fix "extra fields not permitted" error when dataclass with Extra.forbid is validated multiple times (#4354)

Co-authored-by: detachhead <detachhead@users.noreply.github.com>

changes/4343-detachhead.md

@@ -0,0 +1 @@
+fix "extra fields not permitted" error when dataclass with `Extra.forbid` is validated multiple times

pydantic/dataclasses.py

@@ -386,6 +386,10 @@ def create_pydantic_model_from_dataclass(
 
 
 def _dataclass_validate_values(self: 'Dataclass') -> None:
+    # validation errors can occur if this function is called twice on an already initialised dataclass.
+    # for example if Extra.forbid is enabled, it would consider __pydantic_initialised__ an invalid extra property
+    if getattr(self, '__pydantic_initialised__'):
+        return
     if getattr(self, '__pydantic_has_field_info_default__', False):
         # We need to remove `FieldInfo` values since they are not valid as input
         # It's ok to do that because they are obviously the default values!

tests/test_dataclasses.py

@@ -1371,3 +1371,27 @@ def test_kw_only():
         A(1, '')
 
     assert A(b='hi').b == 'hi'
+
+
+def test_extra_forbid_list_no_error():
+    @pydantic.dataclasses.dataclass(config=dict(extra=Extra.forbid))
+    class Bar:
+        ...
+
+    @pydantic.dataclasses.dataclass
+    class Foo:
+        a: List[Bar]
+
+    assert isinstance(Foo(a=[Bar()]).a[0], Bar)
+
+
+def test_extra_forbid_list_error():
+    @pydantic.dataclasses.dataclass(config=dict(extra=Extra.forbid))
+    class Bar:
+        ...
+
+    with pytest.raises(TypeError, match=re.escape("__init__() got an unexpected keyword argument 'a'")):
+
+        @pydantic.dataclasses.dataclass
+        class Foo:
+            a: List[Bar(a=1)]