Raise AttributeError on attempts to access unset oneof fields

This commit modifies `Message.__getattribute__` to raise
`AttributeError` whenever an attempt is made to access an unset `oneof`
field. This provides several benefits over the current approach:

* There is no longer any risk of `betterproto` users accidentally
  relying on values of unset fields.

* Pattern matching with `match/case` on messages containing `oneof`
  groups is now supported. The following is now possible:
  ```
  @dataclasses.dataclass(eq=False, repr=False)
  class Test(betterproto.Message):
    x: int = betterproto.int32_field(1, group="g")
    y: str = betterproto.string_field(2, group="g")

  match Test(y="text"):
    case Test(x=v):
      print("x", v)
    case Test(y=v):
      print("y", v)
  ```
  Before this commit the code above would output `x 0` instead of
  `y text`, but now the output is `y text` as expected. The reason
  this works is because an `AttributeError` in a `case` pattern does
  not propagate and instead simply skips the `case`.

* We now have a type-checkable way to deconstruct `oneof`. When running
  `mypy` for the snippet above `v` has type `int` in the first `case`
  and type `str` in the second `case`. For versions of Python that do
  not support `match/case` (before 3.10) it is now possbile to use
  `try/except/else` blocks to achieve the same result:
  ```
  t = Test(y="text")
  try:
    v0: int = t.x
  except AttributeError:
    v1: str = t.y  # `oneof` contains `y`
  else:
    pass  # `oneof` contains `x`
  ```

This is a breaking change.

src/betterproto/__init__.py

@@ -693,8 +693,28 @@ def __repr__(self) -> str:
         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]}, not {name!r}"
+                            )
+                        else:
+                            raise AttributeError(
+                                f"{group!r} is set to {group_current[group]}, not {name!r}",
+                                name=name,
+                                obj=self,
+                            )
+
             value = super().__getattribute__(name)
             if value is not PLACEHOLDER:
                 return value
@@ -761,7 +781,10 @@ def __bytes__(self) -> bytes:
         """
         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 @@ def __bytes__(self) -> bytes:
             # 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 @@ def parse(self: T, data: bytes) -> T:
                     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 @@ def to_dict(
         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 @@ def from_dict(self: T, value: Mapping[str, Any]) -> T:
 
             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 @@ def _validate_field_groups(cls, values):
         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

tests/inputs/google_impl_behavior_equivalence/test_google_impl_behavior_equivalence.py

@@ -50,8 +50,10 @@ def test_bytes_are_the_same_for_oneof():
 
     # None of these fields were explicitly set BUT they should not actually be null
     # themselves
-    assert isinstance(message.foo, Foo)
-    assert isinstance(message2.foo, Foo)
+    assert not hasattr(message, "foo")
+    assert object.__getattribute__(message, "foo") == betterproto.PLACEHOLDER
+    assert not hasattr(message2, "foo")
+    assert object.__getattribute__(message2, "foo") == betterproto.PLACEHOLDER
 
     assert isinstance(message_reference.foo, ReferenceFoo)
     assert isinstance(message_reference2.foo, ReferenceFoo)

tests/inputs/oneof_enum/test_oneof_enum.py

@@ -18,9 +18,8 @@ def test_which_one_of_returns_enum_with_default_value():
         get_test_case_json_data("oneof_enum", "oneof_enum-enum-0.json")[0].json
     )
 
-    assert message.move == Move(
-        x=0, y=0
-    )  # Proto3 will default this as there is no null
+    assert not hasattr(message, "move")
+    assert object.__getattribute__(message, "move") == betterproto.PLACEHOLDER
     assert message.signal == Signal.PASS
     assert betterproto.which_one_of(message, "action") == ("signal", Signal.PASS)
 
@@ -33,9 +32,8 @@ def test_which_one_of_returns_enum_with_non_default_value():
     message.from_json(
         get_test_case_json_data("oneof_enum", "oneof_enum-enum-1.json")[0].json
     )
-    assert message.move == Move(
-        x=0, y=0
-    )  # Proto3 will default this as there is no null
+    assert not hasattr(message, "move")
+    assert object.__getattribute__(message, "move") == betterproto.PLACEHOLDER
     assert message.signal == Signal.RESIGN
     assert betterproto.which_one_of(message, "action") == ("signal", Signal.RESIGN)
 
@@ -44,5 +42,6 @@ def test_which_one_of_returns_second_field_when_set():
     message = Test()
     message.from_json(get_test_case_json_data("oneof_enum")[0].json)
     assert message.move == Move(x=2, y=3)
-    assert message.signal == Signal.PASS
+    assert not hasattr(message, "signal")
+    assert object.__getattribute__(message, "signal") == betterproto.PLACEHOLDER
     assert betterproto.which_one_of(message, "action") == ("move", Move(x=2, y=3))

tests/test_features.py

@@ -151,17 +151,18 @@ class Foo(betterproto.Message):
     foo.baz = "test"
 
     # Other oneof fields should now be unset
-    assert foo.bar == 0
+    assert not hasattr(foo, "bar")
+    assert object.__getattribute__(foo, "bar") == betterproto.PLACEHOLDER
     assert betterproto.which_one_of(foo, "group1")[0] == "baz"
 
-    foo.sub.val = 1
+    foo.sub = Sub(val=1)
     assert betterproto.serialized_on_wire(foo.sub)
 
     foo.abc = "test"
 
     # Group 1 shouldn't be touched, group 2 should have reset
-    assert foo.sub.val == 0
-    assert betterproto.serialized_on_wire(foo.sub) is False
+    assert not hasattr(foo, "sub")
+    assert object.__getattribute__(foo, "sub") == betterproto.PLACEHOLDER
     assert betterproto.which_one_of(foo, "group2")[0] == "abc"
 
     # Zero value should always serialize for one-of