Improve full diff output for lists

Massage text input for difflib when comparing pformat output of
different line lengths.

Also do not strip ndiff output on the left, which currently already
removes indenting for lines with no differences.

Before:

    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:

    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       ]

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 @@ class TestAssert_reprcompare:
         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