New feature for iterating over fields_dict in marshalling/unmarshalling

jsonschema(and swagger) have specific options for unexpected additional fields.

• additionalProperties
• patternProperties(maybe, jsonschema only)

And, I want to translate this feature to marshmallow schema.

marshmallow’s current implementation

The way of iteration in marshmallow’s current implementation, is just calling iteritems() with fields_dict. So, it is difficult that define schema allowing unexpected field.

for examples, in swagger (openAPI2.0) the schema definition, like bellow.

definitions:
  person:
    type: object
    properties:
      name:
        type: string
    additionalPropeties: integer
    required:
      - name

Then, about person schema, passing {"name": "foo"} is ok, and {"name": "foo", "x": 20, "y": 10} is also ok(unexpected field is treated as integer).

it is a bit difficult, making marshmallow’s version of this. (schema meta’s additional is candidate of expected field names, … and so on.)

proposal

If marshmallow has new hook point that changing the way of iteration, it is enable to implement marshmallow’s version, more easily.

For example, get_marshalling_iterator()(this is not good name), is existed in Schema class.

# def default_iterator(data, fields_dict):
#     return iteritems(fields_dict)


class S(Schema):
    def get_marshalling_iterator(self, additional_field=fields.Int()):
        def iterate_with_additional_field(data, fields_dict):
            for k in data:
                field = fields_dict.get(k, additional_field)
                yield k, field

        return iterate_with_additional_field

    name = fields.String(required=True)

• 💭 get_marshalling_iterator is not good name, maybe
• 💭 the position of hook point(Schema class’s instance method) is not good, maybe

[sloria]

It is now possible to allow additional properties to be included in deserialized data (see #524).

I believe the requested feature here would be covered by #853. Let’s continue discussion there.

[podhmo]

It is decided that supporting additional_properties via Meta attribute, closing this issue, and creating new issue, is better?