renderers: keep the classes a widget template sets

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Fix `RadioSelectButtonGroup` not rendering as a button group. The renderer replaced the class attribute of a widget's enclosing element instead of adding to it, discarding `btn-group btn-group-toggle`. Django's own `RadioSelect` and `CheckboxSelectMultiple` set no class there, so their output is unchanged (#894).
 - Fix `{% buttons layout="horizontal" %}` emitting Bootstrap 3 markup: the label carried `control-label` instead of `col-form-label`, and the wrapper had no `row` for the `col-*` classes to divide, so the button did not line up with the fields above it (#895).
 - **Breaking:** Fix invalid HTML in `RadioSelectButtonGroup`. Every option label carried the widget's own `id`, so a group of N options rendered N+1 identical `id` attributes and no label was associated with its input. Labels now carry `for` pointing at their own input and no `id` (#309).
 - Fix `form_check_class` being ignored for single checkboxes; it was only applied to `RadioSelect` and `CheckboxSelectMultiple` (#290).

src/bootstrap4/renderers.py

@@ -324,20 +324,23 @@ def list_to_class(self, html, klass):
         classes = add_css_class(klass, self.get_size_class())
         soup = BeautifulSoup(html, features="html.parser")
         enclosing_div = soup.find("div")
-        enclosing_div.attrs["class"] = classes
+        # Add to the widget's own classes rather than replacing them. Django's RadioSelect and
+        # CheckboxSelectMultiple templates put no class here, so this is a no-op for them, but a
+        # custom widget template may depend on its classes, as RadioSelectButtonGroup does on
+        # btn-group to be a button group at all.
+        widget_classes = " ".join(enclosing_div.attrs.get("class", []))
+        enclosing_div.attrs["class"] = add_css_class(widget_classes, classes)
         for inner_div in enclosing_div.find_all("div"):
             inner_div.attrs["class"] = inner_div.attrs.get("class", []) + [self.form_check_class]
         # Apply bootstrap4 classes to labels and inputs.
         # A simple 'replace' isn't enough as we don't want to have several 'class' attr definition, which would happen
         # if we tried to 'html.replace("input", "input class=...")'
-        enclosing_div = soup.find("div", {"class": classes})
-        if enclosing_div:
-            for label in enclosing_div.find_all("label"):
-                label.attrs["class"] = label.attrs.get("class", []) + ["form-check-label"]
-                try:
-                    label.input.attrs["class"] = label.input.attrs.get("class", []) + ["form-check-input"]
-                except AttributeError:
-                    pass
+        for label in enclosing_div.find_all("label"):
+            label.attrs["class"] = label.attrs.get("class", []) + ["form-check-label"]
+            try:
+                label.input.attrs["class"] = label.input.attrs.get("class", []) + ["form-check-input"]
+            except AttributeError:
+                pass
         return str(soup)
 
     def get_checkbox_label_class(self):

tests/test_forms.py

@@ -139,6 +139,22 @@ def test_checkbox_form_check_class(self):
         self.assertIn('<div class="custom-control">', res)
         self.assertNotIn('<div class="form-check">', res)
 
+    def test_radio_select_button_group_keeps_widget_classes(self):
+        """The renderer must not strip the classes a custom widget template sets (#894)."""
+        res = render_form_field("category5")
+        soup = BeautifulSoup(res, "html.parser")
+        enclosing_div = soup.select_one(".form-group > div")
+        self.assertIn("btn-group", enclosing_div["class"])
+        self.assertIn("btn-group-toggle", enclosing_div["class"])
+
+    def test_radio_select_keeps_default_classes(self):
+        """Django's own widgets set no class here, so their output is unchanged (#894)."""
+        for field in ("category1", "category2"):
+            with self.subTest(field=field):
+                soup = BeautifulSoup(render_form_field(field), "html.parser")
+                enclosing_div = soup.select_one(".form-group > div")
+                self.assertNotIn("btn-group", enclosing_div["class"])
+
     def test_radio_select_button_group_label_ids(self):
         """Each button group label must point at its own input, with no duplicate ids (#309)."""
         res = render_form_field("category5")