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

tests/oneof_pattern_matching.py

@@ -0,0 +1,46 @@
+from dataclasses import dataclass
+
+import pytest
+
+import betterproto
+
+
+def test_oneof_pattern_matching():
+    @dataclass
+    class Sub(betterproto.Message):
+        val: int = betterproto.int32_field(1)
+
+    @dataclass
+    class Foo(betterproto.Message):
+        bar: int = betterproto.int32_field(1, group="group1")
+        baz: str = betterproto.string_field(2, group="group1")
+        sub: Sub = betterproto.message_field(3, group="group2")
+        abc: str = betterproto.string_field(4, group="group2")
+
+    foo = Foo(baz="test1", abc="test2")
+
+    match foo:
+        case Foo(bar=_):
+            pytest.fail("Matched 'bar' instead of 'baz'")
+        case Foo(baz=v):
+            assert v == "test1"
+        case _:
+            pytest.fail("Matched neither 'bar' nor 'baz'")
+
+    match foo:
+        case Foo(sub=_):
+            pytest.fail("Matched 'sub' instead of 'abc'")
+        case Foo(abc=v):
+            assert v == "test2"
+        case _:
+            pytest.fail("Matched neither 'sub' nor 'abc'")
+
+    foo.sub = Sub(val=1)
+
+    match foo:
+        case Foo(sub=Sub(val=v)):
+            assert v == 1
+        case Foo(abc=v):
+            pytest.fail("Matched 'abc' instead of 'sub'")
+        case _:
+            pytest.fail("Matched neither 'sub' nor 'abc'")