I could include tuples and sets

if not isinstance(data, (list, tuple, Set)):    self.fail('not_a_list', input_type=type(data).__name__)

Thanks for the path.
I don't think it should force to list / tuple / set. Any iterable should do.
The key issue here is that a dictionary is an iterable on the keys.
Questions to be answered:

• do we really want to limit the valid type to a limited set ?
• can't we just exclude dictionaries from theListField ?

You're right, I think excluding dictionaries from the ListField in addition to what was already in that line would do the trick.

Agree - excluding dicts sounds like a good way around to do this.

Note: by dict we should read any mapping type.

Note: by dict we should read any mapping type.

IDK - any chance that could be a bit fuzzy in cases we've not thought of?
No strong opinion tho'

Well, collections.Mapping should be a good base class to test against (as opposed to just dict).

Probably want tests that prove this works as intended.

Yup, having tests would be great so we merge it.

Tests added.

Looks good, thanks!