__name__ and __main__ behavior in Python - Step-by-Step Execution

Concept Flow - __name__ and __main__ behavior

Python file starts

↓

Set __name__ variable

↓

Check if __name__ == "__main__"

Yes No↓

Run main code

↓

Program ends

When a Python file runs, it sets __name__. If __name__ is "__main__", it runs the main code block; otherwise, it skips it.

Execution Sample

Python

def greet():
    print("Hello from greet()")

if __name__ == "__main__":
    greet()

This code runs greet() only if the file is run directly, not when imported.

Execution Table

Step	__name__ value	Condition (__name__ == "__main__")	Action	Output
1	"__main__"	True	Call greet()
2	"__main__"	True	Inside greet(): print message	Hello from greet()
3	"__main__"	True	End of program

💡 Program ends after running greet() because __name__ == "__main__" is True

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
__name__	Set by Python	"__main__"	"__main__"	"__main__"

Key Moments - 2 Insights

Why does the code inside if __name__ == "__main__" run only when the file is run directly?

Because when run directly, Python sets __name__ to "__main__" (see execution_table step 1). When imported, __name__ is the module name, so condition is False and code inside is skipped.

What happens if we import this file in another Python file?

The __name__ variable will be the module's name (not "__main__"), so the condition is False and greet() is not called automatically (see concept_flow No branch).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of __name__ at Step 1?

A "__main__"

B "greet"

C "module"

D Undefined

💡 Hint

Check the __name__ value column in execution_table row 1

At which step does the program print "Hello from greet()"?

A Step 1

B Step 2

C Step 3

D It never prints

💡 Hint

Look at the Output column in execution_table row 2

If this file is imported, what happens to the condition __name__ == "__main__"?

A It becomes True and runs greet()

B It causes an error

C It becomes False and skips greet()

D It runs greet() twice

💡 Hint

Refer to key_moments explanation about import behavior

Concept Snapshot

Python sets __name__ to "__main__" when a file runs directly.
Use if __name__ == "__main__": to run code only when file is executed, not imported.
This helps separate reusable code from script code.
When imported, __name__ is the module name, so main code is skipped.

Full Transcript

When you run a Python file, Python sets a special variable called __name__ to "__main__". This means the file is the main program. If you import the file as a module, __name__ is set to the module's name instead. Using the condition if __name__ == "__main__": lets you write code that runs only when the file is run directly, not when imported. In the example, greet() runs only if the file is the main program. This helps keep code organized and reusable.

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