When does Django's custom model field method to_python get anything other than an instance?

Question

From the documentation for to_python:

As a general rule, [to_python] should deal gracefully with any of the following arguments:

• An instance of the correct type (e.g., Hand in our ongoing example).
• A string (e.g., from a deserializer).
• Whatever the database returns for the column type you're using.

I'm looking at my test coverage (using coverage.py), and the bit of code at the top of my to_python:

def to_python(self, value):
    if isinstance(value, Hand):
        return value

    # More code for handling strings below (I never get here)

is the only bit of my to_python method that gets called. What else should I be testing? I've tested saving and retrieving objects from the database, and I've tested serialization like this:

   cereal = serializers.serialize('json',
                                   Hand.objects.all())
   objects = list(serializers.deserialize("json", cereal))

Answer 1

I've found one place where to_python ends up with something other than an instance - in the case where the value is not provided (for example, if your field is set up to allow null). In that case, because I'd implemented get_prep_value badly, to_python got passed a string containing None.