Odd Behavior for `Function` field

class TestSchema(Schema):
    test = fields.Function(deserialize=lambda x: 1)
    test2 = fields.Constant(11)
    
TestSchema().load({})

yields UnmarshalResult(data={'test2': 11}, errors={}), ie omitting test. The fix seems to be:

class TestSchema2(Schema):
    test = fields.Function(deserialize=lambda x: 1, missing='asdf')
    test2 = fields.Constant(11)

yielding: UnmarshalResult(data={'test': 1, 'test2': 11}, errors={}).

[cancan101]

Name: marshmallow
Version: 2.15.3
Summary: A lightweight library for converting complex datatypes to and from native Python datatypes.
Home-page: https://github.com/marshmallow-code/marshmallow
Author: Steven Loria
Author-email: [e-mail]
License: MIT

[deckar01]

You could use it to invert a boolean value when mapping a field like enabled to disabled. Sometimes a custom field is overkill when the just need to transform the input value a little.

disabled = field.Function(deserialize=lambda x: not x, load_from='enabled')