Use a TypeGuard for dataclasses.is_dataclass(); refine asdict(), astuple(), fields(), replace() (#9362)

test_cases/stdlib/check_dataclasses.py

@@ -0,0 +1,81 @@
+from __future__ import annotations
+
+import dataclasses as dc
+from typing import Any, Dict, Tuple, Type
+from typing_extensions import assert_type
+
+
+@dc.dataclass
+class Foo:
+    attr: str
+
+
+assert_type(dc.fields(Foo), Tuple[dc.Field[Any], ...])
+
+# Mypy correctly emits errors on these
+# due to the fact it's a dataclass class, not an instance.
+# Pyright, however, handles ClassVar members in protocols differently.
+# See https://github.com/microsoft/pyright/issues/4339
+#
+# dc.asdict(Foo)
+# dc.astuple(Foo)
+# dc.replace(Foo)
+
+if dc.is_dataclass(Foo):
+    # The inferred type doesn't change
+    # if it's already known to be a subtype of type[_DataclassInstance]
+    assert_type(Foo, Type[Foo])
+
+f = Foo(attr="attr")
+
+assert_type(dc.fields(f), Tuple[dc.Field[Any], ...])
+assert_type(dc.asdict(f), Dict[str, Any])
+assert_type(dc.astuple(f), Tuple[Any, ...])
+assert_type(dc.replace(f, attr="new"), Foo)
+
+if dc.is_dataclass(f):
+    # The inferred type doesn't change
+    # if it's already known to be a subtype of _DataclassInstance
+    assert_type(f, Foo)
+
+
+def test_other_isdataclass_overloads(x: type, y: object) -> None:
+    # TODO: pyright correctly emits an error on this, but mypy does not -- why?
+    # dc.fields(x)
+
+    dc.fields(y)  # type: ignore
+
+    dc.asdict(x)  # type: ignore
+    dc.asdict(y)  # type: ignore
+
+    dc.astuple(x)  # type: ignore
+    dc.astuple(y)  # type: ignore
+
+    dc.replace(x)  # type: ignore
+    dc.replace(y)  # type: ignore
+
+    if dc.is_dataclass(x):
+        assert_type(dc.fields(x), Tuple[dc.Field[Any], ...])
+        # These should cause type checkers to emit errors
+        # due to the fact it's a dataclass class, not an instance
+        dc.asdict(x)  # type: ignore
+        dc.astuple(x)  # type: ignore
+        dc.replace(x)  # type: ignore
+
+    if dc.is_dataclass(y):
+        assert_type(dc.fields(y), Tuple[dc.Field[Any], ...])
+
+        # Mypy corrextly emits an error on these due to the fact we don't know
+        # whether it's a dataclass class or a dataclass instance.
+        # Pyright, however, handles ClassVar members in protocols differently.
+        # See https://github.com/microsoft/pyright/issues/4339
+        #
+        # dc.asdict(y)
+        # dc.astuple(y)
+        # dc.replace(y)
+
+    if dc.is_dataclass(y) and not isinstance(y, type):
+        assert_type(dc.fields(y), Tuple[dc.Field[Any], ...])
+        assert_type(dc.asdict(y), Dict[str, Any])
+        assert_type(dc.astuple(y), Tuple[Any, ...])
+        dc.replace(y)