Update ImageRendition field to return Image alt text

* Update ImageRendtion to use default_alt_text property of Image
* Update example in documentation
* Add a test for ImageRenditionField
* Resolves #5816

CHANGELOG.txt

@@ -21,6 +21,7 @@ Changelog
  * Add Reddit to oEmbed provider list (Luke Hardwick)
  * Add ability to replace the default Wagtail logo in the userbar, via `branding_logo` block (Meteor0id)
  * Remove sticky footer on small devices, so that content is not blocked and more easily editable (Saeed Tahmasebi)
+ * Add ``alt`` property to ``ImageRenditionField`` api representation (Liam Mullens)
  * Fix: Support IPv6 domain (Alex Gleason, Coen van der Kamp)
  * Fix: Ensure link to add a new user works when no users are visible in the users list (LB (Ben Johnston))
  * Fix: `AbstractEmailForm` saved submission fields are now aligned with the email content fields, `form.cleaned_data` will be used instead of `form.fields` (Haydn Greatnews)

CONTRIBUTORS.rst

@@ -458,6 +458,7 @@ Contributors
 * Rick van Hattem
 * Luke Hardwick
 * Saeed Tahmasebi
+* Liam Mullens
 
 Translators
 ===========

docs/advanced_topics/api/v2/configuration.rst

@@ -240,7 +240,8 @@ This would add the following to the JSON:
         "feed_image_thumbnail": {
             "url": "/media/images/a_test_image.fill-100x100.jpg",
             "width": 100,
-            "height": 100
+            "height": 100,
+            "alt": "image alt text"
         }
     }
 

docs/releases/2.10.rst

@@ -30,6 +30,7 @@ Other features
  * Add Reddit to oEmbed provider list (Luke Hardwick)
  * Remove sticky footer on small devices, so that content is not blocked and more easily editable (Saeed Tahmasebi)
  * Add ability to replace the default Wagtail logo in the userbar, via ``branding_logo`` block (Meteor0id)
+ * Add `alt` property to `ImageRenditionField` api representation (Liam Mullens)
 
 
 Bug fixes

wagtail/images/api/fields.py

@@ -14,7 +14,8 @@ class ImageRenditionField(Field):
     "thumbnail": {
         "url": "/media/images/myimage.max-165x165.jpg",
         "width": 165,
-        "height": 100
+        "height": 100,
+        "alt": "Image alt text"
     }
 
     If there is an error with the source image. The dict will only contain a single
@@ -36,6 +37,7 @@ class ImageRenditionField(Field):
                 ('url', thumbnail.url),
                 ('width', thumbnail.width),
                 ('height', thumbnail.height),
+                ('alt', thumbnail.alt),
             ])
         except SourceImageIOError:
             return OrderedDict([

wagtail/images/models.py

@@ -506,7 +506,7 @@ class AbstractRendition(models.Model):
 
     @property
     def alt(self):
-        return self.image.title
+        return self.image.default_alt_text
 
     @property
     def attrs(self):

wagtail/images/tests/test_api_fields.py

@@ -0,0 +1,22 @@
+from django.test import TestCase
+
+from wagtail.images.api.fields import ImageRenditionField
+
+from .utils import Image, get_test_image_file
+
+
+class TestImageRenditionField(TestCase):
+    def setUp(self):
+        self.image = Image.objects.create(
+            title="Test image",
+            file=get_test_image_file(),
+        )
+
+    def test_api_representation(self):
+        rendition = self.image.get_rendition('width-400')
+        representation = ImageRenditionField('width-400').to_representation(self.image)
+        self.assertEqual(set(representation.keys()), {'url', 'width', 'height', 'alt'})
+        self.assertEqual(representation['url'], rendition.url)
+        self.assertEqual(representation['width'], rendition.width)
+        self.assertEqual(representation['height'], rendition.height)
+        self.assertEqual(representation['alt'], rendition.alt)