Make dataclasses/attrs comparison recursive, fixes #4675

changelog/4675.bugfix.rst

@@ -0,0 +1 @@
+Make dataclasses/attrs comparison recursive.

src/_pytest/assertion/util.py

@@ -148,26 +148,7 @@ def assertrepr_compare(config, op: str, left: Any, right: Any) -> Optional[List[
     explanation = None
     try:
         if op == "==":
-            if istext(left) and istext(right):
-                explanation = _diff_text(left, right, verbose)
-            else:
-                if issequence(left) and issequence(right):
-                    explanation = _compare_eq_sequence(left, right, verbose)
-                elif isset(left) and isset(right):
-                    explanation = _compare_eq_set(left, right, verbose)
-                elif isdict(left) and isdict(right):
-                    explanation = _compare_eq_dict(left, right, verbose)
-                elif type(left) == type(right) and (isdatacls(left) or isattrs(left)):
-                    type_fn = (isdatacls, isattrs)
-                    explanation = _compare_eq_cls(left, right, verbose, type_fn)
-                elif verbose > 0:
-                    explanation = _compare_eq_verbose(left, right)
-                if isiterable(left) and isiterable(right):
-                    expl = _compare_eq_iterable(left, right, verbose)
-                    if explanation is not None:
-                        explanation.extend(expl)
-                    else:
-                        explanation = expl
+            explanation = _compare_eq_any(left, right, verbose)
         elif op == "not in":
             if istext(left) and istext(right):
                 explanation = _notin_text(left, right, verbose)
@@ -187,6 +168,28 @@ def assertrepr_compare(config, op: str, left: Any, right: Any) -> Optional[List[
     return [summary] + explanation
 
 
+def _compare_eq_any(left: Any, right: Any, verbose: int = 0) -> List[str]:
+    explanation: List[str] = []
+    if istext(left) and istext(right):
+        explanation = _diff_text(left, right, verbose)
+    else:
+        if issequence(left) and issequence(right):
+            explanation = _compare_eq_sequence(left, right, verbose)
+        elif isset(left) and isset(right):
+            explanation = _compare_eq_set(left, right, verbose)
+        elif isdict(left) and isdict(right):
+            explanation = _compare_eq_dict(left, right, verbose)
+        elif type(left) == type(right) and (isdatacls(left) or isattrs(left)):
+            type_fn = (isdatacls, isattrs)
+            explanation = _compare_eq_cls(left, right, verbose, type_fn)
+        elif verbose > 0:
+            explanation = _compare_eq_verbose(left, right)
+        if isiterable(left) and isiterable(right):
+            expl = _compare_eq_iterable(left, right, verbose)
+            explanation.extend(expl)
+    return explanation
+
+
 def _diff_text(left: str, right: str, verbose: int = 0) -> List[str]:
     """Return the explanation for the diff between text.
 
@@ -439,7 +442,8 @@ def _compare_eq_cls(
         explanation += ["Differing attributes:"]
         for field in diff:
             explanation += [
-                ("%s: %r != %r") % (field, getattr(left, field), getattr(right, field))
+                ("%s: %r != %r") % (field, getattr(left, field), getattr(right, field)),
+                *_compare_eq_any(getattr(left, field), getattr(right, field), verbose),
             ]
     return explanation
 

testing/example_scripts/dataclasses/test_compare_recursive_dataclasses.py

@@ -0,0 +1,22 @@
+from dataclasses import dataclass
+from dataclasses import field
+
+
+@dataclass
+class SimpleDataObject:
+    field_a: int = field()
+    field_b: int = field()
+
+
+@dataclass
+class ComplexDataObject:
+    field_a: SimpleDataObject = field()
+    field_b: SimpleDataObject = field()
+
+
+def test_recursive_dataclasses():
+
+    left = ComplexDataObject(SimpleDataObject(1, "b"), SimpleDataObject(2, "c"),)
+    right = ComplexDataObject(SimpleDataObject(1, "b"), SimpleDataObject(3, "c"),)
+
+    assert left == right

testing/test_assertion.py

@@ -752,6 +752,24 @@ def test_dataclasses(self, testdir):
                 "*Omitting 1 identical items, use -vv to show*",
                 "*Differing attributes:*",
                 "*field_b: 'b' != 'c'*",
+                "*- c*",
+                "*+ b*",
+            ]
+        )
+
+    @pytest.mark.skipif(sys.version_info < (3, 7), reason="Dataclasses in Python3.7+")
+    def test_recursive_dataclasses(self, testdir):
+        p = testdir.copy_example("dataclasses/test_compare_recursive_dataclasses.py")
+        result = testdir.runpytest(p)
+        result.assert_outcomes(failed=1, passed=0)
+        result.stdout.fnmatch_lines(
+            [
+                "*Omitting 1 identical items, use -vv to show*",
+                "*Differing attributes:*",
+                "*field_b: SimpleDataObject(field_a=2, field_b='c') != SimpleDataObject(field_a=3, field_b='c')*",
+                "*Omitting 1 identical items, use -vv to show*",
+                "*Differing attributes:*",
+                "*field_a: 2 != 3*",
             ]
         )