Improve HTML form rendering of choice fields

Expose basic value on BoundField instances and use it to improve
rendering of choice fields in HTML form output.

Fixes encode#4120, encode#4121

rest_framework/templates/rest_framework/horizontal/select.html

@@ -16,7 +16,7 @@
           {% elif select.end_option_group %}
             </optgroup>
           {% else %}
-            <option value="{{ select.value }}" {% if select.value == field.value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
+            <option value="{{ select.value }}" {% if select.value == field.basic_value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
           {% endif %}
       {% endfor %}
     </select>

rest_framework/templates/rest_framework/horizontal/select_multiple.html

@@ -16,7 +16,7 @@
         {% elif select.end_option_group %}
           </optgroup>
         {% else %}
-          <option value="{{ select.value }}" {% if select.value in field.value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
+          <option value="{{ select.value }}" {% if select.value in field.basic_value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
         {% endif %}
       {% empty %}
           <option>{{ no_items }}</option>

rest_framework/templates/rest_framework/inline/select.html

@@ -15,7 +15,7 @@
         {% elif select.end_option_group %}
           </optgroup>
         {% else %}
-          <option value="{{ select.value }}" {% if select.value == field.value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
+          <option value="{{ select.value }}" {% if select.value == field.basic_value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
         {% endif %}
     {% endfor %}
   </select>

rest_framework/templates/rest_framework/inline/select_multiple.html

@@ -15,7 +15,7 @@
           {% elif select.end_option_group %}
             </optgroup>
           {% else %}
-            <option value="{{ select.value }}" {% if select.value in field.value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
+            <option value="{{ select.value }}" {% if select.value in field.basic_value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
           {% endif %}
       {% empty %}
       <option>{{ no_items }}</option>

rest_framework/templates/rest_framework/vertical/select.html

@@ -15,7 +15,7 @@
         {% elif select.end_option_group %}
           </optgroup>
         {% else %}
-          <option value="{{ select.value }}" {% if select.value == field.value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
+          <option value="{{ select.value }}" {% if select.value == field.basic_value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
         {% endif %}
     {% endfor %}
   </select>

rest_framework/templates/rest_framework/vertical/select_multiple.html

@@ -15,7 +15,7 @@
         {% elif select.end_option_group %}
           </optgroup>
         {% else %}
-          <option value="{{ select.value }}" {% if select.value in field.value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
+          <option value="{{ select.value }}" {% if select.value in field.basic_value %}selected{% endif %} {% if select.disabled %}disabled{% endif %}>{{ select.display_text }}</option>
         {% endif %}
     {% empty %}
         <option>{{ no_items }}</option>

rest_framework/utils/serializer_helpers.py

@@ -8,6 +8,10 @@
 from rest_framework.compat import unicode_to_repr
 
 
+# A singleton helper value to detect unset keyword arguments
+unset = object()
+
+
 class ReturnDict(OrderedDict):
     """
     Return object from `serialier.data` for the `Serializer` class.
@@ -58,10 +62,11 @@ class BoundField(object):
     providing an API similar to Django forms and form fields.
     """
 
-    def __init__(self, field, value, errors, prefix=''):
+    def __init__(self, field, value, errors, prefix='', basic_value=unset):
         self._field = field
         self._prefix = prefix
         self.value = value
+        self.basic_value = value if basic_value is unset else basic_value
         self.errors = errors
         self.name = prefix + self.field_name
 
@@ -79,7 +84,8 @@ def __repr__(self):
 
     def as_form_field(self):
         value = '' if (self.value is None or self.value is False) else force_text(self.value)
-        return self.__class__(self._field, value, self.errors, self._prefix)
+        return self.__class__(self._field, value, self.errors, self._prefix,
+                              self.basic_value)
 
 
 class NestedBoundField(BoundField):
@@ -89,10 +95,11 @@ class NestedBoundField(BoundField):
     `BoundField` that is used for serializer fields.
     """
 
-    def __init__(self, field, value, errors, prefix=''):
+    def __init__(self, field, value, errors, prefix='', basic_value=unset):
         if value is None or value is '':
             value = {}
-        super(NestedBoundField, self).__init__(field, value, errors, prefix)
+        super(NestedBoundField, self).__init__(field, value, errors, prefix,
+                                               basic_value)
 
     def __iter__(self):
         for field in self.fields.values():
@@ -113,7 +120,8 @@ def as_form_field(self):
                 values[key] = value
             else:
                 values[key] = '' if (value is None or value is False) else force_text(value)
-        return self.__class__(self._field, values, self.errors, self._prefix)
+        return self.__class__(self._field, values, self.errors, self._prefix,
+                              self.basic_value)
 
 
 class BindingDict(collections.MutableMapping):