Functions

As programs grow larger, writing everything in one place becomes messy and hard to maintain. Functions let you give a name to a block of code, reuse it anywhere, and break your program into clear, manageable pieces.

Defining a Function

Use the def keyword, followed by the function name, parentheses, and a colon. The function body is indented:

The function does nothing until you call it. You can call it as many times as you want.

Parameters

Parameters let you pass values into a function:

A function can take multiple parameters:

The names a and b only exist inside the function. The values you pass in (3 and 5) are called arguments.

Return Values

A function can send a value back to the caller using return:

Without return, a function returns None by default:

return Exits the Function Immediately

As soon as Python hits a return statement, the function stops — no code below it runs:

Each return exits early as soon as a condition is met. This is a very common pattern for keeping functions readable.

Returning Multiple Values

Python functions can return multiple values as a tuple:

Default Arguments

You can give parameters a default value. If the caller does not provide that argument, the default is used:

Parameters with defaults must come after parameters without defaults:

Passing Lists to Functions

When you pass a list to a function, you are not passing a copy — you are passing a reference to the same list in memory. This means the function can modify the original list:

This is different from passing integers or strings, which are immutable — a function can never change the original integer variable:

If you want to avoid modifying the original list, pass a copy using [:]:

Practical Examples

Checking if a Number is Prime

Sum of a List

Reversing a String

Reading Input into a List

Wrapping common input patterns in a function keeps your CP code clean:

Summary