Testing Python Code

The __name__ attribute tells you whether a module is being run directly (__main__) or imported (module name).

Programming

The Two Ways a File Runs

A Python file can be used in two different ways: a user can run it directly as a program, or another file can import it as a module. Python uses the special __name__ attribute to distinguish these situations. That distinction lets you keep reusable code available for import while preventing test or demonstration code from running unexpectedly.

What do you think happens?

Suppose a file named module_using_name is run directly and then imported by another script. What value does __name__ have in each situation?

__main__ when run directly; module_using_name when imported
module_using_name in both situations
__main__ in both situations

Reveal answer

Answer: __main__ when run directly; module_using_name when imported

Python sets __name__ to __main__ for the file the user runs directly. When another script imports that file, Python sets __name__ to the module's name instead.

Reading the name Value

The __name__ attribute tells your code how the current module is being used. When the user runs the file directly, Python gives that file the value __main__. When another script imports the file, Python gives it the module's name. Your code can check this value and choose whether to execute a particular block.

What value does __name__ contain when the same file is run directly versus imported?

The two underscores on each side are part of the attribute name. Write __name__ exactly, and compare it with the exact string '__main__'.

The Guard as a Gatekeeper

if __name__ == '__main__': print('Standalone test code runs here')

The if __name__ == '__main__': statement acts as a gatekeeper. It allows test or demonstration code through only when the file is being executed directly. When the file is imported, the condition is false, so that protected code does not run automatically.

Which path does the file follow when it runs directly, and which path does it follow when it is imported?

A Calculator Module

Imagine a calculator module containing add and multiply functions. Other programmers should be able to import those functions. The same file can also contain tests that check the functions when the file is run directly. The guard separates these two purposes.

python

def add(first, second):
    return first + second


def multiply(first, second):
    return first * second


if __name__ == '__main__':
    print(add(2, 3))
    print(multiply(2, 3))

def add(first, second):
    return first + second


def multiply(first, second):
    return first * second


if __name__ == '__main__':
    print(add(2, 3))
    print(multiply(2, 3))

Tracing the calculator module

Determine what happens to the test output when the calculator file is run directly and when another script imports it.

Direct execution: Python sets __name__ to '__main__'. The condition is true, so the two print statements inside the guarded block execute and display the test results.

Importing: Python sets __name__ to the calculator module's name rather than '__main__'. The condition is false, so the two test print statements are skipped.

Reusable code: The add and multiply functions remain available for another programmer to use when the module is imported, without the test output running automatically.

Direct execution runs the test code; importing provides the functions without running the guarded test output.

Output

5
6

What Runs in Each Context

Code or value	File run directly	File imported
__name__	__main__	The module's name
Code inside the guard	Executes	Is skipped
Reusable functions	Can be used	Available for the importing program
Test or demonstration output	Runs when placed inside the guard	Does not run when placed inside the guard

Which part remains usable in both contexts, and which part is restricted to direct execution?

Mistakes with the Guard

Using the wrong attribute spelling
The pattern depends on the special __name__ attribute, including both pairs of double underscores.
Fix: Write __name__ exactly.
Comparing with the wrong string
The direct-execution value is the exact string '__main__', including the double underscores.
Fix: Compare __name__ to '__main__'.
Putting test output outside the guard
The test print statement is not protected by the guard, so it is not separated from the reusable module code.
Fix: Place standalone test or demonstration statements inside the if __name__ == '__main__': block.

Use the __name__ pattern when a file is designed to work both as a standalone program and as an importable module. It communicates that the file has reusable code while keeping its test or demonstration behavior controlled.

Apply the Pattern

EASY

Write a small module containing a function named add and a test print statement. Put the test statement inside an if __name__ == '__main__': guard. Then describe what happens when the file is run directly and what happens when another script imports it.

Hints

Use the exact attribute name __name__.
Compare it with the exact string '__main__'.
The test statement belongs inside the indented guarded block.

Your attempt (not saved)

Python sets __name__ to '__main__' for the file the user runs directly. For an imported file, Python sets __name__ to the module's name. The if __name__ == '__main__': pattern checks that distinction and protects standalone test or demonstration code. As a result, reusable functions can be imported without automatically running the module's test output.

Key Takeaways

The __name__ attribute identifies whether a file is being run directly or imported.
A directly run file has __name__ equal to '__main__'.
An imported file has __name__ equal to its module name.
The if __name__ == '__main__': guard runs its block only during direct execution.
The guard separates reusable module code from standalone test or demonstration code.

The Two Ways a File Runs

Reading the __name__ Value

The Guard as a Gatekeeper

A Calculator Module

Tracing the calculator module

What Runs in Each Context

Mistakes with the Guard

Apply the Pattern

Key Takeaways

Related Topics

Reading the name Value