improve failure output

src/_pytest/assertion/util.py

@@ -345,15 +345,13 @@ def _compare_eq_cls(left, right, verbose, type_fns):
     if same and verbose < 2:
         explanation.append(u"Omitting %s identical items, use -vv to show" % len(same))
     elif same:
-        explanation += [u"Common attributes:"]
+        explanation += [u"Matching attributes:"]
         explanation += pprint.pformat(same).splitlines()
     if diff:
-        class_name = left.__class__.__name__
         explanation += [u"Differing attributes:"]
-        for k in diff:
+        for field in diff:
             explanation += [
-                (u"%s(%s=%r) != %s(%s=%r)")
+                (u"%s: %r != %r") % (field, getattr(left, field), getattr(right, field))
-                % (class_name, k, getattr(left, k), class_name, k, getattr(right, k))
             ]
     return explanation
 

testing/test_assertion.py

@@ -559,7 +559,7 @@ class TestAssert_reprcompare_dataclass(object):
             [
                 "*Omitting 1 identical items, use -vv to show*",
                 "*Differing attributes:*",
-                "*SimpleDataObject(field_b='b') != SimpleDataObject(field_b='c')*",
+                "*field_b: 'b' != 'c'*",
             ]
         )
 
@@ -570,10 +570,10 @@ class TestAssert_reprcompare_dataclass(object):
         result.assert_outcomes(failed=1, passed=0)
         result.stdout.fnmatch_lines(
             [
-                "*Common attributes:*",
+                "*Matching attributes:*",
                 "*['field_a']*",
                 "*Differing attributes:*",
-                "*SimpleDataObject(field_b='b') != SimpleDataObject(field_b='c')*",
+                "*field_b: 'b' != 'c'*",
             ]
         )
 
@@ -606,7 +606,7 @@ class TestAssert_reprcompare_attrsclass(object):
 
         lines = callequal(left, right)
         assert lines[1].startswith("Omitting 1 identical item")
-        assert "Common attributes" not in lines
+        assert "Matching attributes" not in lines
         for line in lines[1:]:
             assert "field_a" not in line
 
@@ -620,7 +620,7 @@ class TestAssert_reprcompare_attrsclass(object):
         right = SimpleDataObject(1, "c")
 
         lines = callequal(left, right, verbose=2)
-        assert lines[1].startswith("Common attributes:")
+        assert lines[1].startswith("Matching attributes:")
         assert "Omitting" not in lines[1]
         assert lines[2] == "['field_a']"
 
@@ -634,7 +634,7 @@ class TestAssert_reprcompare_attrsclass(object):
         right = SimpleDataObject(1, "b")
 
         lines = callequal(left, right, verbose=2)
-        assert lines[1].startswith("Common attributes:")
+        assert lines[1].startswith("Matching attributes:")
         assert "Omitting" not in lines[1]
         assert lines[2] == "['field_a']"
         for line in lines[2:]: