__main__ — Top-level code environment¶

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.
    ...

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.

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 below are preferred.