Allow register_type to match subclasses

[dhirschfeld]

To allow factoring out serialization code to a common base class it would be useful if register_type matches all subclasses rather than simply an exact match on the specific type.

It might be possible with a simple change to _serialize_callback:

arrow/python/pyarrow/serialization.pxi

Lines 90 to 95 in 30158ad

	def _serialize_callback(self, obj):
	if type(obj) not in self.type_to_type_id:
	raise SerializationCallbackError(
	"pyarrow does not know how to "
	"serialize objects of type {}.".format(type(obj)), obj)
	type_id = self.type_to_type_id[type(obj)]

def _serialize_callback(self, obj):
    FOUND = False
    for type_ in type(obj).mro():
        if type_ in self.type_to_type_id:
            FOUND = True
            break
    
    if not FOUND:
        raise SerializationCallbackError(
            "pyarrow does not know how to "
            "serialize objects of type {}.".format(type(obj)), obj
        )

    # use the closest match to type(obj)
    type_id = self.type_to_type_id[type_]

[wesm]

I opened https://issues.apache.org/jira/browse/ARROW-1753 for the feature request. Would you like to write a patch?