Replacing `Arg(dict)` with marshmallow?

In webargs < 0.16, I have arguments defined as Arg(dict, ...). How do I convert these to marshmallow fields? The changelog shows examples with strings and nested arguments, but I couldn’t find an example with dict.

[sloria]

@stranbird That is the expected behavior; fields.Dict does not expect to receive strings as input, and webargs does not do any special preprocessing. One solution is to create a custom field:

class JSONDict(fields.Dict):

    def _deserialize(self, value):
        if isinstance(value, str):
            value = json.loads(value)
        super()._deserialize(value)

@use_args({
  'where': JSONDict()
})

[yuanfeiz]

@sloria neat solution, ty!