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PEP: 250
Title: Using site-packages on Windows
Version: 17a68e052d4f
Last-Modified:  2007-06-19 04:20:07 +0000 (Tue, 19 Jun 2007)
Author: Paul Moore <gustav at morpheus.demon.co.uk>
Status: Final
Type: Standards Track
Created: 30-Mar-2001
Python-Version: 2.2
Post-History: 30-Mar-2001

Abstract

    The standard Python distribution includes a directory
    Lib/site-packages, which is used on Unix platforms to hold
    locally-installed modules and packages.  The site.py module
    distributed with Python includes support for locating other
    modules in the site-packages directory.

    This PEP proposes that the site-packages directory should be used
    on the Windows platform in a similar manner.


Motivation

    On Windows platforms, the default setting for sys.path does not
    include a directory suitable for users to install locally
    developed modules.  The "expected" location appears to be the
    directory containing the Python executable itself.  This is also
    the location where distutils (and distutils-generated installers)
    installs packages.  Including locally developed code in the same
    directory as installed executables is not good practice.

    Clearly, users can manipulate sys.path, either in a locally
    modified site.py, or in a suitable sitecustomize.py, or even via
    .pth files.  However, there should be a standard location for such
    files, rather than relying on every individual site having to set
    their own policy.

    In addition, with distutils becoming more prevalent as a means of
    distributing modules, the need for a standard install location for
    distributed modules will become more common.  It would be better
    to define such a standard now, rather than later when more
    distutils-based packages exist which will need rebuilding.

    It is relevant to note that prior to Python 2.1, the site-packages
    directory was not included in sys.path for Macintosh platforms.
    This has been changed in 2.1, and Macintosh includes sys.path now,
    leaving Windows as the only major platform with no site-specific
    modules directory.


Implementation

    The implementation of this feature is fairly trivial.  All that
    would be required is a change to site.py, to change the section
    setting sitedirs.  The Python 2.1 version has

        if os.sep == '/':
            sitedirs = [makepath(prefix,
                                 "lib",
                                 "python" + sys.version[:3],
                                 "site-packages"),
                        makepath(prefix, "lib", "site-python")]
        elif os.sep == ':':
            sitedirs = [makepath(prefix, "lib", "site-packages")]
        else:
            sitedirs = [prefix]

    A suitable change would be to simply replace the last 4 lines with

        else:
            sitedirs == [prefix, makepath(prefix, "lib", "site-packages")]

    Changes would also be required to distutils, to reflect this change
    in policy. A patch is available on Sourceforge, patch ID 445744,
    which implements this change.  Note that the patch checks the Python
    version and only invokes the new behaviour for Python versions from
    2.2 onwards. This is to ensure that distutils remains compatible
    with earlier versions of Python.

    Finally, the executable code which implements the Windows installer
    used by the bdist_wininst command will need changing to use the new
    location.  A separate patch is available for this, currently
    maintained by Thomas Heller.


Notes

    - This change does not preclude packages using the current
      location -- the change only adds a directory to sys.path, it
      does not remove anything.

    - Both the current location (sys.prefix) and the new directory
      (site-packages) are included in sitedirs, so that .pth files
      will be recognised in either location.

    - This proposal adds a single additional site-packages directory
      to sitedirs. On Unix platforms, two directories are added, one
      for version-independent files (Python code) and one for
      version-dependent code (C extensions). This is necessary on
      Unix, as the sitedirs include a common (across Python versions)
      package location, in /usr/local by default. As there is no such
      common location available on Windows, there is also no need for
      having two separate package directories.

    - If users want to keep DLLs in a single location on Windows, rather
      than keeping them in the package directory, the DLLs subdirectory
      of the Python install directory is already available for that
      purpose. Adding an extra directory solely for DLLs should not be
      necessary.


Open Issues

    - Comments from Unix users indicate that there may be issues with
      the current setup on the Unix platform.  Rather than become
      involved in cross-platform issues, this PEP specifically limits
      itself to the Windows platform, leaving changes for other platforms
      to be covered inother PEPs.

    - There could be issues with applications which embed Python. To the
      author's knowledge, there should be no problem as a result of this
      change. There have been no comments (supportive or otherwise) from
      users who embed Python.


Copyright

    This document has been placed in the public domain.