pkg_resources.py 文件源码

def best_match(self, req, working_set, installer=None):
        """Find distribution best matching `req` and usable on `working_set`

        This calls the ``find(req)`` method of the `working_set` to see if a
        suitable distribution is already active.  (This may raise
        ``VersionConflict`` if an unsuitable version of the project is already
        active in the specified `working_set`.)  If a suitable distribution
        isn't active, this method returns the newest distribution in the
        environment that meets the ``Requirement`` in `req`.  If no suitable
        distribution is found, and `installer` is supplied, then the result of
        calling the environment's ``obtain(req, installer)`` method will be
        returned.
        """
        dist = working_set.find(req)
        if dist is not None:
            return dist
        for dist in self[req.key]:
            if dist in req:
                return dist
        return self.obtain(req, installer) # try and download/install