[Python-Dev] PEP 338: Executing modules inside packages with '-m'

Nick Coghlan ncoghlan at iinet.net.au
Sun Dec 12 03:20:01 CET 2004 

Python 2.4's -m command line switch only works for modules directly on sys.path.
Trying to use it with modules inside packages will fail with a "Module not
found" error. This PEP aims to fix that for Python 2.5.

Previously, posting of a draft version of the PEP to python-dev and python-list
didn't actually generate any responses. I'm not sure if that's an indication
that people don't see the restriction to top-level modules as a problem (and
hence think the PEP is unecessary), or think the extension to handle packages is
obvious (and hence see no need to comment).

For c.l.p, it could just be a sign that Python 2.4 hasn't been out long enough 
for anyone to care what I'm yabbering on about :)

Anyway, all comments are appreciated (even a simple "Sounds good to me").

Cheers,
Nick.

***********************************************************************
PEP: 338
Title: Executing modules inside packages with '-m'
Version: $Revision: 1.2 $
Last-Modified: $Date: 2004/12/11 20:31:10 $
Author: Nick Coghlan <ncoghlan at email.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 16-Oct-2004
Python-Version: 2.5
Post-History: 8-Nov-2004


Abstract
========

This PEP defines semantics for executing modules inside packages as
scripts with the ``-m`` command line switch.

The proposed semantics are that the containing package be imported
prior to execution of the script.


Rationale
=========

Python 2.4 adds the command line switch ``-m`` to allow modules to be
located using the Python module namespace for execution as scripts.
The motivating examples were standard library modules such as ``pdb``
and ``profile``.

A number of users and developers have requested extension of the
feature to also support running modules located inside packages.  One
example provided is pychecker's ``pychecker.checker`` module.  This
capability was left out of the Python 2.4 implementation because the
appropriate semantics were not entirely clear.

The opinion on python-dev was that it was better to postpone the
extension to Python 2.5, and go through the PEP process to help make
sure we got it right.


Scope of this proposal
==========================

In Python 2.4, a module located using ``-m`` is executed just as if
its filename had been provided on the command line.  The goal of this
PEP is to get as close as possible to making that statement also hold
true for modules inside packages.

Prior discussions suggest it should be noted that this PEP is **not**
about any of the following:

- changing the idiom for making Python modules also useful as scripts
   (see PEP 299 [1]_).

- lifting the restriction of ``-m`` to modules of type PY_SOURCE or
   PY_COMPILED (i.e. ``.py``, ``.pyc``, ``.pyo``, ``.pyw``).

- addressing the problem of ``-m`` not understanding zip imports or
   Python's sys.metapath.

The issues listed above are considered orthogonal to the specific
feature addressed by this PEP.


Current Behaviour
=================

Before describing the new semantics, it's worth covering the existing
semantics for Python 2.4 (as they are currently defined only by the
source code).

When ``-m`` is used on the command line, it immediately terminates the
option list (like ``-c``).  The argument is interpreted as the name of
a top-level Python module (i.e. one which can be found on
``sys.path``).

If the module is found, and is of type ``PY_SOURCE`` or
``PY_COMPILED``, then the command line is effectively reinterpreted
from ``python <options> -m <module> <args>`` to ``python <options>
<filename> <args>``.  This includes setting ``sys.argv[0]`` correctly
(some scripts rely on this - Python's own ``regrtest.py`` is one
example).

If the module is not found, or is not of the correct type, an error
is printed.


Proposed Semantics
==================

The semantics proposed are fairly simple: if ``-m`` is used to execute
a module inside a package as a script, then the containing package is
imported before executing the module in accordance with the semantics
for a top-level module.

This is necessary due to the way Python's import machinery locates
modules inside packages.  A package may modify its own __path__
variable during initialisation.  In addition, paths may affected by
``*.pth`` files.  Accordingly, the only way for Python to reliably
locate the module is by importing the containing package and
inspecting its __path__ variable.

Note that the package is *not* imported into the ``__main__`` module's
namespace.  The effects of these semantics that will be visible to the
executed module are:

- the containing package will be in sys.modules

- any external effects of the package initialisation (e.g. installed
   import hooks, loggers, atexit handlers, etc.)


Reference Implementation
========================

A reference implementation is available on SourceForge [2]_.  In this
implementation, if the ``-m`` switch fails to locate the requested
module at the top level, it effectively reinterprets the command from
``python -m <script>`` to ``python -m execmodule <script>``.  (There
is one caveat: when reinterpreted in this way, ``sys.argv[0]`` may not
actually contain the filename of ``execmodule``.  This only affects
``execmodule`` itself, not the requested module).

``execmodule`` is a proposed standard library module that contains a
single function (also called ``execmodule``).  When invoked as a
script, this module finds and executes the module supplied as the
first argument.  It adjusts ``sys.argv`` by deleting ``sys.argv[0]``
and replacing the new ``sys.argv[0]`` with the module's filename
instead of its Python name.

The function ``execmodule`` is like ``execfile``, but uses the Python
module namespace to locate the script instead of the filesystem.  It
has an additional optional argument ``set_argv0`` which causes the
filename of the located module to be written to ``sys.argv[0]`` before
the module is executed.

A hybrid C/Python implementation is used as the Python module is much
more flexible and extensible than the equivalent C code would be.  It
also allows the ``execmodule`` function to be made available.  Scripts
which execute other scripts (e.g. ``profile``, ``pdb``) have the
option to use this function to provide ``-m`` style support for
identifying the script to be executed.

The Python code for ``execmodule`` has also been posted as a
cookbook recipe for Python 2.4 [3]_.


Open Issues
===========

- choosing a name for the standard library module containing
   ``execmodule``.  The reference implementation uses ``execmodule``.
   An alternative name proposed on python-dev is ``runpy``.


Alternatives
============

The main alternative implementation considered ignored packages'
__path__ variables, and looked only in the main package directory.  A
Python script with this behaviour can be found in the discussion of
the ``execmodule`` cookbook recipe [3]_.

This approach was not used as it does not meet the main goal of the
``-m`` switch -- to allow the full Python namespace to be used to
locate modules for execution.


References
==========

.. [1] Special __main__() function in modules
    (http://www.python.org/peps/pep-0299.html)

.. [2] Native ``-m`` execmodule support

(http://sourceforge.net/tracker/?func=detail&aid=1043356&group_id=5470&atid=305470 )

.. [3] execmodule Python Cookbook Recipe
    (http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/307772)


Copyright
=========

This document has been placed in the public domain.

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-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
---------------------------------------------------------------
             http://boredomandlaziness.skystorm.net

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