Pass setval calls through SubstructureField to the contained Structure

[rcfox]

(If the value is a Structure, then replace the contained one.)

Currently, setting to a SubstructureField directly will replace the field with whatever you set, which can break the parent structure if what you set isn't a BaseField type.

class PascalString16(Structure):
    length = LengthField(UBInt16())
    value = Payload(length)

 class Name(Structure):
    first = SubstructureField(PascalString16)
    last = SubstructureField(PascalString16)

name = Name()
name.first = 'bob' # Now name.first is a str instead of a PascalString16!

With this change, name.first = 'bob' will call setval on the PascalString16 structure under name.first. This might not be a meaningful operation for many Structure types, but at least it will now raise an exception if setval hasn't been defined.

[posborne]

Stylistically, I think I would prefer:

from suitcase.structure import Structure

[rcfox]

I agree that it would look nicer, but it introduces a circular dependency between fields.py and structure.py.

[mikewadsten]

That could be resolved by adding from suitcase.structure import Structure to the setval method.

[posborne]

Currently, I'm leaning away from overloading the assignment operator in this way. See comments. For PRs to be merged, tests must be passing as well.

[rcfox]

I decided to just implement this locally within my own project:

class PassthroughSubstructureField(SubstructureField):
    ''' Pass assignments of non-Structure values through to the the contained Structure, rather than overwriting it. '''
    def setval(self, value):
        if value is None or isinstance(value, Structure):
            super(PassthroughSubstructureField, self).setval(self, value)
        else:
            self._value.setval(value)