Execute Python modules and compressed modules

Why?

It was somehow mysterius to me how commands like below wotk?

How does Python interpreter know where to find executable code?

  echo '{"json":"obj"}' | python3 -m json.tool
  python3 -m http.server --directory /tmp/http --bind 127.0.0.1 8001
  python3 -m zipfile -c /tmp/zip.zip /etc/fstab
  python3 -m venv /tmp/venv
  python3 -m unittest

So after a little bit of googling I found PEP-0441 which says:

Python has had the ability to execute directories or ZIP-format archives as scripts since version 2.6 [1]. When invoked with a zip file or directory as its first argument the interpreter adds that directory to sys.path and executes the main module.

How?

Executable module

Let’s start with python -m module.name option. Official Python3 doc says:

Searches sys.path for the named module and runs the corresponding .py file as a script.

Unfortunately that doc sentenence doesn’t specify what is that “corresponding .py file”. And as it tunedned out it needs to be __main__.py file, which is an module entry point. So here is the example of empty module which can be called as executable.

Two files in module directory:

$ ls -l module/
total 12
-rw-rw-r--. 1 radek radek  65 Jul 19 16:06 __init__.py
-rw-rw-r--. 1 radek radek  96 Jul 19 16:05 __main__.py

And files content as follow:

# file: module/__init__.py
print("I am package {} from file {}".format(__name__, __file__))

# file: module/__main__.py
if __name__ == "__main__":
    print("I am package {} from file {}".format(__name__, __file__))

So now, let’s execute our new module:

$ python -m module
I am package module from file module/__init__.py
I am package __main__ from file /tmp/module/__main__.py

Compressed executable module

This may be a little bit similar to Java .jar files and should make modules distribution easier.

Let’s compress our module, I am going to use zipfile module but it can be zip in any other way.

$ python3 -m zipfile -c  module.zip module
$ python3 -m zipfile --list compressed_module.zip
File Name                                             Modified             Size
__init__.py                                    2019-07-19 16:06:20           65
__main__.py                                    2019-07-19 16:05:20           96

Now we can execute that compressed module:

$ python3 compressed_module.zip
I am package __main__ from file compressed_module.zip/__main__.py

And it works!