Python 3 cheat sheet

A scannable Python 3 reference: 21 short snippets across 8 topics, each linking back to the lesson it came from.

At a glance

Quick snippets

Python: getting started

Running code

python --version
python hello.py

# interactive REPL - the fastest way to experiment
python
>>> 2 + 3
5

Indentation is syntax

score = 85

if score >= 90:
    grade = 'A'
elif score >= 80:
    grade = 'B'
else:
    grade = 'C'

print(grade)  # B

Core types

x, y = 1, 2          # multiple assignment
x, y = y, x          # swap without a temp

n = 10
print(f'n is {n}')   # f-strings: the modern way to format
print(type(n).__name__)

Full lesson: Python: getting started →

Strings

Creating and slicing

s = 'Python'
print(s[0])     # P
s[-1]           # n      negative index counts from the end
s[0:2]          # 'Py'    end index is exclusive
s[::-1]         # 'nohtyP' reversed
len(s)          # 6

print('py' in s.lower())   # True - membership test

Methods worth memorizing

csv = ' a, b , c '
fields = [f.strip() for f in csv.strip().split(',')]
print(fields)  # ['a', 'b', 'c']

path = '/var/log/app.log'
path.rsplit('/', 1)[-1]     # 'app.log'  - split from the right

Building strings efficiently

# slow: each += allocates a brand new string
out = ''
for line in lines:
    out += line + '\n'

# fast: one pass, one allocation
out = '\n'.join(lines)

Full lesson: Strings →

Lists, dicts and comprehensions

Lists

nums = [3, 1, 2]
nums.append(4)          # [3, 1, 2, 4]
nums.extend([5, 6])
nums.insert(0, 0)
last = nums.pop()       # removes and returns the last item
nums.sort()             # in place; sorted(nums) returns a copy

first, *rest = nums     # unpacking

Comprehensions

squares = [n * n for n in range(10)]
evens   = [n for n in nums if n % 2 == 0]

# dict comprehension
by_id = {u['id']: u for u in users}

# conditional transformation
labels = ['even' if n % 2 == 0 else 'odd' for n in nums]

# set comprehension - deduplicates
unique_tags = {t.lower() for t in tags}

Dicts

user = {'id': 1, 'name': 'Ada'}
user.get('email')            # None instead of KeyError
user.get('email', 'n/a')     # with default
user.setdefault('role', 'guest')

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

merged = {**user, 'role': 'admin'}   # Python 3.5+
counts = dict(Counter(words))        # tallying done for you

Full lesson: Lists, dicts and comprehensions →

Functions and modules

Arguments

def greet(name, greeting='Hello', *extra, punct='!', **opts):
    """Docstring: what this function is for."""
    parts = [greeting, name, *extra]
    sep = opts.get('sep', ' ')
    return sep.join(parts) + punct

greet('Ada')                        # Hello Ada!
greet('Ada', punct='?')             # keyword-only must be named
greet('Ada', 'Hi', punct='.')

Returning values

def divide(a, b):
    return a / b

q, r = divmod(10, 3)      # multiple return values are a tuple
name, _, score = row      # _ conventionally means 'ignored'

def find(users, uid):
    return None            # explicit absence beats raising for 'not found'

Modules and imports

# maths.py
def area(r): return 3.14159 * r * r

if __name__ == '__main__':
    print(area(2))          # only when run directly

# consumer
from maths import area
import maths as m
from pathlib import Path

Full lesson: Functions and modules →

Errors, files and virtualenvs

Reading a traceback

Traceback (most recent call last):
  File 'app.py', line 12, in <module>
    main()
  File 'app.py', line 8, in main
    print(items[5])
          ~~~~~^^^
IndexError: list index out of range

Files with context managers

with open('data.json', encoding='utf-8') as f:
    data = json.load(f)          # file closed even on error

with open('out.txt', 'w', encoding='utf-8') as f:
    json.dump(data, f, indent=2, ensure_ascii=False)

for line in pathlib.Path('app.log').read_text(encoding='utf-8').splitlines():
    if 'ERROR' in line:
        print(line)

Environments and packages

python -m venv .venv
source .venv/bin/activate        # Windows: .venv\Scripts\activate

pip install requests
pip freeze > requirements.txt
pip install -r requirements.txt

Full lesson: Errors, files and virtualenvs →

Type hints and static checking

Running the checker

pip install mypy
mypy src/                      # basic run
mypy --strict src/             # maximum checking
mypy --install-types           # fetch stubs for third-party packages
python -m mypy --show-error-codes src/

Running the checker

[tool.mypy]
python_version = "3.11"
warn_unused_ignores = true
warn_redundant_casts = true
disallow_untyped_defs = true
ignore_missing_imports = true

[[tool.mypy.overrides]]
module = "legacy.*"
ignore_errors = true

Full lesson: Type hints and static checking →

Testing with pytest

Files, discovery and asserts

pytest                        # run everything
pytest tests/test_cart.py     # one file
pytest -k "quantity and not slow"   # filter by name
pytest -x                     # stop at the first failure
pytest -vv -s                 # verbose, do not capture output
pytest --lf                   # rerun only what failed last time

Full lesson: Testing with pytest →

Packaging, project layout and pyproject.toml

pyproject.toml

# src/shop_tools/__init__.py
__version__ = "0.3.1"

# read the version from one place instead of two
# [project] dynamic = ["version"]
# [tool.hatch.version] path = "src/shop_tools/__init__.py"

Building, publishing and pinning

pip install build twine
python -m build                 # writes dist/*.tar.gz and dist/*.whl
twine check dist/*
twine upload --repository testpypi dist/*    # rehearse first
twine upload dist/*                          # then the real index

pip install dist/shop_tools-0.3.1-py3-none-any.whl   # verify locally

Building, publishing and pinning

# applications: pin exactly, reproducibly
pip freeze > requirements.txt
pip install -r requirements.txt

# modern alternatives
pip install -r requirements.txt --require-hashes
uv lock && uv sync
pip-compile requirements.in -o requirements.txt

Full lesson: Packaging, project layout and pyproject.toml →

FAQ

Related cheat sheets

NumPy pandas Matplotlib Jupyter Notebook Flask FastAPI

Last refreshed 2026-09-27.