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 = function_name(parameters).

See also

• Python functions

• List of built-in functions

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

result

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

# Definition of the custom function
def sum_numbers(a, b):
    # Assignment of a variable within the function
    result = a + b
    # Return of the variable as the functions' result
    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 doc string. The doc string follows right after the functions’ header (the first line) 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 if you want to have the docstring shown as Introspection side-by-side in your notebook:

square?

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.