Add tests and improve messages

src/_pytest/assertion/util.py

@@ -292,15 +292,22 @@ def _compare_eq_sequence(left, right, verbose=0):
             explanation += [u"At index %s diff: %r != %r" % (i, left[i], right[i])]
             break
     len_diff = len_left - len_right
+
+    if len_diff:
         if len_diff > 0:
-        explanation += [
+            dir_with_more = "Left"
-            u"Left contains %d more items, first extra item: %s"
+            extra = saferepr(left[len_right])
-            % (len_diff, saferepr(left[len_right]))
-        ]
         elif len_diff < 0:
+            len_diff = 0 - len_diff
+            dir_with_more = "Right"
+            extra = saferepr(right[len_left])
+
+        if len_diff == 1:
+            explanation += [u"%s contains one more item: %s" % (dir_with_more, extra)]
+        else:
             explanation += [
-            u"Right contains %d more items, first extra item: %s"
+                u"%s contains %d more items, first extra item: %s"
-            % (0 - len_diff, saferepr(right[len_left]))
+                % (dir_with_more, len_diff, extra)
             ]
     return explanation
 
@@ -337,14 +344,22 @@ def _compare_eq_dict(left, right, verbose=0):
         for k in diff:
             explanation += [saferepr({k: left[k]}) + " != " + saferepr({k: right[k]})]
     extra_left = set_left - set_right
-    if extra_left:
+    len_extra_left = len(extra_left)
-        explanation.append(u"Left contains %d more items:" % len(extra_left))
+    if len_extra_left:
+        explanation.append(
+            u"Left contains %d more item%s:"
+            % (len_extra_left, "" if len_extra_left == 1 else "s")
+        )
         explanation.extend(
             pprint.pformat({k: left[k] for k in extra_left}).splitlines()
         )
     extra_right = set_right - set_left
-    if extra_right:
+    len_extra_right = len(extra_right)
-        explanation.append(u"Right contains %d more items:" % len(extra_right))
+    if len_extra_right:
+        explanation.append(
+            u"Right contains %d more item%s:"
+            % (len_extra_right, "" if len_extra_right == 1 else "s")
+        )
         explanation.extend(
             pprint.pformat({k: right[k] for k in extra_right}).splitlines()
         )

testing/test_assertion.py

@@ -446,6 +446,50 @@ class TestAssert_reprcompare(object):
         assert "Omitting" not in lines[1]
         assert lines[2] == "{'b': 1}"
 
+    def test_dict_different_items(self):
+        lines = callequal({"a": 0}, {"b": 1, "c": 2}, verbose=2)
+        assert lines == [
+            "{'a': 0} == {'b': 1, 'c': 2}",
+            "Left contains 1 more item:",
+            "{'a': 0}",
+            "Right contains 2 more items:",
+            "{'b': 1, 'c': 2}",
+            "Full diff:",
+            "- {'a': 0}",
+            "+ {'b': 1, 'c': 2}",
+        ]
+        lines = callequal({"b": 1, "c": 2}, {"a": 0}, verbose=2)
+        assert lines == [
+            "{'b': 1, 'c': 2} == {'a': 0}",
+            "Left contains 2 more items:",
+            "{'b': 1, 'c': 2}",
+            "Right contains 1 more item:",
+            "{'a': 0}",
+            "Full diff:",
+            "- {'b': 1, 'c': 2}",
+            "+ {'a': 0}",
+        ]
+
+    def test_sequence_different_items(self):
+        lines = callequal((1, 2), (3, 4, 5), verbose=2)
+        assert lines == [
+            "(1, 2) == (3, 4, 5)",
+            "At index 0 diff: 1 != 3",
+            "Right contains one more item: 5",
+            "Full diff:",
+            "- (1, 2)",
+            "+ (3, 4, 5)",
+        ]
+        lines = callequal((1, 2, 3), (4,), verbose=2)
+        assert lines == [
+            "(1, 2, 3) == (4,)",
+            "At index 0 diff: 1 != 4",
+            "Left contains 2 more items, first extra item: 2",
+            "Full diff:",
+            "- (1, 2, 3)",
+            "+ (4,)",
+        ]
+
     def test_set(self):
         expl = callequal({0, 1}, {0, 2})
         assert len(expl) > 1