Python Cheat Sheet

01Run & Environmentstart here

python --version
Check interpreter version.
python★
Launch the interactive REPL.
python script.py★
Run a script file.
python -m module
Run a module as a script.
python -i script.py
Run then drop into REPL.
if __name__ == "__main__":★
Guard code that should only run when executed directly.

02Variables & Assignmentnames → objects

x = 5★
Bind a name to an object (no declaration needed).
a, b = b, a★
Tuple unpacking — the classic swap.
x = y = 0
Chained assignment — both names, same object.
x += 1★
Augmented assignment (also -=, *=, /=, //=, **=).
first, *rest = nums
Extended (starred) unpacking.
if (n := len(data)) > 3:★
Walrus operator — assign and use in one expression.

03Numbers & Operatorsarithmetic & logic

7 / 2★
3.5 — true division, always returns float.
7 // 2★
3 — floor division.
7 % 2
1 — modulo (remainder).
2 ** 10
1024 — exponentiation.
x == y x is y★
== compares value; is compares identity.
a and b a or b not a
Boolean logic; short-circuits and returns an operand, not just True/False.
0 < x < 10
Chained comparisons work as expected.

04Strings & f-stringsimmutable text

f"{name} is {age}"★
f-string — fastest, most readable interpolation.
f"{value:.2f}"★
Format-spec: 2 decimal places.
f"{age=}"
Debug spec — prints age=2 for quick tracing.
"a b".split()★
Split on whitespace → list.
"-".join(items)★
Join an iterable of strings.
s.strip() s.replace(a,b)
Trim whitespace; substring replace.
s.upper() s.lower() s.title()
Case transforms.
text[1:4] text[::-1]★
Slicing — see the reference card for the full notation.

05Listsordered, mutable

nums.append(x)★
Add one item to the end.
nums.extend(iterable)
Append every item from another iterable.
nums.insert(i, x)
Insert at a specific index.
nums.pop(i)★
Remove & return item at index (default: last).
nums.remove(x)
Remove first matching value (raises if absent).
nums.sort(key=fn)★
Sort in place; sorted(nums) returns a new list.
x in nums len(nums)★
Membership test; length.
nums.reverse()
Reverse in place.

06Tuples & Setsimmutable / unique

point = (3, 4)★
Tuple — ordered, immutable.
single = (1,)
Trailing comma required for a 1-item tuple.
a = {1, 2, 3}★
Set — unordered, unique elements.
a | b a & b a - b a ^ b★
Union · intersection · difference · symmetric difference.
frozenset(a)
Immutable, hashable set — usable as a dict key.

07Dictionarieskey → value

d = {"a": 1}★
Literal mapping.
d.get(k, default)★
Lookup without raising on a missing key.
d.setdefault(k, [])
Get, inserting a default if the key is absent.
d.pop(k)
Remove & return a value.
d.items() d.keys() d.values()★
Views for iteration.
d1 | d2★
Merge two dicts (3.9+) — right side wins on conflicts.
k in d★
Membership test checks keys, not values.

08Control Flowbranching

if … elif … else:★
Standard branching; colon + indentation define blocks.
x if cond else y★
Conditional (ternary) expression.
match cmd: case "quit" | "exit":
Structural pattern matching (3.10+); case _: is the wildcard.
case [x, *rest] if x > 0:
Patterns can destructure sequences and add guards.

09Loopsiteration

for x in iterable:★
The Python loop — works on anything iterable.
for i, x in enumerate(xs):★
Index + value together.
for a, b in zip(xs, ys):★
Walk two iterables in lockstep.
while (line := f.readline()):
Walrus keeps the read-and-check on one line.
break continue★
Exit the loop; skip to the next iteration.
for … else:
else runs only if the loop finished without a break.

10Functionsreusable behavior

def f(x, y=1):★
Positional param + default value.
def f(*args, **kwargs):★
Collect extra positional / keyword args.
f(*nums, **opts)
Unpack a list/dict into a call.
def f(a, /, b, *, c):
/ = positional-only before it; * = keyword-only after.
lambda x: x * 2★
Anonymous single-expression function.
def f(x: int) -> str:
Type-hinted signature (not enforced at runtime).

11Comprehensions & Generatorsbuild collections concisely

[x*x for x in nums if x > 0]★
List comprehension — eager, builds the whole list now.
{k: v for k, v in pairs}★
Dict comprehension.
{x for x in nums}
Set comprehension.
(x*x for x in nums)★
Generator expression — lazy, one value at a time.
def gen(): yield x★
Generator function — pauses/resumes at each yield.
yield from other_gen()
Delegate iteration to another generator.

12OOP Basicsclasses & objects

class Dog: def __init__(self, name):★
Constructor — self is the instance, always first param.
class Puppy(Dog):★
Inheritance; use super().__init__() to call the parent.
def __repr__(self):★
Unambiguous string repr — what you see in the REPL/debugger.
def __eq__(self, other):
Define value equality (pairs with __hash__).
@property def area(self):
Expose a method as a read-only attribute.
@classmethod @staticmethod
Bound to the class, not an instance; static gets no implicit arg.

13Dataclassesless boilerplate

@dataclass class Point: x: int y: int★
Auto-generates __init__, __repr__, __eq__.
@dataclass(frozen=True)
Makes instances immutable & hashable.
field(default_factory=list)
Safe default for a mutable field.

14Decorators & Context Managerswrap behavior

@my_decorator def f(): …★
Sugar for f = my_decorator(f).
functools.wraps(func)
Preserve name/docstring when writing a decorator.
with open("f.txt") as f:★
with guarantees cleanup even if an exception occurs.
@contextmanager def cm(): yield
Turn a generator into a context manager.

15Exception Handlingfail gracefully

try: … except ValueError as e:★
Catch a specific exception type.
except (TypeError, KeyError):
Catch multiple types in one clause.
else: … finally: …★
else runs if no error; finally always runs.
raise ValueError("msg")★
Signal an error explicitly.
class MyError(Exception): pass
Custom exception type.
except* ValueError as e:
Exception groups (3.11+) — handle one type from a batch of failures.

16File I/O & Pathlibread/write

with open("f.txt") as f: data = f.read()★
Read the whole file.
for line in f:★
Iterate a file line by line (memory-efficient).
f.write(text)
Mode "w" overwrites, "a" appends.
Path("dir/file.txt")★
pathlib — modern, cross-platform path handling.
p.exists() p.mkdir(parents=True)
Common Path methods.

17Modules, Packages & Envsorganize & isolate

import module from pkg import x★
Import a module or specific names.
from pkg import x as y
Import with an alias.
python -m venv .venv★
Create an isolated virtual environment.
source .venv/bin/activate★
Activate it (Windows: .venv\Scripts\activate).
pip install pkg★
Install a package into the active environment.
pip freeze > requirements.txt
Snapshot installed packages.

18Standard Library Highlightsbatteries included

collections.Counter(items)★
Frequency count in one call.
collections.defaultdict(list)★
Dict with an auto-created default value.
itertools.chain(a, b)
Iterate several iterables as one.
itertools.groupby(data)
Group consecutive matching items.
functools.lru_cache(maxsize=None)★
Memoize a function's results.
functools.reduce(fn, iterable)
Fold an iterable down to one value.

19Type Hintsfor tooling, not runtime

x: int = 5★
Variable annotation.
def f(x: int) -> bool:★
Parameter & return annotations.
list[int] dict[str, int]★
Built-in generics (3.9+) — no typing.List needed.
int | None★
Union syntax (3.10+) — replaces Optional[int].

20Common Pitfallshandle with care

def f(x=[]):gotcha
Mutable default evaluated once — reused & shared across calls.
a is b (for values)gotcha
Use == for equality; is only for identity (e.g. None).
rows = [[0]*3]*3gotcha
Same inner list repeated 3× — one edit changes every row.
[lambda: i for i in range(3)]gotcha
Late binding — all three lambdas see the final i.
import copy; copy.deepcopy(x)care
copy() is shallow — nested mutables stay shared.

★Slice Notationseq[start:stop:step]

text[0] text[-1]
First item; last item.
text[1:4]
Items at index 1, 2, 3 (stop excluded).
text[:3] text[3:]
From the start; to the end.
text[::2]
Every 2nd item.
text[::-1]
Reversed copy.
text[:]
Shallow copy of the whole sequence.

python cheat sheet

Objects, references & evaluation

aliasing a mutable object

shallow vs. deep copy

eager list vs. lazy generator

decorator call flow

Worth memorizing