req_install.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:
            # get_distribution() will resolve the entire list of requirements
            # anyway, and we've already determined that we need the requirement
            # in question, so strip the marker so that we don't try to
            # evaluate it.
            no_marker = Requirement(str(self.req))
            no_marker.marker = None
            self.satisfied_by = pkg_resources.get_distribution(str(no_marker))
            if self.editable and self.satisfied_by:
                self.conflicts_with = self.satisfied_by
                # when installing editables, nothing pre-existing should ever
                # satisfy
                self.satisfied_by = None
                return True
        except pkg_resources.DistributionNotFound:
            return False
        except pkg_resources.VersionConflict:
            existing_dist = pkg_resources.get_distribution(
                self.req.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