Python: Implementation agnostic way to determine type of the elements of an empty repeated field

[busunkim96]

What language does this apply to?

Python generated code

Describe the problem you are trying to solve.

There is no public method to determine the type of entries in an empty repeated field.

The proto-plus package wraps protobuf messages for many of the Python Google Cloud Client Libraries.
When a repeated field has no entries the proto-plus implementation deep-copies the whole message, adds an entry, and fetches the type of the field from that copy. The deep copying results in a performance hit.

Describe the solution you'd like

An implementation agnostic (works with both C++ and Python implementation types) way to determine type of the elements of an empty repeated field.

Describe alternatives you've considered

To try these examples, pip install googleapis-common-protos to get the built google.api.quota_pb2 proto. Proto file is here.

1. Add an entry to determine the type

This is what we're currently doing but would prefer not to, since we have to copy the whole message first.

>>> from google.api import quota_pb2
>>> q = quota_pb2.Quota()
>>> limit = q.limits.add()
>>> type(limit)
<class 'google.api.quota_pb2.QuotaLimit'>

2. Use _message_descriptor._concrete_class

This only works in the Python implementation and relies on private attributes.

>>> from google.api import quota_pb2
>>> q = quota_pb2.Quota()
>>> q.limits._message_descriptor._concrete_class
<class 'google.api.quota_pb2.QuotaLimit'>

In the C++ implementation the code above results in an error:

>>> q.limits._message_descriptor._concrete_class
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'google.protobuf.pyext._message.RepeatedCompositeCo' object has no attribute '_message_descriptor'

Additional context

See googleapis/proto-plus-python#226 for additional context.

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