INT: avoid accessing self.loc._foo outside of indexing.py

[jbrockmendel]

grepping for .loc. and .iloc. I see two uses of self.loc._convert_to_indexer and two uses of self.loc._setitem_with_indexer in core.frame, and two uses of obj.loc._getitem_axis in io.pytables

[topper-123]

This reminds me: the indexers all have their name prepebded wirh a _:

• _LocIndexer
• _IlocIndexer
• _AtIndexer
• _IatIndexer
• etc.

They are not actually private, so sholdnt we rename them (drop the _)?