__name__ == “__main__”

Python files by default define modules. If you write bare code—like a print in __init__.py or main.py in the previous example—it will run every time the module runs. That isn’t always a good thing! Sometimes we want to define a module but not run the code in it. (For example, doctest and unittest can both be run as code or simply imported as modules.) We need a way to differentiate between the two situations: do we want to load the module definitions and run the code, or do we only want to load the module definitions?

To differentiate, Python offers the __name__ facility. __name__ is a global variable that tells you the name of the current module. If your code is being loaded via import, then __name__ will be the module name (e.g., algorithms.sort). If your code is being run via python or python -m, then __name__ will be __main__.

Here’s an example: suppose we have a file task.py.

# this is task.py

print(f'Hello, my __name__ is {__name__}.')

Here’s what we get when we invoke task as the main module:

$ python task.py
Hello, my __name__ is __main__.
$ python -m task
Hello, my __name__ is __main__.

And here’s what happens when we import task in a Python interpreter:

$ python
Python 3.9.7 (default, Oct 19 2021, 17:23:38)
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import task
Hello, my __name__ is task.

You can read more about __main__ in the Python docs.

Conditional behavior

It’s very common for a Python module to offer a bunch of definitions as well as useful “main program” behavior. We can have the program offer both by conditioning on __name__, as follows:

# definitions here

if __name__ == '__main__':
    # main program code here