choices, defaults and None values

[ppslim]

The API I am accessing returns JSON with a field that will return null for almost all fields. If I POST/PUT this model back into the API, it will interpret the null values as if they are unset.

There is an exception to this. One specific String field (however, there is no reason why in future this could not apply to other value types) in which a POST/PUT operation must be one of x values.

As a result, I apply the choices options and also a default The field is current defined as:
option = fields.String(choices=['FOO', 'BAR', 'ANY', 'NONE'], default='NONE')

Imagine the following though.

import json
from booby import Model, fields

class TestModel(Model):
    fake = fields.String()
    option = fields.String(choices=['FOO', 'BAR', 'ANY', 'NONE'], default='NONE')

test_json_b = "{'fake': 'AMZ',}"
test_json_c = "{'fake': 'AMZ', 'option': null}"

tb = json.loads(test_json_b)
tc = json.loads(test_json_c)

mod_a = TestModel()
mod_b = TestModel(**ta)
mod_c = TestModel(**tc)

assert mod_a.option == 'NONE'
assert mod_a.is_valid == True
assert mod_b.option == 'NONE'
assert mod_b.is_valid == True
assert mod_c.option == None  # Undesired
assert mod_c.is_valid == False  # Undesired

Is there a method to remove the undesired behaviour above, in which a null breaks the option field?

I have added None to the list of choices for the field, but then this also makes the default redundant, given that the API returns null everytime it is unset.