Detect moved items in ordered iterables

README.md

@@ -17,6 +17,18 @@
 
 Tested on Python 3.9+ and PyPy3.
 
+### Detect moved items in lists
+
+DeepDiff reports items that only change position in an ordered iterable under
+the ``iterable_item_moved`` key:
+
+```python
+>>> from deepdiff import DeepDiff
+>>> DeepDiff([1, 2, 3, 4], [4, 2, 3, 1], verbose_level=2)
+{'iterable_item_moved': {'root[0]': {'new_path': 'root[3]', 'value': 1},
+                         'root[3]': {'new_path': 'root[0]', 'value': 4}}}
+```
+
 - **[Documentation](https://zepworks.com/deepdiff/8.6.0/)**
 
 ## What is new?

deepdiff/diff.py

@@ -906,7 +906,7 @@ def _diff_iterable_in_order(self, level, parents_ids=frozenset(), _original_type
                     child_relationship_class=child_relationship_class,
                     local_tree=local_tree_pass2,
                 )
-                if len(local_tree_pass) >= len(local_tree_pass2):
+                if len(local_tree_pass) > len(local_tree_pass2):
                     local_tree_pass = local_tree_pass2
                 else:
                     self._iterable_opcodes[level.path(force=FORCE_DEFAULT)] = opcodes_with_values
@@ -1015,32 +1015,28 @@ def _diff_ordered_iterable_by_difflib(
 
         opcodes = seq.get_opcodes()
         opcodes_with_values = []
+        replace_opcodes: List[Opcode] = []
 
-        # TODO: this logic should be revisted so we detect reverse operations
-        # like when a replacement happens at index X and a reverse replacement happens at index Y
-        # in those cases we have a "iterable_item_moved" operation.
         for tag, t1_from_index, t1_to_index, t2_from_index, t2_to_index in opcodes:
             if tag == 'equal':
-                opcodes_with_values.append(Opcode(
-                    tag, t1_from_index, t1_to_index, t2_from_index, t2_to_index,
-                ))
+                opcodes_with_values.append(
+                    Opcode(tag, t1_from_index, t1_to_index, t2_from_index, t2_to_index)
+                )
                 continue
-            # print('{:7}   t1[{}:{}] --> t2[{}:{}] {!r:>8} --> {!r}'.format(
-            #     tag, t1_from_index, t1_to_index, t2_from_index, t2_to_index, level.t1[t1_from_index:t1_to_index], level.t2[t2_from_index:t2_to_index]))
 
-            opcodes_with_values.append(Opcode(
-                tag, t1_from_index, t1_to_index, t2_from_index, t2_to_index,
-                old_values = level.t1[t1_from_index: t1_to_index],
-                new_values = level.t2[t2_from_index: t2_to_index],
-            ))
+            opcode = Opcode(
+                tag,
+                t1_from_index,
+                t1_to_index,
+                t2_from_index,
+                t2_to_index,
+                old_values=level.t1[t1_from_index:t1_to_index],
+                new_values=level.t2[t2_from_index:t2_to_index],
+            )
+            opcodes_with_values.append(opcode)
 
             if tag == 'replace':
-                self._diff_by_forming_pairs_and_comparing_one_by_one(
-                    level, local_tree=local_tree, parents_ids=parents_ids,
-                    _original_type=_original_type, child_relationship_class=child_relationship_class,
-                    t1_from_index=t1_from_index, t1_to_index=t1_to_index,
-                    t2_from_index=t2_from_index, t2_to_index=t2_to_index,
-                )
+                replace_opcodes.append(opcode)
             elif tag == 'delete':
                 for index, x in enumerate(level.t1[t1_from_index:t1_to_index]):
                     change_level = level.branch_deeper(
@@ -1061,6 +1057,63 @@ def _diff_ordered_iterable_by_difflib(
                         child_relationship_param2=index + t2_from_index,
                     )
                     self._report_result('iterable_item_added', change_level, local_tree=local_tree)
+
+        used: Set[int] = set()
+        for i, opcode_a in enumerate(replace_opcodes):
+            if i in used:
+                continue
+            for j in range(i + 1, len(replace_opcodes)):
+                opcode_b = replace_opcodes[j]
+                if j in used:
+                    continue
+                if (
+                    opcode_a.old_values == opcode_b.new_values
+                    and opcode_a.new_values == opcode_b.old_values
+                    and len(opcode_a.old_values or []) == len(opcode_b.old_values or [])
+                ):
+                    # Mark both opcodes as consumed before any further processing
+                    used.update({i, j})
+                    length = len(opcode_a.old_values or [])
+                    for offset in range(length):
+                        val_a = opcode_a.old_values[offset]
+                        new_index_a = opcode_b.t2_from_index + offset
+                        change_level = level.branch_deeper(
+                            val_a,
+                            val_a,
+                            child_relationship_class=child_relationship_class,
+                            child_relationship_param=opcode_a.t1_from_index + offset,
+                            child_relationship_param2=new_index_a,
+                        )
+                        self._report_result('iterable_item_moved', change_level, local_tree=local_tree)
+
+                        val_b = opcode_b.old_values[offset]
+                        new_index_b = opcode_a.t2_from_index + offset
+                        change_level = level.branch_deeper(
+                            val_b,
+                            val_b,
+                            child_relationship_class=child_relationship_class,
+                            child_relationship_param=opcode_b.t1_from_index + offset,
+                            child_relationship_param2=new_index_b,
+                        )
+                        self._report_result('iterable_item_moved', change_level, local_tree=local_tree)
+
+                    break
+
+        for idx, opcode in enumerate(replace_opcodes):
+            if idx in used:
+                continue
+            self._diff_by_forming_pairs_and_comparing_one_by_one(
+                level,
+                local_tree=local_tree,
+                parents_ids=parents_ids,
+                _original_type=_original_type,
+                child_relationship_class=child_relationship_class,
+                t1_from_index=opcode.t1_from_index,
+                t1_to_index=opcode.t1_to_index,
+                t2_from_index=opcode.t2_from_index,
+                t2_to_index=opcode.t2_to_index,
+            )
+
         return opcodes_with_values
 
 

docs/basics.rst

@@ -106,6 +106,14 @@ List difference
     >>> pprint (ddiff, indent = 2)
     {'iterable_item_removed': {"root[4]['b'][2]": 3, "root[4]['b'][3]": 4}}
 
+List item moved
+    >>> t1 = [1, 2, 3, 4]
+    >>> t2 = [4, 2, 3, 1]
+    >>> pprint(DeepDiff(t1, t2, verbose_level=2), indent=2)
+    { 'iterable_item_moved': {
+        'root[0]': {'new_path': 'root[3]', 'value': 1},
+        'root[3]': {'new_path': 'root[0]', 'value': 4}}}
+
 List that contains dictionary:
     >>> t1 = {1:1, 2:2, 3:3, 4:{"a":"hello", "b":[1, 2, {1:1, 2:2}]}}
     >>> t2 = {1:1, 2:2, 3:3, 4:{"a":"hello", "b":[1, 2, {1:3}]}}

tests/test_diff_text.py

@@ -1819,14 +1819,17 @@ def test_list_item_removed_from_the_middle(self):
         assert {"root[4]"} == diff.affected_paths
         assert {4} == diff.affected_root_keys
 
-    # TODO: we need to support reporting that items have been swapped
-    # def test_item_moved(self):
-    #     # currently all the items in the list need to be hashables
-    #     t1 = [1, 2, 3, 4]
-    #     t2 = [4, 2, 3, 1]
-    #     diff = DeepDiff(t1, t2)
-    #     result = {}  # it should show that those items are swapped.
-    #     assert result == diff
+    def test_item_moved(self):
+        t1 = [1, 2, 3, 4]
+        t2 = [4, 2, 3, 1]
+        diff = DeepDiff(t1, t2, verbose_level=2)
+        result = {
+            'iterable_item_moved': {
+                'root[0]': {'new_path': 'root[3]', 'value': 1},
+                'root[3]': {'new_path': 'root[0]', 'value': 4},
+            }
+        }
+        assert result == diff
 
     def test_list_item_values_replace_in_the_middle(self):
         t1 = [0, 1, 2, 3, 'bye', 5, 6, 7, 8, 'a', 'b', 'c']