This look interesting. I like the general idea for sure.

Given proto's use of default zero values I wonder if it's more useful to know:

1. Has anything ever been set on this?
2. Is the message equivalent to zero (i.e. all fields are their zero value)?

This PR seems to implement 1 unless I'm mistaken. Thoughts?

To elaborate, you can see that a Python list will return to False after you explicitly "set" it by removing the item:

>>> test = []
>>> bool(test)
False

>>> test.append(1)
>>> bool(test)
True

>>> test.pop()
1
>>> bool(test)
False

I think the proto equivalent is setting the int32 to 0 since that should mean it never gets sent over the wire since it's the zero value for the type.

Am I mistaken in thinking that returning self._serialized_on_wire would achieve this?

I should have this working as a collection would now, so if a value is set to anything and then has its value set to its default it will return False

@Gobot1234 I think we can merge this in principal, but it's significant an API feature that it should really be documented. Would you mind?

Ah yes of course.

Sorry just realised I haven't added it to the Sphinx special members directive yet