Functions

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Functions are essential components in computer programs. They allow to define the details of a single task made of multiple statements. The task will be executed every time the corresponding function is called. Functions have at least two important benefits:

  1. Functions allow to write the instructions for a task only once even if the task needs to be executed multiple times.
  2. Functions increase the clarity of your code: they are confined blocks of code that are small enough to be easily understood independently of the rest of your code.

Optionally, a function can take some input, which are generally refers to as arguments, and return some output. Again, a function doesn’t need necessarily to have input and output: a function can also execute without requiring any additional information and without producing any information.

In Python, the syntax to define a the block of a function is

def my_function(my_argument1, my_argument2, etc.):
    statement 1
    statement 2
    return

where my_function is the name of the function, which is used every time we need to call it, my_argument1 is one of the arguments (generally, some value) that we pass to function to be processed. Finally, by convention we use the return command at the end of the function also if we don’t want to return any value after the execution of the function.

A simple function

Let’s say I want to write a function to greet someone (yes, generally you will only define functions for more complicated tasks, buyt bear with me…). I already know that to combine two strings I simply need to add them: so 'cat' + 'tle' will result in 'cattle'. So this is going to be my function

def greet(name, message):
   print('Hello ' + name + ', ' + message)
   return

The function takes two arguments: a name and a message. As you see, after print()ing the message the function return without passing back any value. After running the previous cell, you can test the function with the following code

greet("Pauline", "how are you?")

A function returning something

Now let’s say that I need a function that will compose the same greeting message as before but instead of printing the message out, it will return it a string value. I can simply modify the function such that I store the message in a variable called my_message and then I return the value of that variable.

def greet(name, message):
   my_message = 'Hello ' + name + ', ' + message
   return my_message

returned_value = greet("Pauline", "how are you?")

the previous cell will not output anything (there’s not a print() statement anymore!) Still, if I run the following I get my message back…

print(returned_value)

Exercise

Can you complete the following code so that the function will add two numbers and print the solution?

# Here you define the function
def sumTwoNumbers(num1, num2):
    result = ____ + ____
    print("The result is " + ____)
    
# Here you call the function    
sumTwoNumbers(34, 86)

Did you get an error such as TypeError: Can't convert 'int' object to str implicitly? Remember that you can’t concatenate a string to a number. You need to convert the number to a type string first with str().

Additional resources

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