Fix cursor limit&skip when passed as slice

mongomock/collection.py

@@ -1707,8 +1707,9 @@ def __iter__(self):
         return self
 
     def clone(self):
-        return Cursor(self.collection,
-                      self._spec, self._sort, self._projection, self._skip, self._limit)
+        cursor = Cursor(self.collection,
+                        self._spec, self._sort, self._projection, self._skip, self._limit)
+        return cursor
 
     def __next__(self):
         if self._skip and not self._skipped:
@@ -1792,8 +1793,28 @@ def distinct(self, key):
 
     def __getitem__(self, index):
         if isinstance(index, slice):
-            # Limit the cursor to the given slice
-            self._dataset = (x for x in list(self._dataset)[index])
+            if index.step is not None:
+                raise IndexError("Cursor instances do not support slice steps")
+
+            skip = 0
+            if index.start is not None:
+                if index.start < 0:
+                    raise IndexError("Cursor instances do not support"
+                                     "negative indices")
+                skip = index.start
+
+            if index.stop is not None:
+                limit = index.stop - skip
+                if limit < 0:
+                    raise IndexError("stop index must be greater than start"
+                                     "index for slice %r" % index)
+                if limit == 0:
+                    self.__empty = True
+            else:
+                limit = 0
+
+            self._skip = skip
+            self._limit = limit
             return self
         elif not isinstance(index, int):
             raise TypeError("index '%s' cannot be applied to Cursor instances" % index)

tests/test__collection_api.py

@@ -159,6 +159,20 @@ def test__cursor_clone(self):
         with self.assertRaises(StopIteration):
             next(iterator2)
 
+    def test__cursor_clone_keep_limit_skip(self):
+        self.db.collection.insert([{"a": "b"}, {"b": "c"}, {"c": "d"}])
+        cursor1 = self.db.collection.find()[1:2]
+        cursor2 = cursor1.clone()
+        result1 = list(cursor1)
+        result2 = list(cursor2)
+        self.assertEqual(result1, result2)
+
+        cursor3 = self.db.collection.find(skip=1, limit=1)
+        cursor4 = cursor3.clone()
+        result3 = list(cursor3)
+        result4 = list(cursor4)
+        self.assertEqual(result3, result4)
+
     def test_cursor_returns_document_copies(self):
         obj = {'a': 1, 'b': 2}
         self.db.collection.insert(obj)
@@ -654,6 +668,8 @@ def test__cursor_getitem_slice(self):
         ret = cursor[1:4]
         self.assertIs(ret, cursor)
         count = cursor.count()
+        self.assertEqual(count, 3)
+        count = cursor.count(with_limit_and_skip=True)
         self.assertEqual(count, 2)
 
     def test__cursor_getitem_negative_index(self):