Custom Functions

Custom Functions#

We have used built-in functions in 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

type(result)

NoneType

The pow function has two parameters and returns a result:

result = pow(2, 3)

result

8

Custom functions#

You can define your own functions using the reserved keyword def.

After the def statement, you should specify your functions’ name and in brackets its parameters.

Afterward 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)

7

sum_numbers(5, 6)

11

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

c = sum_numbers(4, 5)
print(c)

9

Simplify code using reusable functions#

Assume you have a complicated algorithm which can tell you if a number is 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 %.

Also see the available operators.

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

print_odd_or_even(3)

3 is odd

print_odd_or_even(10)

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 docstring.

The docstring 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:

print(square.__doc__)

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

Also try to use the ? operator for more information about a function.

In Jupyter, this is shown as Introspection side-by-side with your notebook.

square?

Signature: square(number)
Docstring: Squares a number by multiplying it with itself and returns its result.
File:      /var/folders/9k/tl5h0z310vs7cnztts16_0s80000gn/T/ipykernel_51096/2799921345.py
Type:      function

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

import math
print(math.sqrt.__doc__)

Return the square root of x.

print(math.exp.__doc__)

Return e raised to the power of x.

math.exp?

Signature: math.exp(x, /)
Docstring: Return e raised to the power of x.
Type:      builtin_function_or_method

Exercise#

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.

Contents