__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.