req.py 文件源码

def check_if_exists(self):
        """Find an installed distribution that satisfies or conflicts
        with this requirement, and set self.satisfied_by or
        self.conflicts_with appropriately."""

        if self.req is None:
            return False
        try:
            self.satisfied_by = pkg_resources.get_distribution(self.req)
        except pkg_resources.DistributionNotFound:
            return False
        except pkg_resources.VersionConflict:
            existing_dist = pkg_resources.get_distribution(self.req.project_name)
            if self.use_user_site:
                if dist_in_usersite(existing_dist):
                    self.conflicts_with = existing_dist
                elif running_under_virtualenv() and dist_in_site_packages(existing_dist):
                    raise InstallationError("Will not install to the user site because it will lack sys.path precedence to %s in %s"
                                            %(existing_dist.project_name, existing_dist.location))
            else:
                self.conflicts_with = existing_dist
        return True