What is __main__.py?
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
$ 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
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
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
$ 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 -mbut 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__.pymodule, the contents of which will be executed when the module is run with
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
__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
Particularly relevant to the question -
zipapp causes the zipped directory to execute the
__main__.py by default - and it is executed first, before
$ 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:
and the contents of
import sysprint "hello %s" % sys.argv
Then if we were to run
python test.zip world we would get
hello world out.
__main__.py file run when python is called on a zip file.