def find(self, req):
        """Find a distribution matching requirement `req`

        If there is an active distribution for the requested project, this
        returns it as long as it meets the version requirement specified by
        `req`.  But, if there is an active distribution for the project and it
        does *not* meet the `req` requirement, ``VersionConflict`` is raised.
        If there is no active distribution for the requested project, ``None``
        is returned.
        """
        dist = self.by_key.get(req.key)
        if dist is not None and dist not in req:
            # XXX add more info
            raise VersionConflict(dist, req)
        return dist
Find a distribution matching requirement `req`

        If there is an active distribution for the requested project, this
        returns it as long as it meets the version requirement specified by
        `req`.  But, if there is an active distribution for the project and it
        does *not* meet the `req` requirement, ``VersionConflict`` is raised.
        If there is no active distribution for the requested project, ``None``
        is returned.
        
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_vendor/pkg_resources/__init__.py