Raise AttributeError on attempts to access unset oneof fields (#510)

src/betterproto/__init__.py

@@ -693,8 +693,28 @@ class Message(ABC):
         def __getattribute__(self, name: str) -> Any:
             """
             Lazily initialize default values to avoid infinite recursion for recursive
-            message types
+            message types.
+            Raise :class:`AttributeError` on attempts to access unset ``oneof`` fields.
             """
+            try:
+                group_current = super().__getattribute__("_group_current")
+            except AttributeError:
+                pass
+            else:
+                if name not in {"__class__", "_betterproto"}:
+                    group = self._betterproto.oneof_group_by_field.get(name)
+                    if group is not None and group_current[group] != name:
+                        if sys.version_info < (3, 10):
+                            raise AttributeError(
+                                f"{group!r} is set to {group_current[group]!r}, not {name!r}"
+                            )
+                        else:
+                            raise AttributeError(
+                                f"{group!r} is set to {group_current[group]!r}, not {name!r}",
+                                name=name,
+                                obj=self,
+                            )
+
             value = super().__getattribute__(name)
             if value is not PLACEHOLDER:
                 return value
@@ -761,7 +781,10 @@ class Message(ABC):
         """
         output = bytearray()
         for field_name, meta in self._betterproto.meta_by_field_name.items():
-            value = getattr(self, field_name)
+            try:
+                value = getattr(self, field_name)
+            except AttributeError:
+                continue
 
             if value is None:
                 # Optional items should be skipped. This is used for the Google
@@ -775,9 +798,7 @@ class Message(ABC):
             # Note that proto3 field presence/optional fields are put in a
             # synthetic single-item oneof by protoc, which helps us ensure we
             # send the value even if the value is the default zero value.
-            selected_in_group = (
-                meta.group and self._group_current[meta.group] == field_name
-            )
+            selected_in_group = bool(meta.group)
 
             # Empty messages can still be sent on the wire if they were
             # set (or received empty).
@@ -1016,7 +1037,12 @@ class Message(ABC):
                     parsed.wire_type, meta, field_name, parsed.value
                 )
 
-            current = getattr(self, field_name)
+            try:
+                current = getattr(self, field_name)
+            except AttributeError:
+                current = self._get_field_default(field_name)
+                setattr(self, field_name, current)
+
             if meta.proto_type == TYPE_MAP:
                 # Value represents a single key/value pair entry in the map.
                 current[value.key] = value.value
@@ -1077,7 +1103,10 @@ class Message(ABC):
         defaults = self._betterproto.default_gen
         for field_name, meta in self._betterproto.meta_by_field_name.items():
             field_is_repeated = defaults[field_name] is list
-            value = getattr(self, field_name)
+            try:
+                value = getattr(self, field_name)
+            except AttributeError:
+                value = self._get_field_default(field_name)
             cased_name = casing(field_name).rstrip("_")  # type: ignore
             if meta.proto_type == TYPE_MESSAGE:
                 if isinstance(value, datetime):
@@ -1209,7 +1238,7 @@ class Message(ABC):
 
             if value[key] is not None:
                 if meta.proto_type == TYPE_MESSAGE:
-                    v = getattr(self, field_name)
+                    v = self._get_field_default(field_name)
                     cls = self._betterproto.cls_by_field[field_name]
                     if isinstance(v, list):
                         if cls == datetime:
@@ -1486,7 +1515,6 @@ class Message(ABC):
         field_name_to_meta = cls._betterproto_meta.meta_by_field_name  # type: ignore
 
         for group, field_set in group_to_one_ofs.items():
-
             if len(field_set) == 1:
                 (field,) = field_set
                 field_name = field.name