Improve full diff output for lists

changelog/5924.feature.rst

@@ -0,0 +1,34 @@
+Improve verbose diff output with sequences.
+
+Before:
+
+.. code-block::
+
+    E   AssertionError: assert ['version', '...version_info'] == ['version', '...version', ...]
+    E     Right contains 3 more items, first extra item: ' '
+    E     Full diff:
+    E     - ['version', 'version_info', 'sys.version', 'sys.version_info']
+    E     + ['version',
+    E     +  'version_info',
+    E     +  'sys.version',
+    E     +  'sys.version_info',
+    E     +  ' ',
+    E     +  'sys.version',
+    E     +  'sys.version_info']
+
+After:
+
+.. code-block::
+
+    E   AssertionError: assert ['version', '...version_info'] == ['version', '...version', ...]
+    E     Right contains 3 more items, first extra item: ' '
+    E     Full diff:
+    E       [
+    E        'version',
+    E        'version_info',
+    E        'sys.version',
+    E        'sys.version_info',
+    E     +  ' ',
+    E     +  'sys.version',
+    E     +  'sys.version_info',
+    E       ]

src/_pytest/assertion/util.py

@@ -246,6 +246,18 @@ def _compare_eq_verbose(left, right):
     return explanation
 
 
+def _surrounding_parens_on_own_lines(lines):  # type: (List) -> None
+    """Move opening/closing parenthesis/bracket to own lines."""
+    opening = lines[0][:1]
+    if opening in ["(", "[", "{"]:
+        lines[0] = " " + lines[0][1:]
+        lines[:] = [opening] + lines
+    closing = lines[-1][-1:]
+    if closing in [")", "]", "}"]:
+        lines[-1] = lines[-1][:-1] + ","
+        lines[:] = lines + [closing]
+
+
 def _compare_eq_iterable(left, right, verbose=0):
     if not verbose:
         return ["Use -v to get the full diff"]
@@ -254,9 +266,27 @@ def _compare_eq_iterable(left, right, verbose=0):
 
     left_formatting = pprint.pformat(left).splitlines()
     right_formatting = pprint.pformat(right).splitlines()
+
+    # Re-format for different output lengths.
+    lines_left = len(left_formatting)
+    lines_right = len(right_formatting)
+    if lines_left != lines_right:
+        if lines_left > lines_right:
+            max_width = min(len(x) for x in left_formatting)
+            right_formatting = pprint.pformat(right, width=max_width).splitlines()
+            lines_right = len(right_formatting)
+        else:
+            max_width = min(len(x) for x in right_formatting)
+            left_formatting = pprint.pformat(left, width=max_width).splitlines()
+            lines_left = len(left_formatting)
+
+    if lines_left > 1 or lines_right > 1:
+        _surrounding_parens_on_own_lines(left_formatting)
+        _surrounding_parens_on_own_lines(right_formatting)
+
     explanation = ["Full diff:"]
     explanation.extend(
-        line.strip() for line in difflib.ndiff(left_formatting, right_formatting)
+        line.rstrip() for line in difflib.ndiff(left_formatting, right_formatting)
     )
     return explanation
 

testing/test_assertion.py

@@ -413,6 +413,55 @@ def test_list_different_lengths(self):
         expl = callequal([0, 1, 2], [0, 1])
         assert len(expl) > 1
 
+    def test_list_wrap_for_multiple_lines(self):
+        long_d = "d" * 80
+        l1 = ["a", "b", "c"]
+        l2 = ["a", "b", "c", long_d]
+        diff = callequal(l1, l2, verbose=True)
+        assert diff == [
+            "['a', 'b', 'c'] == ['a', 'b', 'c...dddddddddddd']",
+            "Right contains one more item: '" + long_d + "'",
+            "Full diff:",
+            "  [",
+            "   'a',",
+            "   'b',",
+            "   'c',",
+            "+  '" + long_d + "',",
+            "  ]",
+        ]
+
+        diff = callequal(l2, l1, verbose=True)
+        assert diff == [
+            "['a', 'b', 'c...dddddddddddd'] == ['a', 'b', 'c']",
+            "Left contains one more item: '" + long_d + "'",
+            "Full diff:",
+            "  [",
+            "   'a',",
+            "   'b',",
+            "   'c',",
+            "-  '" + long_d + "',",
+            "  ]",
+        ]
+
+    def test_list_wrap_for_width_rewrap_same_length(self):
+        long_a = "a" * 30
+        long_b = "b" * 30
+        long_c = "c" * 30
+        l1 = [long_a, long_b, long_c]
+        l2 = [long_b, long_c, long_a]
+        diff = callequal(l1, l2, verbose=True)
+        assert diff == [
+            "['aaaaaaaaaaa...cccccccccccc'] == ['bbbbbbbbbbb...aaaaaaaaaaaa']",
+            "At index 0 diff: 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' != 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'",
+            "Full diff:",
+            "  [",
+            "-  'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa',",
+            "   'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb',",
+            "   'cccccccccccccccccccccccccccccc',",
+            "+  'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa',",
+            "  ]",
+        ]
+
     def test_dict(self):
         expl = callequal({"a": 0}, {"a": 1})
         assert len(expl) > 1