Convert dataframe into list of edges, assuming that source and target ids are stored in _SRC_VID_COLUMN, and _DST_VID_COLUMN respectively.

def _dataframe_to_edge_list(df):
    """
    Convert dataframe into list of edges, assuming that source and target ids are stored in _SRC_VID_COLUMN, and _DST_VID_COLUMN respectively.
    """
    cols = df.columns
    if len(cols):
        assert _SRC_VID_COLUMN in cols, "Vertex DataFrame must contain column %s" % _SRC_VID_COLUMN
        assert _DST_VID_COLUMN in cols, "Vertex DataFrame must contain column %s" % _DST_VID_COLUMN
        df = df[cols].T
        ret = [Edge(None, None, _series=df[col]) for col in df]
        return ret
    else:
        return []