Functions

A function in Python is a reusable block of code designed to perform a specific task. A function can accept one or more parameters, and may or may not return a value. Here are some of Python’s built-in functions we’ve used up to this point:

print - output a value to the console
list.append - add an element to the end of a list
input - prompt the user for input and return the string entered by the user
type - return the type of a value (int, float, str, list, bool)

The data types themselves can also be called as functions to convert a value from one type to another (when possible). Remember we did this to convert a string to an integer:

int('45')

Output:

Another built-in function is len, which accepts one parameter. If the parameter is a string, len returns the the number of characters in the string:

greeting = "Hello"
len(greeting)

Output:

If the parameter is a list, len returns the number of items in the list:

groceries = ['apples', 'pears', 'grapes']
len(groceries)

Output:

We can also define our own functions using the def keyword. Similar to if statements and while loops, we must end the line with a colon : and indent the lines of code within the function. Use the return keyword to have a function return a value. For example, here is a function that takes one string parameter and returns a string with a personalized greeting:

def make_greeting(first_name):
    s = f"Hello, {first_name}!"
    return s

Now we can call the function we just created:

greeting = make_greeting('Bob')
print(greeting)

Output:

Hello, Bob!

Exercises

Change the make_greeting function above to accept two parameters, first_name and last_name, and return a greeting with the user’s full name.