Often, a Python program is run by naming a .py file on the command line:

$ python my_program.py

You can also create a directory or zipfile full of code, and include a __main__.py. Then you can simply name the directory or zipfile on the command line, and it executes the __main__.py automatically:

$ python my_program_dir$ python my_program.zip# Or, if the program is accessible as a module$ python -m my_program

You'll have to decide for yourself whether your application could benefit from being executed like this.

Note that a __main__ module usually doesn't come from a __main__.py file. It can, but it usually doesn't. When you run a script like python my_program.py, the script will run as the __main__ module instead of the my_program module. This also happens for modules run as python -m my_module, or in several other ways.

If you saw the name __main__ in an error message, that doesn't necessarily mean you should be looking for a __main__.py file.

What is the __main__.py file for?

When creating a Python module, it is common to make the module execute some functionality (usually contained in a main function) when run as the entry point of the program. This is typically done with the following common idiom placed at the bottom of most Python files:

if __name__ == '__main__':    # execute only if run as the entry point into the program    main()

You can get the same semantics for a Python package with __main__.py, which might have the following structure:

.└── demo    ├── __init__.py    └── __main__.py

To see this, paste the below into a Python 3 shell:

from pathlib import Pathdemo = Path.cwd() / 'demo'demo.mkdir()(demo / '__init__.py').write_text("""print('demo/__init__.py executed')def main():    print('main() executed')""")(demo / '__main__.py').write_text("""print('demo/__main__.py executed')from demo import mainmain()""")

We can treat demo as a package and actually import it, which executes the top-level code in the __init__.py (but not the main function):

>>> import demodemo/__init__.py executed

When we use the package as the entry point to the program, we perform the code in the __main__.py, which imports the __init__.py first:

$ python -m demodemo/__init__.py executeddemo/__main__.py executedmain() executed

You can derive this from the documentation. The documentation says:

__main__ — Top-level script environment

'__main__' is the name of the scope in which top-level code executes.A module’s __name__ is set equal to '__main__' when read from standardinput, a script, or from an interactive prompt.

A module can discover whether or not it is running in the main scopeby checking its own __name__, which allows a common idiom forconditionally executing code in a module when it is run as a script orwith python -m but not when it is imported:

if __name__ == '__main__':     # execute only if run as a script     main()

For a package, the same effect can be achieved by including a__main__.py module, the contents of which will be executed when the module is run with -m.

Zipped

You can also zip up this directory, including the __main__.py, into a single file and run it from the command line like this - but note that zipped packages can't execute sub-packages or submodules as the entry point:

from pathlib import Pathdemo = Path.cwd() / 'demo2'demo.mkdir()(demo / '__init__.py').write_text("""print('demo2/__init__.py executed')def main():    print('main() executed')""")(demo / '__main__.py').write_text("""print('demo2/__main__.py executed')from __init__ import mainmain()""")

Note the subtle change - we are importing main from __init__ instead of demo2 - this zipped directory is not being treated as a package, but as a directory of scripts. So it must be used without the -m flag.

Particularly relevant to the question - zipapp causes the zipped directory to execute the __main__.py by default - and it is executed first, before __init__.py:

$ python -m zipapp demo2 -o demo2zip$ python demo2zipdemo2/__main__.py executeddemo2/__init__.py executedmain() executed

Note again, this zipped directory is not a package - you cannot import it either.

__main__.py is used for python programs in zip files. The __main__.py file will be executed when the zip file in run. For example, if the zip file was as such:

test.zip     __main__.py

and the contents of __main__.py was

import sysprint "hello %s" % sys.argv[1]

Then if we were to run python test.zip world we would get hello world out.

So the __main__.py file run when python is called on a zip file.