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Question/feature request related to only and exclude

See original GitHub issue

Hi, first off this project it has helped us a lot, so a huge thanks!

I have a question / request related to the only and exclude parameters to schemas (and nested fields).

Provided a schema looks like this:

class TestSchema(Schema):
    foo = Int()
    bar = Str()
    baz = Dict()

The following happens: schema = TestSchema(only=('foo', 'bar', )) // works as expected schema = TestSchema(only=('foo', 'not_part_of_schema', )) // adds not_part_of_schema schema = TestSchema(exclude=('bar', 'baz', )) // works as expected schema = TestSchema(exclude=('bar', 'not_part_of_schema', )) // not_part_of_schema does nothing

The following should happen in my opinion (to avoid leaking input data when schema forbids it): schema = TestSchema(only=('foo', 'bar', )) // works schema = TestSchema(only=('foo', 'not_part_of_schema', )) // raises exception schema = TestSchema(exclude=('bar', 'baz', )) // works schema = TestSchema(exclude=('bar', 'not_part_of_schema', )) // raises exception

To get the behaviour above we’ve implemented the following override to a base schema:

class BaseSchema(Schema):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs) // sorry py3 only

        valid_fields = set(self.declared_fields.keys()) | set(self.opts.additional)
        excluded_fields = set(self.exclude) | set(self.opts.exclude)
        only_fields = set(self.only) if self.only else set()

        if not excluded_fields.issubset(valid_fields):
            raise LookupError('Excluded field(s) not part of valid fields')

        valid_fields -= excluded_fields
        if not only_fields.issubset(valid_fields):
            raise LookupError('Only field(s) not part of valid fields')

Is this behaviour possible in the current Marshmallow code? If not, is our solution even recommended (am I missing some really obvious use case for the injection)? If yes, is this something that could be useful in Marshmallow?