fix comparison of dataclasses with `InitVar`

changelog/9820.bugfix.rst

@@ -0,0 +1 @@
+Fix comparison of  ``dataclasses`` with ``InitVar``.

src/_pytest/assertion/util.py

@@ -437,8 +437,10 @@ def _compare_eq_cls(left: Any, right: Any, verbose: int) -> List[str]:
     if not has_default_eq(left):
         return []
     if isdatacls(left):
-        all_fields = left.__dataclass_fields__
-        fields_to_check = [field for field, info in all_fields.items() if info.compare]
+        import dataclasses
+
+        all_fields = dataclasses.fields(left)
+        fields_to_check = [info.name for info in all_fields if info.compare]
     elif isattrs(left):
         all_fields = left.__attrs_attrs__
         fields_to_check = [field.name for field in all_fields if getattr(field, "eq")]

testing/example_scripts/dataclasses/test_compare_initvar.py

@@ -0,0 +1,12 @@
+from dataclasses import dataclass
+from dataclasses import InitVar
+
+
+@dataclass
+class Foo:
+    init_only: InitVar[int]
+    real_attr: int
+
+
+def test_demonstrate():
+    assert Foo(1, 2) == Foo(1, 3)

testing/test_assertion.py

@@ -882,6 +882,13 @@ class TestAssert_reprcompare_dataclass:
         result.assert_outcomes(failed=1, passed=0)
         result.stdout.no_re_match_line(".*Differing attributes.*")
 
+    def test_data_classes_with_initvar(self, pytester: Pytester) -> None:
+        p = pytester.copy_example("dataclasses/test_compare_initvar.py")
+        # issue 9820
+        result = pytester.runpytest(p, "-vv")
+        result.assert_outcomes(failed=1, passed=0)
+        result.stdout.no_re_match_line(".*AttributeError.*")
+
 
 class TestAssert_reprcompare_attrsclass:
     def test_attrs(self) -> None: