Global scope in Python - Step-by-Step Execution

Concept Flow - Global scope

Start Program

↓

Define global variable

↓

Define function

↓

Call function

↓

Inside function: Access global variable

↓

Function ends

↓

Program ends

The program starts by defining a global variable, then a function that uses it. When the function is called, it accesses the global variable from the global scope.

Execution Sample

Python

x = 10

def print_x():
    print(x)

print_x()

This code defines a global variable x and a function that prints x, then calls the function.

Execution Table

Step	Action	Variable 'x' Value	Output
1	Define global variable x = 10	10
2	Define function print_x()	10
3	Call print_x()	10
4	Inside print_x(), print(x)	10	10
5	Function print_x() ends	10
6	Program ends	10

💡 Program ends after function call and output printed

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
x	undefined	10	10	10

Key Moments - 2 Insights

Why does the function print_x() print the value of x even though x is not defined inside it?

Because x is defined in the global scope before the function is called, the function can access it. See execution_table step 4 where print(x) outputs 10.

What if we try to change x inside the function without declaring it global?

Without declaring x as global inside the function, Python treats it as a new local variable. This can cause errors or unexpected behavior. This example only reads x, so no problem occurs.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what value does print_x() print?

A 0

B 10

C undefined

D Error

💡 Hint

Check the 'Output' column at step 4 in execution_table.

At which step is the global variable x first defined?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in execution_table for when x is assigned.

If we added 'x = 5' inside print_x() without 'global' keyword, what would happen?

A The global x changes to 5

B An error occurs immediately

C A new local x is created inside print_x()

D The function prints 10

💡 Hint

Recall that assigning to a variable inside a function creates a local variable unless declared global.

Concept Snapshot

Global scope means variables defined outside functions.
Functions can read global variables directly.
To modify global variables inside functions, use 'global' keyword.
Without 'global', assignments create local variables.
Global variables exist throughout program execution.

Full Transcript

This example shows a global variable x set to 10. A function print_x() prints x. When called, print_x() accesses the global x and prints 10. The variable tracker shows x stays 10 throughout. Key points: functions can read global variables without declaring them. But to change them, you must declare 'global x' inside the function. The quiz checks understanding of when x is defined, what print_x() outputs, and what happens if you assign x inside the function without 'global'.

Practice

1. What does the global keyword do inside a Python function?

easy

A. It deletes a global variable.

B. It creates a new local variable inside the function.

C. It makes the function run faster.

D. It allows the function to modify a variable defined outside the function.

Solution

1. Step 1: Understand the role of global keyword

   The global keyword tells Python that the variable inside the function refers to the variable defined outside (in global scope).

2. Step 2: Effect on variable modification

   Without global, assigning a value creates a new local variable. With global, it modifies the existing global variable.

3. Final Answer:

   It allows the function to modify a variable defined outside the function. -> Option D

4. Quick Check:

   global keyword modifies outer variable [OK]

Hint: global lets functions change outside variables [OK]

Common Mistakes:

• Thinking global creates a new local variable
• Assuming global deletes variables
• Believing global affects performance

2. Which of the following is the correct way to declare a global variable inside a function?

easy

A. def global x:

B. global = x

C. global x

D. x global

Solution

1. Step 1: Recall correct syntax for global declaration

   The correct syntax is to write the keyword global followed by the variable name, separated by a space.

2. Step 2: Check each option

   global x matches the correct syntax. Others are invalid Python syntax.

3. Final Answer:

   global x -> Option C

4. Quick Check:

   global keyword followed by variable name [OK]

Hint: Use 'global' then variable name to declare global inside function [OK]

Common Mistakes:

• Using '=' with global keyword
• Trying to define global like a function
• Placing global after variable name

3. What will be the output of this code?

count = 5
def increment():
    global count
    count += 1
increment()
print(count)

medium

A. 6

B. None

C. Error

D. 5

Solution

1. Step 1: Understand the global variable usage

   The variable count is defined globally with value 5. Inside increment(), global count allows modifying this global variable.

2. Step 2: Trace the function call and print

   The function adds 1 to count, changing it from 5 to 6. Then print(count) outputs 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   global lets function change count to 6 [OK]

Hint: global lets function update outside variable [OK]

Common Mistakes:

• Expecting original value 5 to print
• Thinking global causes error here
• Assuming function returns None

4. Find the error in this code:

total = 10
def add():
    total += 5
add()
print(total)

medium

A. Missing global declaration inside add()

B. Syntax error in function definition

C. total is not defined globally

D. print statement is incorrect

Solution

1. Step 1: Identify variable scope issue

   Inside add(), total += 5 tries to modify total. Without global total, Python treats total as local but it's used before assignment, causing an error.

2. Step 2: Fix by adding global declaration

   Adding global total inside add() tells Python to use the global total variable, fixing the error.

3. Final Answer:

   Missing global declaration inside add() -> Option A

4. Quick Check:

   Modify global variable needs global keyword [OK]

Hint: Add global keyword to modify global variable inside function [OK]

Common Mistakes:

• Ignoring need for global keyword
• Thinking total is local automatically
• Assuming no error occurs

5. You want to count how many times a function is called using a global variable. Which code correctly updates the global counter?

hard

A. calls = 0
def func():
calls += 1
func()
print(calls)

B. calls = 0
def func():
global calls
calls += 1
func()
print(calls)

C. calls = 0
def func():
calls = calls + 1
func()
print(calls)

D. calls = 0
def func():
global calls
calls = calls
func()
print(calls)

Solution

1. Step 1: Understand global variable modification

   To update a global variable inside a function, you must declare it with global inside the function.

2. Step 2: Analyze each option

   calls = 0 def func(): calls += 1 func() print(calls) tries to increment without global, causing error. calls = 0 def func(): global calls calls += 1 func() print(calls) correctly uses global calls and increments. calls = 0 def func(): calls = calls + 1 func() print(calls) tries to read and assign without global, causing UnboundLocalError. calls = 0 def func(): global calls calls = calls func() print(calls) declares global but does not increment.

3. Final Answer:

   calls = 0 def func(): global calls calls += 1 func() print(calls) -> Option B

4. Quick Check:

   global keyword needed to update global variable [OK]

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

Common Mistakes:

• Forgetting global keyword causes UnboundLocalError
• Assigning without global causes UnboundLocalError when reading variable
• Declaring global but not updating variable