Turn off automatic skipping of unknown fields

When loading/deserializing, unknown field names are skipped. It would be nice to be able to raise an error instead, particularly when calling .validate(). Currently this doesnt seem possible, even with a custom validator, because the field filtering appears to already have occurred by the time the validator gets a hold of it, e.g.

class MySchema:
    foo = field.String()

@MySchema.validator
def check_unknown_fields(schema, data):
    for k in data:
        if k not in schema.fields:
            raise ValidationError("Unknown field name: '{}'".format(k))

errors = MySchema().validate({'bar': 'abc'})

doesnt work because data will be an empty {} by the time check_unknown_fields() is called.

[sloria]

This is now possible in 3.0.0b12.

https://marshmallow.readthedocs.io/en/3.0/quickstart.html#handling-unknown-fields

[norbertpy]

Would have been great if setting strict=True would actually raise an error on unknown fields.