107 lines
3.0 KiB
Plaintext
107 lines
3.0 KiB
Plaintext
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PEP: 299
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Title: Special __main__() function in modules
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Version: $Revision$
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Last-Modified: $Date$
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Author: Jeff Epler <jepler@unpythonic.net>
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Status: Draft
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Type: Standards Track
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Created: 12-Aug-2002
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Python-version: 2.3
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Post-History:
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Abstract
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Many Python modules are also intended to be callable as standalone
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scripts. This PEP proposes that a special function called
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__main__() should serve this purpose.
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Motivation
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There should be one simple and universal idiom for invoking a
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module as a standalone script.
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The semi-standard idiom
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if __name__ == '__main__':
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perform "standalone" functionality
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is unclear to programmers of languages like C and C++. It also
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does not permit invocation of the standalone function when the
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module is imported. The variant
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if __name__ == '__main__':
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main_function()
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is sometimes seen, but there exists no standard name for the
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function, and because arguments are taken from sys.argv it is not
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possible to pass specific arguments without changing the argument
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list seen by all other modules. (Imagine a threaded Python
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program, with two threads wishing to invoke the standalone
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functionality of different modules with different argument lists)
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Proposal
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The standard name of the 'main function' should be '__main__'.
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When a module is invoked on the command line, such as
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python mymodule.py
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then the module behaves as though the following lines existed at
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the end of the module (except that the attribute __sys may not be
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used or assumed to exist elsewhere in the script):
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if globals().has_key("__main__"):
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import sys as __sys
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__sys.exit(__main__(__sys.argv))
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Other modules may execute
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import mymodule
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mymodule.__main__(['mymodule', ...])
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It is up to mymodule to document thread-safety issues or other
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issues which might restrict use of __main__. (Other issues might
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include use of mutually exclusive GUI modules, non-sharable
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resources like hardware devices, reassignment of sys.stdin/stdout,
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etc)
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Implementation
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In modules/main.c, the block near line 385 (after the
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PyRun_AnyFileExFlags call) will be changed so that the above code
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(or its C equivalent) is executed.
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Open Issues
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- Should the return value from __main__ be treated as the exit value?
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Yes. Many __main__ will naturally return None, which sys.exit
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translates into a "success" return code. In those that return a
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numeric result, it behaves just like the argument to sys.exit()
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or the return value from C's main().
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- Should the argument list to __main__ include argv[0], or just the
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"real" arguments argv[1:]?
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argv[0] is included for symmetry with sys.argv and easy
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transition to the new standard idiom.
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Copyright
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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End:
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