Special Variable Name

▶

What Is __name__?

__name__ is a special built-in variable in Python that stores the name of the current module. Its value changes depending on how the Python file is executed.

• If a file is run directly, __name__ is set to "__main__"
• If a file is imported as a module, __name__ is set to the module’s filename (without .py)

This behavior allows Python programs to distinguish between direct execution and import usage.

Why Is __name__ Important?

When a Python file is imported:

• All top-level statements execute immediately.
• This can cause unexpected output or side effects.

To avoid this, Python provides the __name__ check to control what code runs in different contexts.

__name__ and globals()

print(globals())
print(__name__)

• globals() displays all global variables in the current module
• __name__ appears as part of the global namespace
• Helps in debugging and understanding runtime context

The if __name__ == "__main__": Guard

This construct ensures that certain code runs only when the file is executed directly, not when imported.

if __name__ == "__main__":
    # code runs only when file is executed directly

Key Benefits:

• Prevents unwanted execution during imports
• Keeps modules clean and reusable
• Separates test/debug code from reusable logic

Example

calc.py

print("In calc", __name__)

demo.py

import calc
print("In demo", __name__)

Output:

$ python calc.py
In calc __main__

$ python demo.py
In calc calc
In demo __main__

Explanation:

• calc.py run directly → __name__ == "__main__"
• calc.py imported → __name__ == "calc"
• demo.py is the entry point → __name__ == "__main__"

Practical Use Case: Protecting Test Code

calc.py

def add():
    return "addition"

def sub():
    return "subtraction"

if __name__ == "__main__":
    print(add())
    print(sub())

demo.py

from calc import *

result = add()
print("In demo", result)

Output:

$ python calc.py
addition
subtraction

$ python demo.py
In demo addition

Explanation:

• Test code runs only when calc.py is executed directly
• Importing calc does not trigger test output

Summary

• __name__ identifies the execution context of a Python module at runtime.
• A value of "__main__" indicates that the script is being executed directly as the program’s entry point.
• Helps prevent unintended code execution when modules are imported into other programs.
• Promotes clean separation between reusable logic and test or execution-specific code.
• Enables writing modular, reusable, and maintainable Python applications following best practices.