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Python Enhancement Proposals

PEP 299 – Special __main__() function in modules

Jeff Epler <jepler at>
Standards Track

Table of Contents


Many Python modules are also intended to be callable as standalone scripts. This PEP proposes that a special function called __main__() should serve this purpose.


There should be one simple and universal idiom for invoking a module as a standalone script.

The semi-standard idiom:

if __name__ == '__main__':
    perform "standalone" functionality

is unclear to programmers of languages like C and C++. It also does not permit invocation of the standalone function when the module is imported. The variant:

if __name__ == '__main__':

is sometimes seen, but there exists no standard name for the function, and because arguments are taken from sys.argv it is not possible to pass specific arguments without changing the argument list seen by all other modules. (Imagine a threaded Python program, with two threads wishing to invoke the standalone functionality of different modules with different argument lists)


The standard name of the ‘main function’ should be __main__. When a module is invoked on the command line, such as:


then the module behaves as though the following lines existed at the end of the module (except that the attribute __sys may not be used or assumed to exist elsewhere in the script):

if globals().has_key("__main__"):
    import sys as __sys

Other modules may execute:

import mymodule mymodule.__main__(['mymodule', ...])

It is up to mymodule to document thread-safety issues or other issues which might restrict use of __main__. (Other issues might include use of mutually exclusive GUI modules, non-sharable resources like hardware devices, reassignment of sys.stdin/stdout, etc)


In modules/main.c, the block near line 385 (after the PyRun_AnyFileExFlags call) will be changed so that the above code (or its C equivalent) is executed.

Open Issues

  • Should the return value from __main__ be treated as the exit value?

    Yes. Many __main__ will naturally return None, which sys.exit translates into a “success” return code. In those that return a numeric result, it behaves just like the argument to sys.exit() or the return value from C’s main().

  • Should the argument list to __main__ include argv[0], or just the “real” arguments argv[1:]?

    argv[0] is included for symmetry with sys.argv and easy transition to the new standard idiom.


In a short discussion on python-dev [1], two major backwards compatibility problems were brought up and Guido pronounced that he doesn’t like the idea anyway as it’s “not worth the change (in docs, user habits, etc.) and there’s nothing particularly broken.”



Last modified: 2023-09-09 17:39:29 GMT