi {name}, {n:.2f}"f-string with an embedded expression and format spec

s[1:4] s[::-1]Slice characters 1–3 / reverse a string

s.split(",")Split into a list on a separator

", ".join(items)Join a list of strings with a separator

s.strip()Remove leading and trailing whitespace

s.replace("a", "b")Replace every occurrence

s.startswith("x")Prefix test (endswith for suffixes)

s.lower() / s.upper()Change case

Lists, tuples, and sets

xs.append(x)Add one item to the end

xs.extend(ys)Add every item from another iterable

xs.pop()Remove and return the last item

sorted(xs, key=len, reverse=True)Return a new sorted list

len(xs)Number of items

a, b = b, aSwap two variables with tuple unpacking

first, *rest = xsUnpack the head and the remaining items

set(xs)Unique items; use | & - for union, intersection, difference

Dictionaries

d.get("k", default)Read a key without raising KeyError

d.items()Iterate over (key, value) pairs

d.setdefault("k", [])Insert a default if missing and return the value

{**a, **b} / a | bMerge dicts (later keys win)

"k" in dTest whether a key exists

del d["k"]Remove a key

Comprehensions

[x * 2 for x in xs]List comprehension

[x for x in xs if x > 0]Filter while building the list

{k: v for k, v in pairs}Dict comprehension

{x % 3 for x in xs}Set comprehension

sum(x * x for x in xs)Generator expression — lazy, no list built

Control flow

for i, x in enumerate(xs):Loop with an index

for a, b in zip(xs, ys):Loop over two iterables in step

for i in range(0, 10, 2):Loop over 0, 2, 4, 6, 8

x if cond else yConditional expression

match cmd:
  case "go": ...

Structural pattern matching (3.10+)

while (line := f.readline()):Walrus operator: assign inside an expression

Functions and classes

def f(a, b=2, *args, **kwargs):Defaults, extra positional and keyword arguments

lambda x: x + 1Anonymous one-expression function

def f(x: int) -> str:Type hints (not enforced at runtime)

class Dog(Animal):Define a class that inherits from Animal

def __init__(self, name):Constructor

@dataclassAuto-generate __init__, __repr__, and __eq__

@propertyExpose a method as a read-only attribute

Files and errors

with open("f.txt", encoding="utf-8") as f:Open a file and close it automatically

f.read() / f.readlines()Read the whole file / a list of lines

open("f.txt", "w").write(s)Write text (mode "a" appends)

try: ... except ValueError as e: ...Catch a specific exception

finally:Runs whether or not an exception occurred

raise ValueError("bad input")Raise an exception

import json; json.loads(s)Parse JSON (json.dumps to serialise)

Related guides

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More cheat sheets

Regex →Markdown →SQL →JS Arrays →TypeScript →JS Strings →