Freature request: allow handle_error to suppress an error.

In do_load you have the following code block:

        if errors:
            exc = ValidationError(errors, data=data, valid_data=result)
            self.handle_error(exc, data, many=many, partial=partial)
            raise exc

So the exception is raised unconditionally. This means that there is no way for a custom schema to correct or suppress an error raised by a sub-schema.

Our use case is wanting to have a schema in which some fields are validated, but the remainder are passed “as is” (they are massaged and validated later on in our data pipeline). In Marshmallow 2 we set our schema to strict=False and had our handle_error raise on the fields we wanted validated. As far as I can see there is no equivalent functionality in Marshmallow 3. This could be solved by having the code that calls handle_error allow it to signal that the exception be ignored. For example, it could treat a return value of True from handle_error as signalling that the error has been successfully handled and should not be raised.

(By the way, this use case is similar to that mentioned in #1198, but we aren’t serializing an object, just a json data structure. We are currently using a @post_dump method to include the unknown fields in the dump, but I haven’t actually gotten far enough yet to know if that will actually work in Marshmallow 3 😃

[deckar01]

The validation error that is raised has a valid_data property you can use to recover the valid data.

https://marshmallow.readthedocs.io/en/stable/quickstart.html#validation

I think this is what @bitdancer ended up doing:

from marshmallow import Schema, fields, ValidationError

class Test(Schema):
    foo = fields.String()
    bar = fields.String()
    
    def load(self, *args, **kwargs):
        try:
            return super().load(*args, **kwargs)
        except ValidationError as e:
            return e.valid_data

Test().load({'foo': 123, 'bar': 'ok'})
# {'bar': 'ok'}

[bitdancer]

Ah, nevermind. What I requested wouldn’t do what we wanted because _do_load is called from validate as well.

What I ended up with was adding a ‘load’ method to our schema that calls super inside a try/except and does what we want. Sorry for wasting your time 😦