See protocolbuffers/protobuf#16596 breaking change

proto/marshal/compat.py

@@ -19,6 +19,9 @@
 # not be included.
 
 from google.protobuf.internal import containers
+import google.protobuf
+
+PROTOBUF_VERSION = google.protobuf.__version__
 
 # Import protobuf 4.xx first and fallback to earlier version
 # if not present.
@@ -36,15 +39,17 @@
 repeated_composite_types = (containers.RepeatedCompositeFieldContainer,)
 repeated_scalar_types = (containers.RepeatedScalarFieldContainer,)
 map_composite_types = (containers.MessageMap,)
+map_composite_types_str = ('MessageMapContainer')
 
 if _message:
     repeated_composite_types += (_message.RepeatedCompositeContainer,)
     repeated_scalar_types += (_message.RepeatedScalarContainer,)
-    map_composite_types += (_message.MessageMapContainer,)
-
+    if PROTOBUF_VERSION[0:2] in ["3.", "4."]:
+        map_composite_types += (_message.MessageMapContainer,)
 
 __all__ = (
     "repeated_composite_types",
     "repeated_scalar_types",
     "map_composite_types",
+    "map_composite_types_str",
 )

proto/marshal/marshal.py

@@ -188,7 +188,11 @@ def to_python(self, proto_type, value, *, absent: bool = None):
                 return Repeated(value, marshal=self)
 
         # Same thing for maps of messages.
-        if value_type in compat.map_composite_types:
+        # See https://github.com/protocolbuffers/protobuf/issues/16596
+        # We need to look up the name of the type in compat.map_composite_types_str
+        # as class `MessageMapContainer` is no longer exposed
+        # This is done to avoid taking a breaking change in proto-plus
+        if value_type in compat.map_composite_types or value_type.__name__ in compat.map_composite_types_str:
             return MapComposite(value, marshal=self)
         return self.get_rule(proto_type=proto_type).to_python(value, absent=absent)