Python Functions Cheat Sheet

Python function fundamentals covering default arguments, *args/**kwargs, lambdas, type hints, scope rules, and higher-order functions.

1 PageBeginnerApr 2, 2026

Defining Functions

Default arguments and variable-length argument lists.

python

def greet(name, greeting="Hello"):    return f"{greeting}, {name}!"def add_all(*args):          # args is a tuple    return sum(args)def make_profile(**kwargs):  # kwargs is a dict    return kwargsgreet("Alice")                     # 'Hello, Alice!'add_all(1, 2, 3)                   # 6make_profile(name="Bob", age=30)   # {'name': 'Bob', 'age': 30}

Lambda Functions

Anonymous single-expression functions.

python

square = lambda x: x ** 2square(5)   # 25data = [(1, 'c'), (2, 'a'), (3, 'b')]sorted(data, key=lambda x: x[1])  # sort by second element

Type Hints

Optional static typing annotations for parameters and return values.

python

def add(a: int, b: int) -> int:    return a + bdef process(items: list[str], count: int = 0) -> None:    ...from typing import Optionaldef find(name: str) -> Optional[int]:    return None

Scope: global & nonlocal

Modifying variables from an enclosing scope.

python

def make_counter():    count = 0    def increment():        nonlocal count   # refers to enclosing scope's count        count += 1        return count    return incrementcounter = make_counter()counter()  # 1counter()  # 2

Built-in Higher-Order Functions

Functions that take or return other functions.

map(fn, iterable)- applies fn to every item, returns a map object (lazy)
filter(fn, iterable)- keeps items where fn(item) is truthy
functools.reduce(fn, iterable)- cumulatively applies fn to reduce to one value
sorted(iterable, key=fn)- returns a new sorted list using fn as the sort key
any(iterable)- True if at least one element is truthy
all(iterable)- True if every element is truthy
zip(a, b)- pairs up elements from two or more iterables

Pro Tip

Never use a mutable object like a list or dict as a default argument value — defaults are evaluated once when the function is defined, so all calls share the same object. Use `None` as the default and create the mutable object inside the function body instead.

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