When we use the use_kwargs method with a callable (that accepts a request and returns a Schema instance), like in this example with use_kwargs: an error raises here, as the argmap is now a callable, not a dict.
argmap = <function factory at 0x1090227d0>
if isinstance(argmap, ma.Schema):
all_field_names = set([fname for fname, fobj in iteritems(argmap.fields)
if not fobj.dump_only])
> all_field_names = set(argmap.keys())
E AttributeError: 'function' object has no attribute 'keys'
get_field_names_for_argmap is called by fill_in_missing_args.