From 5ef96fdb5390b1c232b42559f3460947739282d3 Mon Sep 17 00:00:00 2001
From: Anthony Sottile <asottile@umich.edu>
Date: Fri, 8 Apr 2022 20:08:51 -0400
Subject: [PATCH] [7.1.x] fix comparison of dataclasses with `InitVar`

---
 changelog/9820.bugfix.rst                            |  1 +
 src/_pytest/assertion/util.py                        |  6 ++++--
 .../dataclasses/test_compare_initvar.py              | 12 ++++++++++++
 testing/test_assertion.py                            |  7 +++++++
 4 files changed, 24 insertions(+), 2 deletions(-)
 create mode 100644 changelog/9820.bugfix.rst
 create mode 100644 testing/example_scripts/dataclasses/test_compare_initvar.py

diff --git a/changelog/9820.bugfix.rst b/changelog/9820.bugfix.rst
new file mode 100644
index 00000000000..03af940f2c4
--- /dev/null
+++ b/changelog/9820.bugfix.rst
@@ -0,0 +1 @@
+Fix comparison of  ``dataclasses`` with ``InitVar``.
diff --git a/src/_pytest/assertion/util.py b/src/_pytest/assertion/util.py
index 03167ddd471..b1f168767be 100644
--- a/src/_pytest/assertion/util.py
+++ b/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")]
diff --git a/testing/example_scripts/dataclasses/test_compare_initvar.py b/testing/example_scripts/dataclasses/test_compare_initvar.py
new file mode 100644
index 00000000000..d859634ddd5
--- /dev/null
+++ b/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)
diff --git a/testing/test_assertion.py b/testing/test_assertion.py
index d37ee72a2ec..40a3e0021d0 100644
--- a/testing/test_assertion.py
+++ b/testing/test_assertion.py
@@ -882,6 +882,13 @@ def test_data_classes_with_custom_eq(self, pytester: Pytester) -> None:
         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: