New submission from Larry Hastings <la...@hastings.org>: The attached patch adds support for a new environment variable, PYTHONPREFIXES. PYTHONPREFIXES is similar to PYTHONUSERBASE: it lets you add "prefix directories" to be culled for site packages. It differs from PYTHONUSERBASE in three ways:
* PYTHONPREFIXES has an empty default value. PYTHONUSERBASE has a default, e.g. ~/.local on UNIX-like systems. * PYTHONPREFIXES supports multiple directories, separated by the site-specific directory separator character (os.pathsep). Earlier directories take precedence. PYTHONUSERBASE supports specifying at most one directory. * PYTHONPREFIXES adds its directories to site.PREFIXES, so it reuses the existing mechanisms for site package directories, exactly simulating a real prefix directory. PYTHONUSERBASE only adds a single directory, using its own custom code path. This last point bears further discussion. PYTHONUSERBASE's custom code to inspect only a single directory has resulted in at least one bug, if not more, as follows: * The bona-fide known bug: the Debian package mantainer for Python decided to change "site-packages" to "dist-packages" in 2.6, for reasons I still don't quite understand. He made this change in site.addsitepackages and distutils.sysconfig.get_python_lib, and similarly in setuptools, but he missed changing it in site.addusersitepackages. This meant that if you used setup.py to install a package to a private prefix directory, PYTHONUSERBASE had no hope of ever finding the package. (Happily this bug is fixed.) * I suspect there's a similar bug with PYTHONUSERBASE on the "os2emx" and "riscos" platforms. site.addsitepackages on those platforms looks in "{prefix}/Lib/site-packages", but site.addusersitepackages looks in "{prefix}/lib/python{version}/site-packages" as it does on any non-Windows platform. Presumably setup.py on those two platforms installs site packages to the directory site.addsitepackages inspects, which means that PYTHONUSERBASE doesn't work on those two platforms. PYTHONUSERBASE's custom code path to add site package directories is a source of unnecessary complexity and bugs. I cannot fathom why its implementors chose this approach; in any case I think reusing site.addsitepackages is a clear win. I suspect it's too late to change the semantics of PYTHONUSERBASE to simply call site.addsitepackages, though if that idea found support I'd be happy to contribute a patch. A few more notes on PYTHONPREFIXES: * PYTHONPREFIXES is gated by the exact same mechanisms that shut off PYTHONUSERBASE. * Specifying "-s" on the Python command line shuts it off. * Setting the environment variable PYTHONNOUSERSITE to a non-empty string shuts it off. * If the effective uid / gid doesn't match the actual uid / gid it automatically shuts off. * I'm not enormously happy with the name. Until about an hour or two ago I was calling it "PYTHONUSERBASES". I'm open to other suggestions. * I'm not sure that PYTHONPREFIX should literally modify site.PREFIXES. If that's a bad idea I'd be happy to amend the patch so it didn't touch site.PREFIXES. * Reaction in python-ideas has been reasonably positive, though I gather Nick Coughlan and Scott David Daniels think it's unnecessary. (To read the discussion, search for the old name: "PYTHONUSERBASES".) * Ben Finney prefers a counter-proposal he made in the python-ideas discussion: change the existing PYTHONUSERBASE to support multiple directories. I don't like this approach, because: a) it means you have to explicitly add the local default if you want to use it, and b) PYTHONUSERBASE only inspects one directory, whereas PYTHONPREFIX inspects all the directories Python might use for site packages. I do admit this approach would be preferable to no change at all. The attached patch is thrillingly simple and works fine. However it's not ready to be merged because I haven't touched the documentation. I figured I'd hold off until I see which way the wind blows. I'd also be happy to whip out a PEP if that's what is called for. ---------- files: lch.pythonprefixes.r71812.diff keywords: patch messages: 86352 nosy: lhastings severity: normal status: open title: Add PYTHONPREFIXES environment variable Added file: http://bugs.python.org/file13745/lch.pythonprefixes.r71812.diff _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue5819> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com