Functions basics

Functions are a basic tool for organizing programs in Python (and in other programming languages). A function is essentially a block a code with a name. It we want to use a function then we can simply call it by its name. The syntax for defining functions is as follows:

def <function name>(<function arguments>):

Example. The following function multiplies two numbers and prints the result:

def multiply(a, b):
    c = a*b
    print('{0}*{1} = {2}'.format(a, b, c))

3*2 = 6


12*13 = 156

A function can return one or more values using the return statement. This statement is optional since in general a function does not need to return anything. However, a function will stop executing as soon as the return statement is encountered.

Example. The following function takes as its argument a string, and returns the first vowel in the string:

def first_vowel(s):
    for letter in s:
        if letter in ['a', 'e', 'i', 'o', 'u']:
            return letter
    return 0




Positional and keyword arguments

There are two ways to pass arguments to a function: by position and by keyword. For example, here is a function which computes the volume of a cylinder with a given height and radius:

def cyl_vol(height, radius):
    pi = 3.1415926
    vol = pi*height*radius**2
    return vol

If we call this function by cyl_vol(3,5) it will pass the arguments by position: 3 becomes the value of the first argument height, and 5 the value of the second argument radius:



Alternatively, we can explicitly use the keywords to set values of the arguments:

cyl_vol(height = 3, radius = 5)


While this takes more typing the advantage is that the code is more readable. In addition, arguments specified by keywords can be given in any order, so we don’t need to remember their positions:

cyl_vol(radius = 5, height = 3)


It is also possible to specify some arguments of a function by position and some by keyword.

Example. The following function coverts temperatures between Celsius, Fahrenheit, and Kelvin scales. It takes three arguments: t is the temperature to be converted, in_temp specifies the scale from which we are converting, and out_temp sets the scale we are converting to:

def temp_convert(t, in_temp, out_temp):
    if in_temp == 'C' and out_temp == 'F':
        return (9.0/5.0)*t + 32
    if in_temp == 'C' and out_temp == 'K':
        return t + 273.15
    if in_temp == 'F' and out_temp == 'C':
        return (5.0/9.0)*(t-32)
    if in_temp == 'F' and out_temp == 'K':
        return (5.0/9.0)*(t+459.67)
    if in_temp == 'K' and out_temp == 'C':
        return t - 273.15
    if in_temp == 'K' and out_temp == 'F':
        return (9.0/5.0)*t - 459.67

It is convenient to specify the first argument of this function by position, and the remaining ones by keywords. In this way we don’t need to remember which position is the input and which is the output scale:

temp_convert(47, in_temp = 'C', out_temp = 'K')


temp_convert(12, out_temp = 'F', in_temp = 'C')


Default values

In the definition of a function we can give default values of some function arguments. These arguments can be then ommitted in function calls since in such case the default values will be used.

Example. Here we modify the function first_vowel defined above so that for a given integer n it returns the n-th vowel in a string :

def nth_vowel(s, n=1):
    count = 0
    for letter in s:
        if letter in ['a', 'e', 'i', 'o', 'u']:
            count += 1
            if count == n:
                return letter
    return 0

In the definition of this function the argument n is given the default value 1, so this will be the value used if we don’t specify a value for n:



On the other hand, if we specify the value of n this value will be used instead:

nth_vowel('profitable', 3)


Note. In the definition of a function arguments with default values must be listed after arguments without defaults.

Why use functions

Here are some reasons why functions are useful:

  • Reusability. If some code will be used more than once in a program it is better to write is as a function rather than copy and paste the code several times.

  • Code readibility. It is usually easier to understand what a program does if it is split into short functions.

  • Debugging. Since each function can be tested separately it is easier to fix problems in a program if it is organized into functions.