Support slicing on StreamValue (#7013)

Fixes #7012

CHANGELOG.txt

@@ -22,6 +22,7 @@ Changelog
  * Add a new switch input widget as an alternative to checkboxes (Karl Hobley)
  * Allow `{% pageurl %}` fallback to be a direct URL or an object with a `get_absolute_url` method (Andy Babic)
  * Add support for exporting redirects (Martin Sandström)
+ * Support slicing on StreamField / StreamBlock values (Matt Westcott)
  * Fix: StreamField required status is now consistently handled by the `blank` keyword argument (Matt Westcott)
  * Fix: Show 'required' asterisks for blocks inside required StreamFields (Matt Westcott)
  * Fix: Make image chooser "Select format" fields translatable (Helen Chapman, Thibaud Colas)

docs/releases/2.13.rst

@@ -45,6 +45,8 @@ Other features
 * Add a new switch input widget as an alternative to checkboxes (Karl Hobley)
 * Allow ``{% pageurl %}`` fallback to be a direct URL or an object with a ``get_absolute_url`` method (Andy Babic)
 * Add support for exporting redirects (Martin Sandström)
+* Support slicing on StreamField / StreamBlock values (Matt Westcott)
+
 
 Bug fixes
 ~~~~~~~~~

wagtail/core/blocks/stream_block.py

@@ -505,6 +505,10 @@ class StreamValue(MutableSequence):
         return StreamValue.StreamChild(block_def, value, id=block_id)
 
     def __getitem__(self, i):
+        if isinstance(i, slice):
+            start, stop, step = i.indices(len(self._bound_blocks))
+            return [self[j] for j in range(start, stop, step)]
+
         if self._bound_blocks[i] is None:
             raw_value = self._raw_data[i]
             self._prefetch_blocks(raw_value['type'])

wagtail/core/tests/test_streamfield.py

@@ -27,6 +27,11 @@ class TestLazyStreamField(TestCase):
         self.no_image = StreamModel.objects.create(body=json.dumps([
             {'type': 'text', 'value': 'foo'}]))
         self.nonjson_body = StreamModel.objects.create(body="<h1>hello world</h1>")
+        self.three_items = StreamModel.objects.create(body=json.dumps([
+            {'type': 'text', 'value': 'foo'},
+            {'type': 'image', 'value': self.image.pk},
+            {'type': 'text', 'value': 'bar'},
+        ]))
 
     def test_lazy_load(self):
         """
@@ -50,6 +55,29 @@ class TestLazyStreamField(TestCase):
             self.assertEqual(body[0].value, self.image)
             self.assertEqual(body[1].value, 'foo')
 
+    def test_slice(self):
+        with self.assertNumQueries(1):
+            instance = StreamModel.objects.get(pk=self.three_items.pk)
+
+        with self.assertNumQueries(1):
+            # Access the image item from the stream. The image is fetched now
+            instance.body[1].value
+
+        with self.assertNumQueries(0):
+            # taking a slice of a StreamValue should re-use already-fetched values
+            values = [block.value for block in instance.body[1:3]]
+            self.assertEqual(values, [self.image, 'bar'])
+
+        with self.assertNumQueries(0):
+            # test slicing with negative indexing
+            values = [block.value for block in instance.body[-2:]]
+            self.assertEqual(values, [self.image, 'bar'])
+
+        with self.assertNumQueries(0):
+            # test slicing with skips
+            values = [block.value for block in instance.body[0:3:2]]
+            self.assertEqual(values, ['foo', 'bar'])
+
     def test_lazy_load_no_images(self):
         """
         Getting a single item whose StreamField never accesses the database