"__main__" --- Top-level code environment
*****************************************

======================================================================

In Python, the special name "__main__" is used for two important
constructs:

1. the name of the top-level environment of the program, which can be
   checked using the "__name__ == '__main__'" expression; and

2. the "__main__.py" file in Python packages.

Both of these mechanisms are related to Python modules; how users
interact with them and how they interact with each other.  They are
explained in detail below.  If you're new to Python modules, see the
tutorial section Modules for an introduction.


"__name__ == '__main__'"
========================

When a Python module or package is imported, "__name__" is set to the
module's name.  Usually, this is the name of the Python file itself
without the ".py" extension:

   >>> import configparser
   >>> configparser.__name__
   'configparser'

If the file is part of a package, "__name__" will also include the
parent package's path:

   >>> from concurrent.futures import process
   >>> process.__name__
   'concurrent.futures.process'

However, if the module is executed in the top-level code environment,
its "__name__" is set to the string "'__main__'".


What is the "top-level code environment"?
-----------------------------------------

"__main__" is the name of the environment where top-level code is run.
"Top-level code" is the first user-specified Python module that starts
running. It's "top-level" because it imports all other modules that
the program needs. Sometimes "top-level code" is called an *entry
point* to the application.

The top-level code environment can be:

* the scope of an interactive prompt:

     >>> __name__
     '__main__'

* the Python module passed to the Python interpreter as a file
  argument:

        $ python3 helloworld.py
        Hello, world!

* the Python module or package passed to the Python interpreter with
  the "-m" argument:

        $ python3 -m tarfile
        usage: tarfile.py [-h] [-v] (...)

* Python code read by the Python interpreter from standard input:

        $ echo "import this" | python3
        The Zen of Python, by Tim Peters

        Beautiful is better than ugly.
        Explicit is better than implicit.
        ...

* Python code passed to the Python interpreter with the "-c" argument:

        $ python3 -c "import this"
        The Zen of Python, by Tim Peters

        Beautiful is better than ugly.
        Explicit is better than implicit.
        ...

In each of these situations, the top-level module's "__name__" is set
to "'__main__'".

As a result, a module can discover whether or not it is running in the
top-level environment by checking its own "__name__", which allows a
common idiom for conditionally executing code when the module is not
initialized from an import statement:

   if __name__ == '__main__':
       # Execute when the module is not initialized from an import statement.
       ...

See also:

  For a more detailed look at how "__name__" is set in all situations,
  see the tutorial section Modules.


Idiomatic Usage
---------------

Some modules contain code that is intended for script use only, like
parsing command-line arguments or fetching data from standard input.
If a module like this was imported from a different module, for
example to unit test it, the script code would unintentionally execute
as well.

This is where using the "if __name__ == '__main__'" code block comes
in handy. Code within this block won't run unless the module is
executed in the top-level environment.

Putting as few statements as possible in the block below "if __name___
== '__main__'" can improve code clarity and correctness. Most often, a
function named "main" encapsulates the program's primary behavior:

   # echo.py

   import shlex
   import sys

   def echo(phrase: str) -> None:
      """A dummy wrapper around print."""
      # for demonstration purposes, you can imagine that there is some
      # valuable and reusable logic inside this function
      print(phrase)

   def main() -> int:
       """Echo the input arguments to standard output"""
       phrase = shlex.join(sys.argv)
       echo(phrase)
       return 0

   if __name__ == '__main__':
       sys.exit(main())  # next section explains the use of sys.exit

Note that if the module didn't encapsulate code inside the "main"
function but instead put it directly within the "if __name__ ==
'__main__'" block, the "phrase" variable would be global to the entire
module.  This is error-prone as other functions within the module
could be unintentionally using the global variable instead of a local
name.  A "main" function solves this problem.

Using a "main" function has the added benefit of the "echo" function
itself being isolated and importable elsewhere. When "echo.py" is
imported, the "echo" and "main" functions will be defined, but neither
of them will be called, because "__name__ != '__main__'".


Packaging Considerations
------------------------

"main" functions are often used to create command-line tools by
specifying them as entry points for console scripts.  When this is
done, pip inserts the function call into a template script, where the
return value of "main" is passed into "sys.exit()". For example:

   sys.exit(main())

Since the call to "main" is wrapped in "sys.exit()", the expectation
is that your function will return some value acceptable as an input to
"sys.exit()"; typically, an integer or "None" (which is implicitly
returned if your function does not have a return statement).

By proactively following this convention ourselves, our module will
have the same behavior when run directly (i.e. "python3 echo.py") as
it will have if we later package it as a console script entry-point in
a pip-installable package.

In particular, be careful about returning strings from your "main"
function. "sys.exit()" will interpret a string argument as a failure
message, so your program will have an exit code of "1", indicating
failure, and the string will be written to "sys.stderr".  The
"echo.py" example from earlier exemplifies using the
"sys.exit(main())" convention.

See also:

  Python Packaging User Guide contains a collection of tutorials and
  references on how to distribute and install Python packages with
  modern tools.


"__main__.py" in Python Packages
================================

If you are not familiar with Python packages, see section Packages of
the tutorial.  Most commonly, the "__main__.py" file is used to
provide a command-line interface for a package. Consider the following
hypothetical package, "bandclass":

   bandclass
     ├── __init__.py
     ├── __main__.py
     └── student.py

"__main__.py" will be executed when the package itself is invoked
directly from the command line using the "-m" flag. For example:

   $ python3 -m bandclass

This command will cause "__main__.py" to run. How you utilize this
mechanism will depend on the nature of the package you are writing,
but in this hypothetical case, it might make sense to allow the
teacher to search for students:

   # bandclass/__main__.py

   import sys
   from .student import search_students

   student_name = sys.argv[2] if len(sys.argv) >= 2 else ''
   print(f'Found student: {search_students(student_name)}')

Note that "from .student import search_students" is an example of a
relative import.  This import style can be used when referencing
modules within a package.  For more details, see Intra-package
References in the Modules section of the tutorial.


Idiomatic Usage
---------------

The contents of "__main__.py" typically isn't fenced with "if __name__
== '__main__'" blocks.  Instead, those files are kept short, functions
to execute from other modules.  Those other modules can then be easily
unit-tested and are properly reusable.

If used, an "if __name__ == '__main__'" block will still work as
expected for a "__main__.py" file within a package, because its
"__name__" attribute will include the package's path if imported:

   >>> import asyncio.__main__
   >>> asyncio.__main__.__name__
   'asyncio.__main__'

This won't work for "__main__.py" files in the root directory of a
.zip file though.  Hence, for consistency, minimal "__main__.py" like
the "venv" one mentioned above are preferred.

See also:

  See "venv" for an example of a package with a minimal "__main__.py"
  in the standard library. It doesn't contain a "if __name__ ==
  '__main__'" block. You can invoke it with "python3 -m venv
  [directory]".

  See "runpy" for more details on the "-m" flag to the interpreter
  executable.

  See "zipapp" for how to run applications packaged as *.zip* files.
  In this case Python looks for a "__main__.py" file in the root
  directory of the archive.


"import __main__"
=================

Regardless of which module a Python program was started with, other
modules running within that same program can import the top-level
environment's scope (*namespace*) by importing the "__main__" module.
This doesn't import a "__main__.py" file but rather whichever module
that received the special name "'__main__'".

Here is an example module that consumes the "__main__" namespace:

   # namely.py

   import __main__

   def did_user_define_their_name():
       return 'my_name' in dir(__main__)

   def print_user_name():
       if not did_user_define_their_name():
           raise ValueError('Define the variable `my_name`!')

       if '__file__' in dir(__main__):
           print(__main__.my_name, "found in file", __main__.__file__)
       else:
           print(__main__.my_name)

Example usage of this module could be as follows:

   # start.py

   import sys

   from namely import print_user_name

   # my_name = "Dinsdale"

   def main():
       try:
           print_user_name()
       except ValueError as ve:
           return str(ve)

   if __name__ == "__main__":
       sys.exit(main())

Now, if we started our program, the result would look like this:

   $ python3 start.py
   Define the variable `my_name`!

The exit code of the program would be 1, indicating an error.
Uncommenting the line with "my_name = "Dinsdale"" fixes the program
and now it exits with status code 0, indicating success:

   $ python3 start.py
   Dinsdale found in file /path/to/start.py

Note that importing "__main__" doesn't cause any issues with
unintentionally running top-level code meant for script use which is
put in the "if __name__ == "__main__"" block of the "start" module.
Why does this work?

Python inserts an empty "__main__" module in "sys.modules" at
interpreter startup, and populates it by running top-level code. In
our example this is the "start" module which runs line by line and
imports "namely". In turn, "namely" imports "__main__" (which is
really "start"). That's an import cycle! Fortunately, since the
partially populated "__main__" module is present in "sys.modules",
Python passes that to "namely". See Special considerations for
__main__ in the import system's reference for details on how this
works.

The Python REPL is another example of a "top-level environment", so
anything defined in the REPL becomes part of the "__main__" scope:

   >>> import namely
   >>> namely.did_user_define_their_name()
   False
   >>> namely.print_user_name()
   Traceback (most recent call last):
   ...
   ValueError: Define the variable `my_name`!
   >>> my_name = 'Jabberwocky'
   >>> namely.did_user_define_their_name()
   True
   >>> namely.print_user_name()
   Jabberwocky

Note that in this case the "__main__" scope doesn't contain a
"__file__" attribute as it's interactive.

The "__main__" scope is used in the implementation of "pdb" and
"rlcompleter".
