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

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))