merge.py 文件源码

def _left_join_on_index(left_ax, right_ax, join_keys, sort=False):
    if len(join_keys) > 1:
        if not ((isinstance(right_ax, MultiIndex) and
                 len(join_keys) == right_ax.nlevels)):
            raise AssertionError("If more than one join key is given then "
                                 "'right_ax' must be a MultiIndex and the "
                                 "number of join keys must be the number of "
                                 "levels in right_ax")

        left_indexer, right_indexer = \
            _get_multiindex_indexer(join_keys, right_ax, sort=sort)
    else:
        jkey = join_keys[0]

        left_indexer, right_indexer = \
            _get_single_indexer(jkey, right_ax, sort=sort)

    if sort or len(left_ax) != len(left_indexer):
        # if asked to sort or there are 1-to-many matches
        join_index = left_ax.take(left_indexer)
        return join_index, left_indexer, right_indexer

    # left frame preserves order & length of its index
    return left_ax, None, right_indexer