[Distutils] Unexpected VersionConflict

[Townsend, Scott E. (GRC-RTM0)[Vantage Partners, LLC]]

During easy_install of an egg where two versions of pyparsing were available (1.5.2 and 1.5.6), a VersionConflict was raised:

pkg_resources.VersionConflict: (pyparsing 1.5.6 (/usr/lib/python2.7/dist-packages), Requirement.parse('pyparsing==1.5.2'))

This was unexpected since sys.path (via virtualenv) has version 1.5.2 before 1.5.6.  And the system gets 1.5.2 from 'import pyparsing', not 1.5.6.

I've traced this to the line calling _sort_dists(dists), line 801 in my copy of pkg_resources.py:

    def __getitem__(self,project_name):
        """Return a newest-to-oldest list of distributions for `project_name`
        """
        try:
            return self._cache[project_name]
        except KeyError:
            project_name = project_name.lower()
            if project_name not in self._distmap:
                return []

        if project_name not in self._cache:
            dists = self._cache[project_name] = self._distmap[project_name]
            _sort_dists(dists)

        return self._cache[project_name]

The problem is that one dependent package of the egg has a requirement of 'pyparsing' while a subsequent dependent package has a requirement of 'pyparsing==1.5.2'.  The intent was that by using virtualenv with a correct sys.path version 1.5.2 would be used for both requirements.  Unfortunately, because of the call to _sort_dists(), the 'pyparsing' requirement is resolved to 1.5.6 by env.best_match() in WorkingSet.resolve().  Once that resolution was made, the more explicit requirement fails.

Note that without the _sort_dists() call the egg loads and runs correctly, using pyparsing 1.5.2.

Its not clear to me that removing the _sort_dists() call is correct in general, but it appears to be a bug that an egg which would load and run correctly reports a VersionConflict.

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