Scope: local vs. global

Let’s take a look at the following simple code.

def assign_x(num):
    x = num

x = 10
assign_x(8)
print('The value of x is:', x)

The function assign_x(num) is defined to assign x with the given value num. After defining the function, x is set to be 10, and assign_x(8) is called, which runs x=8. Then, the last line prints the value of x to the screen. Now, one may expect that the output would show The value of x is: 8. However, our actual output is

The value of x is: 10

What is happening here? The value of x hasn’t been changed although print function was called after assign_x(8). This is because x in assign_x function was treated as a local variable, which exists only in the local scope of assign_x function. It’s about time to learn what local/global scope is.

A parameter or variable that is assigned or declared in a function is called a local variable, and it is said to be in the local scope of that function. When a function is returned, every local variable in that function is forgotten. On the contrary, a variable that is declared outside all functions is called a global variable, which is in the global scope. The global scope is created when the program begins. While there can be as many local scopes as there are functions, there is only one global scope.

Please note that a variable cannot be both local and global, even if a variable of the same name appeared in each other’s scope. In the example above, when x=10 is implemented, x is a global variable. However, when assign_x(8) is called, a new local variable named x is created and assigned with num. Python has its own way to tell apart the global x and the local x. After assign_x(8) is returned, the data of the local variable x is removed. When the last line with print function is called, x is the global one, which is why our result showed that the value of x is 10.

Now that we have figured out how local and global scopes are different, let’s think about the following code.

def assign_x(num):
    x = num
    print('The value of x is:', x)

x = 10
assign_x(8)
print('The value of x is:', x)

This is the same as the previous example except that assign_x function also has a line to print the value of x. The output is as follows.

The value of x is: 8
The value of x is: 10

The first line shows that x is 8, because print inside the call of assign_x function deals with the local variable x, the value of which is 8. The next line reads that x is 10, since it is using the global variable x.