Python Global and Local Variables

In this blog post, we will learn the scope of Functions, namely Global and Local variables, scopes and namespace expressions. Let’s start.

As in all programming languages, every class, function and variable has a scope. To put it simply, a block and namespace in which the variables we create are valid are stored in that namespace and are deleted from memory when you leave that block.

Local Variable

Let’s show it with a simple example;

def function():
    a = 5
   
function()
print(a)

When we run the code, we get the following error.

Traceback (most recent call last):
  File "/Users/baran/Documents/work/baransel.dev/functions.py", line 7, in <module>
    print(a)
NameError: name 'a' is not defined

So we got the error variable a could not be found because the scope of variable a is valid only inside the function(). Because variable a is a local variable. Variables defined for a function are specific to that function only.

Global Variable

We learned what a local variable is, now let’s look at what a global variable is. Let me just show you with an example.

a = 3
def function():
    a = 5
    print(a)
function()
print(a)

# 5
# 3

When we run the code, we output 5 and 3. Let’s see how it works now;

When we run the program, since Python is an interpretive language, it reads line by line first, the function() will run, and as a result, we got the value of 5 in the function. After the function() is finished, the print() function will run, and as a result, we got the value 3. Here, since the variable a in the function() function is a local variable, its scope is only within the function. The print() function cannot reach it and prints the global variable a. Then we can define global variables as follows;

Global variables are variables that do not belong to a single function and can be called and used from anywhere in the program once they are defined.

Now a question mark has appeared in your mind. The global variable can be called and used everywhere, but can we call these variables inside the function? Of course, you can use global variables in a function, let me show you right away;

a = 3
def function():
    print(a)
function()

# 3

Global Expression

Now that we have learned this, there is something that I have just said to you so that you do not get confused. Just before, while we were defining global variables, we made a definition like this; we used the definition of global variables only variables that do not belong to a function. And we also used global variables inside the function. Can we do something like this, can we create a global variable inside the function? Let’s try it now and see;

def function():
    global a
    a = 3
    print(a)
function()
print(a)

# 3
# 3

As you can see, we got two values of 3, so let’s see how we got them.

First, function() will run and print the value of a in the function, then the print() function will run and since we have defined the variable a globally in the function, the print() function will still print the value 3 on the screen, as it can reach the variable a.

Here, we learned how to create a global variable in a function with the global expression. But here’s a point I have to tell you, because global variables are accessible everywhere, your code can cause complexity, so using global variable and global expression is not recommended. It’s better and readable to define and use locally for the function.