[Python-checkins] python/nondist/peps pep-0299.txt,NONE,1.1

2002年8月26日 11:11:51 -0700

Update of /cvsroot/python/python/nondist/peps
In directory usw-pr-cvs1:/tmp/cvs-serv26432
Added Files:
	pep-0299.txt 
Log Message:
PEP 299, Special __main__() function in modules, Jeff Epler
--- NEW FILE: pep-0299.txt ---
PEP: 299
Title: Special __main__() function in modules
Version: $Revision: 1.1 $
Last-Modified: $Date: 2002年08月26日 18:11:49 $
Author: Jeff Epler <jepler@unpythonic.net>
Status: Draft
Type: Standards Track
Created: 12-Aug-2002
Python-version: 2.3
Post-History:
Abstract
 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.
Motivation
 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__':
 main_function()
 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)
Proposal
 The standard name of the 'main function' should be '__main__'.
 When a module is invoked on the command line, such as
 python mymodule.py
 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
 __sys.exit(__main__(__sys.argv))
 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)
Implementation
 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.
Copyright
 This document has been placed in the public domain.

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