Make it possible to generate srcset with the thumbnail template tag

docs/configuration.rst

@@ -24,6 +24,15 @@ Settings
     The directory to which image files will be cached.
 
 
+.. attribute:: IMAGEKIT_DEFAULT_THUMBNAIL_SRCSET_SCALES
+
+    :default: ``None``
+
+    A list of scale factors, for example ``[2, 3]``. If specified, every
+    ``<img>`` generated by the ``thumbnail`` template tag will have a ``srcset``
+    attribute with the given scales. To prevent this, set ``srcset=None``.
+
+
 .. attribute:: IMAGEKIT_DEFAULT_FILE_STORAGE
 
     :default: ``None``

imagekit/templatetags/imagekit.py

@@ -3,6 +3,7 @@
 from django.utils.encoding import force_str
 from django.utils.html import escape
 from django.utils.safestring import mark_safe
+from django.conf import settings
 
 from ..cachefiles import ImageCacheFile
 from ..registry import generator_registry
@@ -13,6 +14,7 @@
 ASSIGNMENT_DELIMETER = 'as'
 HTML_ATTRS_DELIMITER = '--'
 DEFAULT_THUMBNAIL_GENERATOR = 'imagekit:thumbnail'
+default_thumbnail_srcset_scales = getattr(settings, 'IMAGEKIT_DEFAULT_THUMBNAIL_SRCSET_SCALES', None)
 
 
 def get_cachefile(context, generator_id, generator_kwargs, source=None):
@@ -126,11 +128,27 @@ def render(self, context):
             # recursion errors when anchor is set to a SafeString instance.
             # This converts the SafeString into a str instance.
             kwargs['anchor'] = kwargs['anchor'][:]
+        srcset_scales = default_thumbnail_srcset_scales
+        if "srcset" in kwargs:
+            if kwargs['srcset'] is not None:
+                srcset_scales = list(map(float, kwargs['srcset'].split()))
+            else:
+                srcset_scales = None
+            kwargs.pop("srcset")
         if kwargs.get('format'):
             kwargs['format'] = kwargs['format'][:]
         generator = generator_registry.get(generator_id, **kwargs)
 
         file = ImageCacheFile(generator)
+        srcset = []
+        if srcset_scales:
+            for scale in srcset_scales:
+                scaled_kwargs = dict(kwargs)
+                if scaled_kwargs.get("height"):
+                    scaled_kwargs["height"] = int(scaled_kwargs["height"] * scale)
+                if scaled_kwargs.get("width"):
+                    scaled_kwargs["width"] = int(scaled_kwargs["width"] * scale)
+                srcset.append(ImageCacheFile(generator_registry.get(generator_id, **scaled_kwargs)))
 
         attrs = {k: v.resolve(context) for k, v in self._html_attrs.items()}
 
@@ -140,6 +158,9 @@ def render(self, context):
             attrs.update(width=file.width, height=file.height)
 
         attrs['src'] = file.url
+        if len(srcset) > 0:
+            attrs['srcset'] = f'{file.url} 1x , ' + ' , '.join(
+                f'{entry[0].url} {entry[1]}x' for entry in zip(srcset, srcset_scales))
         attr_str = ' '.join('%s="%s"' % (escape(k), escape(v)) for k, v in
                 attrs.items())
         return mark_safe('<img %s />' % attr_str)
@@ -245,7 +266,11 @@ def thumbnail(parser, token):
 
     The thumbnail tag supports the "--" and "as" bits for adding html
     attributes and assigning to a variable, respectively. It also accepts the
-    kwargs "format", "anchor", and "crop".
+    kwargs "format", "srcset", "anchor", and "crop".
+
+    To use "srcset" (generating multiple thumbnails for different pixel densities) list the scale factors::
+
+        {% thumbnail '100x100' mymodel.profile_image srcset='2 3' %}
 
     To use "smart cropping" (the ``SmartResize`` processor)::
 

tests/test_thumbnail_tag.py

@@ -1,4 +1,5 @@
 import pytest
+import re
 from django.template import TemplateSyntaxError
 
 from . import imagegenerators  # noqa
@@ -89,6 +90,14 @@ def test_alternate_generator():
     assert html == '1'
 
 
+def test_srcset_arg():
+    ttag = r"""{% thumbnail '100x' img srcset="1.5 2" %}"""
+    clear_imagekit_cache()
+    html = render_tag(ttag)
+    srcset_regex = re.compile('srcset=".* 1x ,.* 1\\.5x ,.* 2.0x"')
+    assert srcset_regex.search(html) is not None
+
+
 def test_alternate_format():
     ttag = r"""{% thumbnail '100x' img format='webp' as th %}{{ th.url }}"""
     clear_imagekit_cache()