Modifying Variables Out of Scope

00:00 When you want to modify variables, then you need to be aware of the scope that the variable you need to modify is in. Sometimes, a function is able to modify a variable outside of its local scope, and sometimes it is not.

00:17 So, how can you know whether it’s possible to modify a variable outside the function’s local scope? Well, a function can never modify an immutable variable outside its local scope,

00:34 and a function is able to modify a mutable variable outside its local scope in place.

00:44 Let’s start with an example that shows you it is not possible for a function to redefine the value of a variable that is already defined outside its scope.

00:58 This module declares a variable called x and it assigns a value 20. Then inside the function f(), the function declares another variable x to a value 40, after which it prints x.

01:17 Now, when you load the module and call the function f(), you will see that the value of x is 40, but when you look at the value of x itself, then you see that the value of x is still 20.

01:35 That is because when this assignment is executed, a new local reference to an integer object whose value is 40 is declared in the local namespace of the function f() and the value of x in the global namespace is not affected at all.

01:59 Out of scope objects of a mutable type, on the other hand, can be modified in place. In this example, a list—which is a mutable object—is created in the global scope.

02:14 And the first item of this list is replaced with another value in the function f(). After running f() and looking at my list,

02:27 you will see that the contents of the list changed.

02:33 But if the function tries to replace the list entirely, then it will create a new object in the local scope and won’t modify the list that was created in the global scope.

02:46 So, after running f() and looking at my list, you will see that the list hasn’t changed at all.

02:57 But what if you really need to modify an immutable value in the global scope? We will have a look at that in the next lesson.