Global keyword in Python - Time & Space Complexity

Time Complexity: Global keyword

O(n)

Understanding Time Complexity

Let's explore how using the global keyword affects the time it takes for a program to run.

We want to see if accessing or changing global variables changes how long the program takes as it grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

count = 0

def increment(n):
    global count
    for _ in range(n):
        count += 1

increment(5)

This code increases a global variable by 1, n times inside a loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that runs n times.
• How many times: Exactly n times, where n is the input number.

How Execution Grows With Input

Each time n grows, the loop runs that many more times, increasing work linearly.

Input Size (n)	Approx. Operations
10	10 increments
100	100 increments
1000	1000 increments

Pattern observation: The work grows directly with n; double n means double work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line as the input number n grows.

Common Mistake

[X] Wrong: "Using the global keyword makes the loop slower or faster."

[OK] Correct: The global keyword only changes where the variable lives, not how many times the loop runs. The time depends on the loop count, not on global or local variables.

Interview Connect

Understanding how global variables affect performance helps you write clear and efficient code, a skill valued in many coding challenges and real projects.

Self-Check

"What if we replaced the for-loop with a recursive function that increments the global variable? How would the time complexity change?"

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