[1.11.x] Fixed #27920 -- Restored empty RadioSelect choice producing value=""

Regression in b52c73008a.
Thanks Tim Graham for the review.
Backport of 540ae68a5c from master.

django/contrib/admin/widgets.py

@@ -350,13 +350,9 @@ class AdminURLFieldWidget(forms.URLInput):
         context = super(AdminURLFieldWidget, self).get_context(name, value, attrs)
         context['current_label'] = _('Currently:')
         context['change_label'] = _('Change:')
-        context['widget']['href'] = smart_urlquote(context['widget']['value'])
+        context['widget']['href'] = smart_urlquote(context['widget']['value']) if value else ''
         return context
 
-    def format_value(self, value):
-        value = super(AdminURLFieldWidget, self).format_value(value)
-        return force_text(value)
-
 
 class AdminIntegerFieldWidget(forms.NumberInput):
     class_name = 'vIntegerField'

django/forms/jinja2/django/forms/widgets/input.html

@@ -1 +1 @@
-<input type="{{ widget.type }}" name="{{ widget.name }}"{% if widget.value != None and widget.value != "" %} value="{{ widget.value }}"{% endif %}{% include "django/forms/widgets/attrs.html" %} />
+<input type="{{ widget.type }}" name="{{ widget.name }}"{% if widget.value != None %} value="{{ widget.value }}"{% endif %}{% include "django/forms/widgets/attrs.html" %} />

django/forms/templates/django/forms/widgets/input.html

@@ -1 +1 @@
-<input type="{{ widget.type }}" name="{{ widget.name }}"{% if widget.value != None and widget.value != "" %} value="{{ widget.value }}"{% endif %}{% include "django/forms/widgets/attrs.html" %} />
+<input type="{{ widget.type }}" name="{{ widget.name }}"{% if widget.value != None %} value="{{ widget.value }}"{% endif %}{% include "django/forms/widgets/attrs.html" %} />

django/forms/widgets.py

@@ -195,8 +195,8 @@ class Widget(six.with_metaclass(RenameWidgetMethods)):
         """
         Return a value as it should appear when rendered in a template.
         """
-        if value is None:
-            value = ''
+        if value == '' or value is None:
+            return None
         if self.is_localized:
             return formats.localize_input(value)
         return force_text(value)

tests/forms_tests/widget_tests/test_radioselect.py

@@ -7,8 +7,10 @@ class RadioSelectTest(WidgetTest):
     widget = RadioSelect
 
     def test_render(self):
-        self.check_html(self.widget(choices=self.beatles), 'beatle', 'J', html=(
+        choices = (('', '------'),) + self.beatles
+        self.check_html(self.widget(choices=choices), 'beatle', 'J', html=(
             """<ul>
+            <li><label><input type="radio" name="beatle" value="" /> ------</label></li>
             <li><label><input checked type="radio" name="beatle" value="J" /> John</label></li>
             <li><label><input type="radio" name="beatle" value="P" /> Paul</label></li>
             <li><label><input type="radio" name="beatle" value="G" /> George</label></li>

tests/forms_tests/widget_tests/test_widget.py

@@ -8,6 +8,13 @@ from .base import WidgetTest
 
 class WidgetTests(WidgetTest):
 
+    def test_format_value(self):
+        widget = Widget()
+        self.assertIsNone(widget.format_value(None))
+        self.assertIsNone(widget.format_value(''))
+        self.assertEqual(widget.format_value('español'), 'español')
+        self.assertEqual(widget.format_value(42.5), '42.5')
+
     def test_value_omitted_from_data(self):
         widget = Widget()
         self.assertIs(widget.value_omitted_from_data({}, {}, 'field'), True)