Introduce PEP 499 from Cameron Simpson

pep-0499.txt

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+PEP: 499
+Title: ``python -m foo`` should bind ``sys.modules['foo']`` in additon to ``sys.modules['__main__']``
+Version: $Revision$
+Last-Modified: $Date$
+Author: Cameron Simpson <cs@zip.com.au>
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 07-Aug-2015
+Python-Version: 3.6
+
+Abstract
+========
+
+When a module is used as a main program on the Python command line,
+such as by:
+
+    python -m module.name ...
+
+it is easy to accidentally end up with two independent instances
+of the module if that module is again imported within the program.
+This PEP proposes a way to fix this problem.
+
+When a module is invoked via Python's -m option the module is bound
+to ``sys.modules['__main__']`` and its ``.__name__`` attribute is set to
+``'__main__'``.
+This enables the standard "main program" boilerplate code at the
+bottom of many modules, such as::
+
+    if __name__ == '__main__':
+        sys.exit(main(sys.argv))
+
+However, when the above command line invocation is used it is a
+natural inference to presume that the module is actually imported
+under its official name ``module.name``,
+and therefore that if the program again imports that name
+then it will obtain the same module instance.
+
+That actuality is that the module was imported only as ``'__main__'``.
+Another import will obtain a distinct module instance, which can
+lead to confusing bugs.
+
+
+Proposal
+========
+
+It is suggested that to fix this situation all that is needed is a
+simple change to the way the ``-m`` option is implemented: in addition
+to binding the module object to ``sys.modules['__main__']``, it is also
+bound to ``sys.modules['module.name']``.
+
+Nick Coghlan has suggested that this is as simple as modifying the
+``runpy`` module's ``_run_module_as_main`` function as follows::
+
+    main_globals = sys.modules["__main__"].__dict__
+
+to instead be::
+
+    main_module = sys.modules["__main__"]
+    sys.modules[mod_spec.name] = main_module
+    main_globals = main_module.__dict__
+
+
+Considerations and Prerequisites
+================================
+
+Pickling Modules
+----------------
+
+Nick has mentioned `issue 19702`_ which proposes (quoted from the issue):
+
+- runpy will ensure that when __main__ is executed via the import
+  system, it will also be aliased in sys.modules as __spec__.name
+- if __main__.__spec__ is set, pickle will use __spec__.name rather
+  than __name__ to pickle classes, functions and methods defined in
+  __main__
+- multiprocessing is updated appropriately to skip creating __mp_main__
+  in child processes when __main__.__spec__ is set in the parent
+  process
+
+The first point above covers this PEP's specific proposal.
+
+
+Background
+==========
+
+`I tripped over this issue`_ while debugging a main program via a
+module which tried to monkey patch a named module, that being the
+main program module.  Naturally, the monkey patching was ineffective
+as it imported the main module by name and thus patched the second
+module instance, not the running module instance.
+
+However, the problem has been around as long as the ``-m`` command
+line option and is encountered regularly, if infrequently, by others.
+
+In addition to `issue 19702`_, the discrepancy around `__main__`
+is alluded to in PEP 451 and a similar proposal (predating PEP 451)
+is described in PEP 395 under `Fixing dual imports of the main module`_.
+
+
+References
+==========
+
+.. _issue 19702: http://bugs.python.org/issue19702
+
+.. _I tripped over this issue: https://mail.python.org/pipermail/python-list/2015-August/694905.html
+
+.. _Fixing dual imports of the main module: https://www.python.org/dev/peps/pep-0395/#fixing-dual-imports-of-the-main-module
+
+
+Copyright
+=========
+
+This document has been placed in the public domain.
+
+
+..
+   Local Variables:
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+   coding: utf-8
+   End: