Optional fields with value None raises exception on load

[sveinse]

Running on python 3.8.0 on Windows 10, 32-bit version in a venv. desert retrieved from master commit a9b41e1.

desert does not support reading input data containing None from a schema with Optional type. The following test shows the issue:

def test_optional_present(module):
    """Including optional with None set the field."""

    @module.dataclass
    class A:
        x: t.Optional[int]

    data = desert.schema_class(A)().load({"x": None})
    assert data == A(None)

This will fail with the error: marshmallow.exceptions.ValidationError: {'x': ['Field may not be null.']}. The documentation for typing.Optional states that it is equivalent of Union[X, None] indicates that None is valid input to a Optional field.

[python-desert]

Thanks for reporting. I can reproduce the issue. I'll try to get it fixed over the weekend.