selectable.py 文件源码

def _froms(self):
        # would love to cache this,
        # but there's just enough edge cases, particularly now that
        # declarative encourages construction of SQL expressions
        # without tables present, to just regen this each time.
        froms = []
        seen = set()
        translate = self._from_cloned

        for item in itertools.chain(
            _from_objects(*self._raw_columns),
            _from_objects(self._whereclause)
            if self._whereclause is not None else (),
            self._from_obj
        ):
            if item is self:
                raise exc.InvalidRequestError(
                    "select() construct refers to itself as a FROM")
            if translate and item in translate:
                item = translate[item]
            if not seen.intersection(item._cloned_set):
                froms.append(item)
            seen.update(item._cloned_set)

        return froms