Global Variables

We can create a variable outside of a function. This is called a global variable. We can access the global variable inside or outside of a function. Let’s see an example.

A variable’s scope is the region of the program where it is visible. A variable created outside every function lives in the global scope; one created inside a function lives in that function’s local scope.

Diagram:

diagram global vs local scope mermaid

  graph TD
    A[Global scope] --> B[name = 'John']
    A --> C[Function display]
    C --> D[Local scope: its own names]
    D -->|can read| B
    D -->|cannot change unless 'global'| B

Where variables are visible

Example:

variable.py

# Global variable
name = "John"
def display():
    print("Hello, " + name)
display()
print("Hello, " + name)

Output:

command

C:\Users\Your Name> python variable.py
Hello, John
Hello, John

In the above example, we created a global variable name outside of the function display(). We can access the global variable name inside or outside of the function display().

Tip

We can access the global variable inside or outside of a function.

Global Keyword

We can create a local variable inside a function. This is called a local variable. We can access the local variable inside the function. We can’t access the local variable outside the function. Let’s see an example.

Example:

variable.py

# Global variable
name = "John"
def display():
    # Local variable
    name = "Smith"
    print("Hello, " + name)
display()
print("Hello, " + name)

Output:

command

C:\Users\Your Name> python variable.py
Hello, Smith
Hello, John

In this example, we created a local variable name inside the function display(). We can access the local variable name inside the function display(). We can’t access the local variable name outside the function display().

Tip

We can access the local variable inside the function. We can’t access the local variable outside the function.

We can use the global keyword to access the global variable inside the function. Let’s see an example.

Example:

variable.py

# Global variable
name = "John"
def display():
    # Local variable
    global name
    name = "Smith"
    print("Hello, " + name)
display() 
print("Hello, " + name) 

Output:

command

C:\Users\Your Name> python variable.py
Hello, Smith
Hello, Smith

In this example, we used the global keyword to access the global variable name inside the function display(). We can access the global variable name inside the function display(). We can also access the global variable name outside the function display().

Caution

Use global sparingly. Functions that quietly modify global state are hard to test and debug, because their result depends on data outside the function. Prefer passing values in as parameters and returning results:

prefer_return.py

# Better than mutating a global
def increment(value):
    return value + 1
 
count = increment(0)   # caller controls the state

Note

global reaches the module-level scope. To reassign a variable in an enclosing function (not the global scope), use the nonlocal keyword instead. Both are covered in depth in the Closures and Scope (LEGB) tutorial.

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Try it: Global Variables Exercises

Exercise 1 – Access Global Variable

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Exercise 2 – Modify Global with global Keyword

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Exercise 3 – Global vs Local

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