Master Python Functions: Definitions, Scope, Closures, and Lambdas Explained

1. Function Definition

Functions are organized, reusable code blocks that implement a single or related functionality.

Function blocks start with the def keyword, followed by the function name and parentheses ().

All input parameters must be placed inside the parentheses; they can be defined there.

The first line of a function may optionally contain a docstring to describe the function.

The function body starts after a colon and is indented.

A return [expression] ends the function, optionally returning a value; a bare return returns None.

When using default parameters, if a call omits a value, the default is used.

Variable-length arguments. A name prefixed with an asterisk (*) collects all unnamed positional arguments; you may also omit extra arguments.

All arguments in Python are passed by reference; modifying a parameter inside the function also changes the original argument.

The return statement exits the function, optionally returning an expression; a bare return returns None.

Inner functions. Functions can be defined inside other functions.

2. Function Variable Scope

Variables defined inside a function have local scope; variables defined outside have global scope.

Local variables are accessible only within the function where they are declared, while global variables can be accessed throughout the program. When a function is called, all variable names declared inside are added to its scope.

3. Python Closures

If an inner function references a variable from the outer (non‑global) scope, the inner function is considered a closure. A closure occurs when function A returns function B; the returned function B is the closure, and the arguments passed to A become free variables.

4. Anonymous Functions (lambda)

Python uses lambda expressions to create anonymous functions. lambda is a single expression, making the function body much simpler than def.

The body of a lambda is an expression, not a code block, and can only encapsulate limited logic.

A lambda has its own namespace and cannot access variables outside its parameter list or the global namespace.

Although a lambda appears on one line, it is not equivalent to C/C++ inline functions, which aim to reduce stack usage for performance.

Syntax: lambda [arg1 [, arg2, ... argn]]: expression Example:

5. Using the Built‑in filter Function

Official documentation: filter(function, iterable) Constructs an iterator from elements of iterable for which function returns true. iterable may be a sequence, a container supporting iteration, or an iterator. If function is None, the identity function is assumed, removing elements that are false.

The function argument can be None, a function, or a lambda expression; iterable must be an iterable object.

If function is None, it returns all elements of the iterable that are not False.

If function is a function or lambda, it returns elements for which the function returns True.