We have used functions is past lessons. Functions have a name and parameters. Some of them return a result, others don’t. We typically call them using result = name(parameters).

See also

Let’s take a look at some functions, for example print(text) and pow(x, y). The print function takes a parameter (or multiple parameters) and returns nothing:

result = print('Hello world')
Hello world

The pow function has two parameters and returns a result:

result = pow(2, 3)

Custom functions#

You can DEFine your own functions using the def statement. After the def statement, you should specify your functions’ name and in brackets its parameters. Afterwards follows a colon : and all following lines of code which are indented are part of this function. A final return statement sends the result back to from where the function was called.

def sum_numbers(a, b):
    result = a + b
    return result

You can then call your function as often as you like

sum_numbers(3, 4)
sum_numbers(5, 6)

Sometimes, you want to save the result of your function in a variable.

c = sum_numbers(4, 5)

Simplify code using functions#

Assume you have a complicated algorithm which can tell you if a number if odd or even. Let’s put this algorithm in a function and call it later on. For our algorithm, we will use the modulo operator %.

def print_odd_or_even(number):
    if number % 2 == 0:
        print(number, "is even")
        print(number, "is odd")
3 is odd
4 is even
10 is even

Thus, instead of writing the same if-else block again and again, we can just call our custom print_odd_or_even function.

Documenting functions#

You can document what a function does in its so called doc string. The doc string follows right after the functions header and looks like this:

def square(number):
    Squares a number by multiplying it with itself and returns its result.

    return number * number

You can then later read the documentation of the function like this:

    Squares a number by multiplying it with itself  and returns its result.

Also try this if you want to have the docstring shown side-by-side in your notebook:

Signature: square(number)
Docstring: Squares a number by multiplying it with itself  and returns its result.
File:      /var/folders/p1/6svzckgd1y5906pfgm71fvmr0000gn/T/ipykernel_11914/
Type:      function

By the way, you can do this with any function:

import math
Return the square root of x.
Return e raised to the power of x.


Write a function that takes two parameters: number_of_points_in_exam and number_of_total_points_in_exam and returns a grade from 1 to 5. Students with > 95% of the points get grade 1, above 80% they get grade 2, above 60% grade 3 and above 50% grade 4. Students with less than 50% get grade 5 and have to repeat the exam. Then, call the function for three students who had 15, 25 and 29 points in an exam with 30 total points.