Functions

You've been calling functions since Day 1: print(), len(), sorted(). Today you'll write your own. A function is a named, reusable recipe: give it inputs, it does some work, and it hands back a result.

Defining and calling

def starts the definition, the name follows the same rules as variables, and the body is indented. Nothing runs until you call the function with parentheses.

Parameters and arguments

Parameters are the names in the definition; arguments are the values you pass in:

Default values

Give a parameter a default and callers can leave it out:

return vs print

print only shows a value. return hands it back so the caller can use it:

Returning several values

Return a tuple and unpack it (Day 7):

Scope: where variables live

Variables created inside a function are local: they exist only during that call. The function can read variables from outside, but assigning to a name inside a function creates a new local one.

Docstrings

A string right after the def line documents the function. Tools and help() show it:

Functions calling functions

Small functions combine into bigger ones:

Recap

• def name(params): defines a function; call it with name(args).
• Arguments can be positional, keyword, or left out when there's a default.
• return hands a value back; without it a function returns None.
• Variables inside a function are local; pass inputs in and return results out.
• Add a docstring to explain what a function does.