Can't set any_pb2.Any field via assignment or from constructor

[software-dov]

import proto
import protobuf.any_pb2 as any_pb2

class Container(proto.Message):
    contents = proto.Field(any_pb2.Any, number=1)
    
class Thing(proto.Message):
    name = proto.Field(proto.STRING, number=1)
    
a = any_pb2.Any()
a.Pack(Thing(name="Steve"))
c = Container(contents=a)   # This works
c = Container()
c.contents = a    # As does this

c = Container(contents=Thing(name="Alice"))    # This fails
c = Container()
c.contents = Thing(name="Alice")    # This also fails

Assignment to an Any field does not do automatic packing/unpacking. This is annoying and unintuitive.

[maffoo]

I agree using any_pb2.Any with proto-plus is quite awkward, but I'm not sure that automatic packing and unpacking is the best solution, since sometimes one wants to preserve an Any message and work with it without repeated packing and unpacking. I think the root of the problem is that any_pb2.Any has an awkward API that is not very pythonic (can't pack or unpack in a single expression for example, instead one has to create the any message and call the Pack method as in the issue description). If proto-plus had it's own proto.Any type this could be improved significantly, something like:

import proto

class Container(proto.Message):
    contents = proto.Field(proto.Any, number=1)

class Thing(proto.Message):
    name = proto.Field(proto.String, number=1)

c = Container(contents=proto.Any(Thing(name="Steve")))
c.contents = proto.Any(Thing("Steph"))

thing = c.contents.unpack()  # unpacks the message in the Any

[vam-google]

Changed the status of it from bug to feature request, since it is about usability and unintuitive behavior, which, nonetheless works by design now. Also, status of proto-plus may change in the future (we may remove it from the surface or get it integrated with protobuf more naively, because currently it causes a lot of compatibility and performance issues). Making major changes in proto-plus surface is not a priority this time.