Filtering is supported through the OMIT value: http://glom.readthedocs.io/en/latest/api.html#glom.OMIT (another example: http://glom.readthedocs.io/en/latest/snippets.html#filtered-... )

Lambdas and functions are always a safe fallback, but glom does its best to keep your specs readable and roundtrippable (gotta love a nice repr()).