Why Global keyword in Python? - Purpose & Use Cases

The Big Idea

What if your function could magically update variables everywhere without confusing copies?

The Scenario

Imagine you have a program where you want to change a number stored outside a function, but inside the function, you create a new number instead of changing the original one.

The Problem

Without the global keyword, changing a variable inside a function only changes a local copy. This means your original number stays the same, causing confusion and bugs. You have to write extra code or use tricky workarounds.

The Solution

The global keyword tells Python you want to use the variable defined outside the function. This way, you can change the original number directly inside the function, making your code clearer and easier to manage.

Before vs After

✗ Before

count = 0

def increase():
    count = 1  # This creates a new local variable

increase()
print(count)  # Still 0

✓ After

count = 0

def increase():
    global count
    count = count + 1  # Changes the global variable

increase()
print(count)  # Now 1

What It Enables

It lets you safely and clearly update shared data across different parts of your program.

Real Life Example

Think of a game where you keep track of the player's score. Using global lets you update the score from different functions like when the player earns points or loses lives.

Key Takeaways

Without global, functions can't change variables outside them.

Global keyword links a function variable to the outside one.

This makes updating shared data simple and clear.

Practice

1.

What does the global keyword do in Python?

easy

A. It creates a new local variable inside a function.

B. It allows a function to modify a variable defined outside it.

C. It deletes a variable from the global scope.

D. It imports a module globally.

Solution

1. Step 1: Understand variable scopes

   Variables defined outside functions are global, inside functions are local by default.

2. Step 2: Role of global keyword

   The global keyword tells Python to use the global variable inside the function, allowing modification.

3. Final Answer:

   It allows a function to modify a variable defined outside it. -> Option B

4. Quick Check:

   global lets function change outside variable [OK]

Hint: Global lets functions change outside variables directly [OK]

Common Mistakes:

• Thinking global creates new local variables
• Confusing global with import statements
• Assuming global deletes variables

2.

Which of the following is the correct way to modify a global variable count inside a function?

count = 0
def increment():
    ?
    count += 1

easy

A. def count

B. local count

C. global count

D. nonlocal count

Solution

1. Step 1: Identify the need to modify global variable

   The function wants to increase the global variable count.

2. Step 2: Use correct keyword to access global variable

   Using global count inside the function tells Python to use the global count, not create a local one.

3. Final Answer:

   global count -> Option C

4. Quick Check:

   Use 'global' to modify global variables inside functions [OK]

Hint: Use 'global' before variable to modify it inside function [OK]

Common Mistakes:

• Using 'local' which is not a Python keyword
• Using 'nonlocal' which applies to enclosing functions, not globals
• Trying to define variable with 'def'

3.

What is the output of this code?

value = 5
def change():
    global value
    value = 10
change()
print(value)

medium

A. 5

B. Error

C. None

D. 10

Solution

1. Step 1: Analyze the function change()

   The function declares value as global and sets it to 10.

2. Step 2: Effect of calling change()

   Calling change() updates the global value from 5 to 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   global lets function update outside variable [OK]

Hint: global changes outside variable, so print shows updated value [OK]

Common Mistakes:

• Thinking print shows original value 5
• Expecting an error without global keyword
• Confusing local and global scopes

4.

Find the error in this code and choose the fix:

counter = 0
def add():
    counter += 1
add()
print(counter)

medium

A. Add global counter inside add() before incrementing.

B. Change counter to a local variable inside add().

C. Remove the increment line counter += 1.

D. Define counter inside add() without global.

Solution

1. Step 1: Identify the error cause

   Trying to increment counter inside add() without declaring it global causes UnboundLocalError.

2. Step 2: Fix by declaring global variable

   Adding global counter inside add() tells Python to use the global counter variable.

3. Final Answer:

   Add global counter inside add() before incrementing. -> Option A

4. Quick Check:

   Declare global to modify outside variable inside function [OK]

Hint: Declare global before modifying global variable inside function [OK]

Common Mistakes:

• Ignoring the need for global declaration
• Trying to create local variable with same name
• Removing increment instead of fixing scope

5.

You want to count how many times a function is called using a global variable calls. Which code correctly updates calls each time track() runs?

calls = 0
def track():
    ?
    calls += 1

track()
track()
print(calls)

hard

A. global calls

B. nonlocal calls

C. local calls

D. calls = 0

Solution

1. Step 1: Understand the goal

   We want to update the global variable calls inside the function track().

2. Step 2: Use the correct keyword

   Using global calls inside track() allows incrementing the global calls variable.

3. Step 3: Confirm output

   Calling track() twice increments calls from 0 to 2, so print(calls) outputs 2.

4. Final Answer:

   global calls -> Option A

5. Quick Check:

   Use global to update global counter inside function [OK]

Hint: Use 'global' to update global counter inside function [OK]

Common Mistakes:

• Using 'nonlocal' which is for nested functions
• Trying to assign local variable without global
• Resetting calls inside function