What is __main__.py?

Question

What is the __main__.py file for, what sort of code should I put into it and when should I have one at all? :)

Answer 1

__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 sys
print "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.

Answer 2

If your script is a directory or ZIP file rather than a single python file, __main__.py will be executed when the "script" is passed as an argument to the python interpreter.

Answer 3

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

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