__name__ and __main__ behavior in Python - Mini Project: Build & Apply

__name__ and __main__ behavior

📖 Scenario: Imagine you are creating a Python script that can be used both as a program and as a module imported by other scripts. You want to control what code runs only when the script is executed directly.

🎯 Goal: Build a Python script that defines a function and uses the if __name__ == '__main__' check to run code only when the script is run directly, not when imported.

📋 What You'll Learn

Create a function called greet that prints a greeting message.

Create a variable called message with the value 'Hello from the function!'.

Use the if __name__ == '__main__' statement to call the greet function only when the script is run directly.

Print 'Script is being run directly' inside the if __name__ == '__main__' block.

💡 Why This Matters

🌍 Real World

Many Python scripts are written to be both reusable modules and standalone programs. Using <code>if __name__ == '__main__'</code> helps control code execution depending on how the script is used.

💼 Career

Understanding this concept is essential for writing clean, reusable Python code in software development, automation scripts, and data science projects.

1

Create the greet function and message variable

Create a function called greet that prints the variable message. Also, create a variable called message with the value 'Hello from the function!'.

Python

# Create the message variable and greet function
# Your code here

Hint

Define message first, then define greet() that prints message.

2

Add the if __name__ == '__main__' block

Add the if __name__ == '__main__' statement below the function. Inside this block, print 'Script is being run directly'.

Python

message = 'Hello from the function!'

def greet():
    print(message)

# Add the if __name__ == '__main__' block below
# Your code here

Hint

Use the exact line if __name__ == '__main__': and indent the print statement inside it.

3

Call the greet function inside the if __name__ == '__main__' block

Inside the if __name__ == '__main__' block, call the greet() function after the print statement.

Python

message = 'Hello from the function!'

def greet():
    print(message)

if __name__ == '__main__':
    print('Script is being run directly')
    # Call greet() here
    # Your code here

Hint

Simply add greet() indented inside the if __name__ == '__main__' block after the print.

4

Print the output when running the script

Run the script and print the output. The output should show Script is being run directly followed by Hello from the function!.

Python

# Run the script and print the output
# Your code here

Hint

Just run the script as is. The output should match exactly.

Practice

1. What does the special variable __name__ contain when a Python file is run directly?

easy

A. The file's directory path

B. The file's extension

C. "__main__"

D. The Python version

Solution

1. Step 1: Understand the role of __name__

   When a Python file runs directly, __name__ is set to the string "__main__".

2. Step 2: Differentiate direct run vs import

   If the file is imported, __name__ is the module's name, not "__main__".

3. Final Answer:

   "__main__" -> Option C

4. Quick Check:

   __name__ = "__main__" when run directly [OK]

Hint: Direct run sets __name__ to "__main__" [OK]

Common Mistakes:

• Thinking __name__ holds file path
• Confusing __name__ with Python version
• Assuming __name__ is always module name

2. Which of the following is the correct syntax to run code only when a Python file is executed directly?

easy

A. if __name__ == '__main__':

B. if __main__ == '__name__':

C. if main == '__name__':

D. if __name__ = '__main__':

Solution

1. Step 1: Check correct variable and string

   The variable is __name__ and the string to compare is "__main__".

2. Step 2: Verify syntax correctness

   Use double equals == for comparison, and colons to start the block.

3. Final Answer:

   if __name__ == '__main__': -> Option A

4. Quick Check:

   Correct syntax uses == and exact names [OK]

Hint: Use if __name__ == '__main__': exactly [OK]

Common Mistakes:

• Using single equals (=) instead of double (==)
• Swapping __name__ and __main__
• Missing colon at end of if statement

3. What will be the output when running this Python file directly?

def greet():
    print('Hello!')

if __name__ == '__main__':
    greet()

medium

A. Hello! Hello!

B. No output

C. Error: greet() undefined

D. Hello!

Solution

1. Step 1: Check if condition when run directly

   Since the file runs directly, __name__ == '__main__' is True, so greet() is called.

2. Step 2: Understand greet() function output

   The function prints "Hello!" once.

3. Final Answer:

   Hello! -> Option D

4. Quick Check:

   Function called once prints "Hello!" [OK]

Hint: Direct run triggers greet() printing once [OK]

Common Mistakes:

• Thinking greet() runs twice
• Assuming no output without main guard
• Confusing function call with definition

4. Identify the error in this code snippet:

def main():
    print('Running main')

if __name__ = '__main__':
    main()

medium

A. IndentationError in function definition

B. SyntaxError due to single equals (=) in if condition

C. NameError because main() is undefined

D. No error, code runs fine

Solution

1. Step 1: Check the if condition syntax

   The condition uses single equals (=) which is assignment, not comparison, causing SyntaxError.

2. Step 2: Confirm function and indentation

   The function main() is defined correctly and indentation is fine.

3. Final Answer:

   SyntaxError due to single equals (=) in if condition -> Option B

4. Quick Check:

   Use == for comparison, not = [OK]

Hint: Use == in if condition, not = [OK]

Common Mistakes:

• Using = instead of ==
• Assuming function undefined error
• Ignoring indentation correctness

5. You have two Python files:

# file1.py
def greet():
    print('Hi from file1')

if __name__ == '__main__':
    greet()

# file2.py
import file1
print('In file2')

What will be the output when you run file2.py?

hard

A. In file2

B. No output

C. Hi from file1

D. Hi from file1
In file2

Solution

1. Step 1: Understand import behavior with __name__

   When file1 is imported, its __name__ is "file1", not "__main__", so greet() inside the if block does NOT run.

2. Step 2: Check what runs in file2.py

   Only print('In file2') runs, so output is just "In file2".

3. Final Answer:

   In file2 -> Option A

4. Quick Check:

   Import skips main block, prints only file2 message [OK]

Hint: Imported file skips if __name__ == '__main__' block [OK]

Common Mistakes:

• Assuming greet() runs on import
• Expecting both prints always
• Confusing __name__ values on import