Python 3.14 Cheat Sheet

01Run & Environmentpython3.14

python3.14 script.py★
Run a file with the 3.14 interpreter.
python3.14★
Open the REPL — now with syntax colour & import autocomplete.
python -m venv .venv★
Create an isolated virtual environment.
source .venv/bin/activate
Activate it (.venv\Scripts\activate on Windows).
python -m pip install rich★
Install a package into the active env.
python -c "print(1+1)"
Run a one-liner (auto-dedented in 3.14).
python -m module
Run a module as a script (e.g. http.server).

02Variables & Typesbind a name

x = 42★
Assignment binds a name to an object — no declaration.
a, b = 1, 2★
Tuple unpacking; a, *rest = seq grabs the tail.
x = y = 0
Chained assignment — both names to one object.
type(x) · isinstance(x, int)
Inspect / test an object's type.
int float complex bool str
Core scalars; 3+4j is complex, True/False are ints.
v = None
The null singleton — test with is None.
if (n := len(a)) > 10:
Walrus := assigns inside an expression.

03Operatorscombine values

+ - * / # / always gives a float
Arithmetic; / returns float even for ints.
// % **★
Floor-divide, modulo, power (2**10 → 1024).
== != < <= > >=
Comparison — chainable: 0 <= x < 10.
and or not★
Boolean ops; short-circuit & return an operand.
x is y · x is not y
Identity — same object? Use for None, not equality.
k in seq · k not in seq★
Membership test on any container.
a if cond else b★
Conditional (ternary) expression.

04Stringsimmutable text

s = "hi" · 'hi' · """multi"""
Any quote style; triple quotes span lines.
f"{name} is {age}"★
f-string interpolation — the default way to format.
f"{x=}" · f"{x:.2f}" · f"{x:,}"★
Self-doc x=…, 2 decimals, thousands separators.
s[0] · s[1:4] · s[::-1]★
Index & slice; [::-1] reverses.
.strip() .lower() .replace(a,b)
Return new strings — the original never mutates.
.split(sep) · ",".join(parts)★
Split to list; join an iterable back to a string.
.startswith() .find() f"{v!r}"
Prefix test, index-of, !r for the repr form.

05Listsordered · mutable

xs = [1, 2, 3]★
Ordered, mutable, mixed-type sequence.
xs[0] · xs[-1] · xs[1:3]★
Index from front / back; slice returns a new list.
.append(v) · .extend(it)★
Add one item / splice in many.
.insert(i,v) .pop(i) .remove(v)
Insert at index; pop/return; delete first match.
.sort() · sorted(xs, reverse=True)★
.sort() mutates in place; sorted() returns new.
[x*2 for x in xs if x>0]★
List comprehension — build & filter in one line.
len(xs) sum(xs) min(xs) max(xs)
Common aggregate built-ins.

06Tuples & Setsfixed · unique

t = (1, 2) · single = (1,)★
Immutable sequence; the comma makes the tuple.
x, y, z = t
Unpack — great for multiple returns & swaps.
st = {1, 2, 3} · set()★
Unordered, unique; set() for an empty one ({} is a dict).
a | b · a & b · a - b · a ^ b
Union · intersection · difference · symmetric-diff.
.add(v) · .discard(v)
Insert; remove without error if absent.
v in st★
O(1) membership — the reason to reach for a set.
frozenset(xs)
Immutable set — hashable, usable as a dict key.

07Dictionarieskey → value

d = {"a": 1, "b": 2}★
Insertion-ordered map (guaranteed since 3.7).
d["a"] · d.get("z", 0)★
Index raises on miss; .get returns a default.
d["c"] = 3 · .setdefault(k, [])
Assign; get-or-create in one step.
for k, v in d.items():★
Iterate pairs; also .keys() / .values().
d1 | d2 · d1 |= d2★
Merge into a new dict / update in place.
{k: v for k, v in pairs}
Dict comprehension.
.pop(k) · del d[k] · k in d
Remove & return; delete; test a key.

08Control Flowindent = block

if c: … elif d: … else: …★
Branching; blocks are defined by indentation.
for item in iterable:★
Iterate any sequence, range, or generator.
while cond:
Loop until the condition is falsy.
break · continue · pass★
Exit loop · skip to next · do-nothing placeholder.
for … else:
else runs only if the loop wasn't break-ed.
with open(f) as fh:★
Context manager — auto-cleanup on exit.
assert cond, "msg"
Sanity check; stripped away under -O.

09match / casestructural · 3.10+

match command.split():★
Structural pattern matching on shape, not just value.
case ["go", direction]:
Match a sequence & capture parts into names.
case Point(x=0, y=y):
Class pattern — destructure attributes.
case n if n > 0:
Guard — extra boolean condition on a pattern.
case 1 | 2 | 3:
Or-pattern — match any of several literals.
case _:★
Wildcard — the default / catch-all arm.

10Comprehensions & Generatorsbuild lazily

[f(x) for x in xs]★
List comp — the idiomatic map+filter.
{x for x in xs} · {k: v for …}
Set & dict comprehensions.
(x*x for x in xs)★
Generator expression — lazy, memory-light.
def gen(): yield v★
A function with yield is a generator.
yield from other
Delegate to a sub-iterable / sub-generator.
next(g) · list(g)
Pull one value / drain the whole generator.

11Functionsdef & return

def f(a, b=1): return a+b★
Positional args with defaults (defaults eval once!).
f(*args, **kwargs)★
Collect extra positionals / keywords; also splat to call.
def f(a, *, key):
* forces key to be keyword-only.
def f(a, /, b):
/ makes a positional-only.
lambda x: x + 1★
Anonymous one-expression function.
def f(x: int) -> str:
Type hints — lazy in 3.14, no quoting needed. 3.14
global g · nonlocal n
Rebind an outer-scope name from inside.

12Iterating Wellskip range(len())

enumerate(xs, start=1)★
Index + item together — beats manual counters.
zip(a, b) · zip(a, b, strict=True)★
Walk sequences in lockstep; strict checks equal length.
sorted(xs, key=lambda r: r.age)★
Sort by a computed key; add reverse=True.
range(start, stop, step)★
Lazy integer sequence — stop is exclusive.
reversed(xs) · map(f, xs, strict=…)
Reverse iterator; map gains strict in 3.14. 3.14
any(xs) · all(xs)★
Short-circuit truth tests over an iterable.

13Classes & OOPeverything's an object

class Dog: …★
Define a new type; instances via Dog().
def __init__(self, name):★
Constructor; self is the instance, always first.
self.name = name
Instance attribute set in the initializer.
class Pup(Dog): super().__init__(…)
Inheritance; super() calls the parent.
@property★
Expose a method as a read-only attribute.
@classmethod · @staticmethod
Bind to the class / to nothing (no self).
__repr__ __eq__ __len__
Dunder methods hook into built-in behaviour.

14Dataclassesstructs, minus boilerplate

from dataclasses import dataclass★
Auto-generates __init__, __repr__, __eq__.
@dataclass class Point: x: int; y: int★
Fields are just annotated class attributes.
tags: list = field(default_factory=list)
Use a factory for mutable defaults — never = [].
@dataclass(frozen=True)
Immutable & hashable instances.
@dataclass(slots=True)
Slot layout — less memory, faster attribute access.

15Modules & Importsreuse code

import math★
Import a module; use as math.sqrt(2).
from math import sqrt, pi★
Bring specific names into scope.
import numpy as np★
Alias a long module name.
from . import sibling
Relative import inside a package.
if __name__ == "__main__":★
Run this block only when executed directly.
dir(mod) · help(obj)
Explore a module's names / read its docs.

16Exceptionstry · except · finally

try: … except ValueError as e:★
Catch a type; bind the instance with as.
except TypeError, ValueError:3.14
No brackets needed for multiple types (no as).
else: … · finally: …
else = no error; finally always runs.
raise ValueError("bad")★
Signal an error; bare raise re-raises.
raise E(…) from cause
Chain exceptions to preserve the root cause.
except* TypeError:
Handle one type from an ExceptionGroup.
except: # bare — avoidavoid
Swallows everything incl. KeyboardInterrupt.

17Files & Pathsread · write · path

with open(p) as f: f.read()★
with closes the file even on error.
with open(p, "w") as f: f.write(s)★
Modes: r read · w write · a append · b binary.
for line in f:
Stream a file line-by-line, no full load.
from pathlib import Path★
Object paths: Path("a") / "b.txt".
p.read_text() · p.write_text(s)
One-shot text I/O without an explicit handle.
p.copy(dst) · p.move(dst)3.14
Path gains recursive copy/move (& *_into).

18Type Hintsoptional, checked offline

name: str · count: int = 0★
Annotate variables & attributes.
list[int] · dict[str, int]★
Built-in generics — no typing.List needed.
x: int | None★
Modern union; | None replaces Optional[int].
from typing import Callable, Any
Callable[[int], str], Any, Literal, TypedDict.
def f(n: Node): # no quotes3.14
Deferred eval — forward refs work unquoted.
$ pip install mypy; mypy app.py
Hints are ignored at runtime — a checker enforces them.

19Standard Library Picksbatteries included

collections★
Counter, defaultdict, deque, namedtuple.
itertools★
chain, groupby, product, combinations, count.
functools
cache, reduce, partial, wraps, Placeholder. 3.14
pathlib · os · sys · shutil★
Paths, environment, argv, high-level file ops.
json · re · datetime★
Parse/dump JSON, regex, dates & times.
random · math · statistics
Randomness, numeric functions, mean/median/stdev.
compression.zstd3.14
Zstandard compress/decompress in the stdlib.

python 3.14 cheat sheet

What's new in 3.14 headline features

Template strings (t-strings) PEP 750

Deferred annotations PEP 649 / 749

Free-threaded Python PEP 779

Multiple interpreters PEP 734

`except` without brackets PEP 758 / 765

Stdlib & tooling polish 3.14

Four pictures worth holding in your head

names & objects — mutable vs immutable

slicing — seq[start:stop:step]

scope resolution — LEGB

f-string vs t-string 3.14

Worth memorizing