Working with Scope

Learning Objectives

Understand global variables in Python
Understand local variables in Python and why they are a good feature of Python

Introduction

In our introduction to functions, we casually introduced something that is quite odd. Take a look at the following.

def sample_function():
    words = 'function body'

words

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

<ipython-input-2-993ed7d20d5f> in <module>()
----> 1 words


NameError: name 'words' is not defined

Somehow, our words variable is suddenly inaccessible. Python has various rules about when and how to access variables and data throughout a file. As we'll see in this lesson, accessibility of a variable depends on whether that variable is a global variable or a local variable. We will also see how a function's return statement allows us to send data inside of a function into global scope.

Global variables

Before our introduction to functions in the recent lesson, we always had access to any variable we had declared in Python.

number = 1

number

We weren't thinking about it, but we were operating in the global scope. Whenever we declare a variable that is not declared inside of a function, we are operating in global scope. This means that the variable is available anywhere in current file.

Accordingly, we can access the variable from outside of a function...

number

...or inside of a function.

def access_to_globals():
    # we can access global variables from inside of our function
    return number

access_to_globals()

Global variables are a privileged bunch. Once declared, they can be referenced either inside or outside of a function.

Local variables

Local variables are resigned to a different fate than global variables. Below, the variable trapped is a local variable.

def locals_stay_local():
    trapped = 'not escaping'

locals_stay_local()

Unlike our global variable number, trapped is first declared from inside a function, making it a local variable.

trapped

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

<ipython-input-4-956155b55e40> in <module>()
----> 1 trapped


NameError: name 'trapped' is not defined

Because the variable trapped is declared inside a function, it can only be referenced from that function. Believe it or not, this is a helpful feature. By declaring a local variable, we know that we only have to pay attention to that variable from inside the body of the function. We do not have to search our file to see what that variable may have been assigned or reassigned to.

And from inside of the function, we can use that variable to make our code more expressive.

def no_return_full_name():
    first_name = 'bob'
    last_name = 'smith'
    full_name = first_name + ' ' + last_name

Return statements

Of course we want our function to have some impact outside of itself. To do that, we use a return statement.

Let's execute our function no_return_full_name.

no_return_full_name()

full_name

If you press shift + enter on the two code blocks above, you have executed our function and then tried to reference the variable full_name. However, because the variable full_name is local, it is only available from inside of the function. That's not very useful.

So let's write another function called return_full_name that has a return statement.

def return_full_name():
    first_name = 'bob'
    last_name = 'smith'
    full_name = first_name + ' ' + last_name
    return full_name

return_full_name()

'bob smith'

Now the string 'bob smith' is returned from the function. Notice that the full_name is still not available globally.

full_name

---------------------------------------------------------------------------

NameError                                 Traceback (most recent call last)

<ipython-input-5-1ef4381f016d> in <module>()
----> 1 full_name


NameError: name 'full_name' is not defined

However, we did throw the variable's value over the wall, and if we wish to use it with more code we can. For example, we can combine the return value with another expression:

'Hello ' + return_full_name()

Or we can assign the return value to a global variable, and access it from that variable.

a_fine_name = return_full_name()
a_fine_name

'bob smith'

So variables declared inside of a function are still only available locally. However, by using a return statement we can throw the data over the wall of the function and into global scope.

Another thing to note about a return statement is that once a function reaches the return statement, no other lines of the function are executed.

def return_statements():
    print("this is executed")
    return "this is the function's output"
    print("this is not executed")
return_statements()

this is executed





"this is the function's output"

So, return statements are an important feature of functions as they terminate the execution of a function and throw the result of the function into global scope.

Summary

In this section we learned about scope. We saw how when we declare a variable outside of a function, we are declaring that variable in global scope. This means that the variable is available throughout the file it is declared in - inside and outside of functions. Variables declared inside of functions are local variables and are available to be referenced from inside of the function in which they are declared. However, we can access the local variable outside of the function by using the return keyword at the end of our function. The combination of local variables and returning specific data allows us to encapsulate our code inside of a function and explicitly state what should be returned, and thus accessible from outside of the function.