def best_match(self, req, working_set, installer=None):
        dist = working_set.find(req)
        if dist is not None:
            return dist
        for dist in self[req.key]:
            if dist in req:
                return dist
        # try to download/install
        return self.obtain(req, installer)
Find distribution best matching `req` and usable on `working_set`
dataset/ETHPy150Open anzev/hedwig/build/pip/pip/_vendor/pkg_resources/__init__.py/Environment.best_match