Functions

What you'll learn

How to define a function with def, pass values in, get a value out with return, and use default and keyword arguments.

Why it matters

Functions stop you repeating yourself. You write the steps once, give them a name, and use that name whenever you need them. They are also how large programs are broken into understandable pieces.

Explanation

A function is a named block of code. You create it with def, and run it by calling its name with brackets.

• name is a parameter: a placeholder. The value you pass when calling is an argument.
• return sends a result back and ends the function. Without return, the function gives back None.
• Default values: def greet(name="friend") is used when no argument is given.
• Keyword arguments: call with names, greet(name="Asha"), so order does not matter.
• Variables created inside a function are local: they do not exist outside it.
• A docstring (a string as the first line) documents what the function does.

Example

Step-by-step walkthrough

1. area(4, 5) multiplies 4 by 5 and returns 20.
2. area(7) uses the default height=1, so it returns 7.
3. describe(age=30, name="Asha") uses keyword arguments, so the order does not matter: Asha is 30 years old.
4. stats returns three values at once. Python packs them into a tuple, and we unpack them into low, high, total: 3 9 16.

Try it yourself

Write a function is_even(n) that returns True when n is even, then print is_even(10) and is_even(7).

FunctionsPython

Editable codedef area(width, height=1): """Return the area of a rectangle.""" return width * height print(area(4, 5)) print(area(7)) def is_even(n): # your code here pass print(is_even(10))

Run ResetCopyCtrl+Enter to run

Common mistakes

• Forgetting return and getting None.
• Calling without brackets: greet is the function, greet("Asha") runs it.
• Mutable default arguments such as def add(item, items=[]). The same list is shared between calls. Use None instead.
• Printing instead of returning, which makes the result impossible to reuse.

Quick quiz

Summary

• def name(params): defines a function; return sends back a value.
• Default and keyword arguments make calls flexible.
• Variables inside are local.

Next: Python's built-in collections.

def greet(name):
    return "Hello, " + name

def area(width, height=1):
    """Return the area of a rectangle."""
    return width * height

def describe(name, age):
    return f"{name} is {age} years old"

print(area(4, 5))
print(area(7))
print(describe(age=30, name="Asha"))

def stats(numbers):
    return min(numbers), max(numbers), sum(numbers)

low, high, total = stats([3, 9, 4])
print(low, high, total)