Functions II

Now that you know how to use functions in practice, there are several more concepts that will help you understand behaviour of functions not only in Python, but other languages as well.

Scope

Global and Local Variables

Imagine you want to use the slightly modified printBirthdayGreeting() function from before and you want to increment age every time the function is called:

name = "Johann"
age = 32

def printBirthdayGreeting():
        age += 1
    return "Happy Birthday " + name + ", you are " + str(age) + " years old"

printBirthdayGreeting()

Can you spot what is wrong? If you try to run it, you’ll probably get this message: `` UnboundLocalError: local variable ‘age’ referenced before assignment``.

To understand this, we have to talk about scope. Scope is an ‘area’ in which a variable is defined, can be accessed and written to. From this point of view we know two types of variables: global and local. By default, all variables defined within a function are local - you cannot access them outside of the function. And since the scope within the function is different from the global one, it’s possible to use the same name for two different variables.

Can you explain what happened in the code snippet above now?

age outside of printBirthdayGreeting() function is a global variable. However, when we want to access it inside the function, Python considers it to be a new local variable. How do we solve this? We can declare the variable age as global:

name = "Johann"
age = 32

def printBirthdayGreeting():
        global age
        age += 1
        return "Happy Birthday " + name + ", you are " + str(age) + " years old"

This will let Python now, that the age variable we mean is the one in global namespace.

Warning

Using global variables is generally a bad practice and you should avoid it, since it makes the purpose of your functions less obvious and you can end up with ‘spaghetti’ code. A better way to do this is to pass variable age as one of the arguments of the function (example below).

Here is an example for a function that passes variables as arguments:

def printBirthdayGreeting(name, age):
        age += 1
        return "Happy Birthday " + name + ", you are " + str(age) + " years old"

Tip

You will be hearing about ‘best practices’ a lot. How do you determine what is a best practice and what is not? In general, best practice is what makes your code more readable to others. You can look at style guides for a language you’re coding in, but in the end it’s always about good judgment, since no rule applies to all cases.

Source: xkcd https://xkcd.com/1513/

Nonlocal variables

A curious case arises with the use of nested functions. So let’s say you want to change a local variable of the justAnExample() function using the nested function:

def justAnExample():
        def continuingExample():
                variable = "Inner function that changes everything!"

        variable = "Outer function"
        continuingExample()

        print(variable)

justAnExample()

You already know why this does not work. But how do you fix it? You cannot declare the variable global, because it’s within a function - it’s local and there is another local scope within the continuingExample() function. To resolve this situation, you can declare a variable to be nonlocal:

def justAnExample():
        def continuingExample():
                nonlocal variable
                variable = "Inner function that changes everything!"

        variable = "Outer function"
        continuingExample()

        print(variable)

justAnExample()

Now the code should print "Inner function that changes everything!" exactly the way we wanted.

Note

To learn more about namespace and scope in Python, look at the documentation.

Passing parameters

An important concept that will have a visible impact on the working of your functions is passing parameters. This describes the way a variable is treated as it’s passed in a function - in a pass-by-value scenario, the argument is treated as a new local variable and has no influence on the original variable (if a variable was passed as an argument). In the case of pass-by-reference, the variable passed as an argument can be affected within a function. In Python, the method of parameter passing is a specific combination of the two - parameter are passed by value of object reference.

For a good explanation of passing parameters and the difference between different techniques, I would recommend you to read this blogpost by Robert Heaton.