deserialize is not called for null values

[Ted-Barrett]

I would like to handle null values when deserializing. I have the following custom attribute:

class TestAttribute(Attribute):
    attr_type = BINARY

    def serialize(self, value: Any) -> Any:
        return json.dumps(value)
    def deserialize(self, value: Any) -> Any:
        raise RuntimeError("STOP!!")

being used in a model:

class MyModel(Model):
    test_field = TestAttribute(null=True)

If the field has data in it, I get the runtime error STOP!! as expected.

If the field is empty, there is no such error, and e.test_field is None.

How can I handle the behaviour when a value is null?

[ikonst]

Unfortunately this check simply won't let you have that:

PynamoDB/pynamodb/attributes.py

Lines 431 to 432 in 4b86304

	attribute_value = attribute_values.get(attr.attr_name)
	if attribute_value and NULL not in attribute_value:

(Curiously, the NULL DynamoDB attribute value would also have the same effect, making it indistinguishable from absence of value. A shame.)

Typically I'd override a model's deserialize if I want the opportunity to work with the raw data before field-by-field deserialization.