[1.9.x] Fixed #25942 -- Fixed TypedChoiceField.has_changed with nullable field

This fixes a regression introduced by 871440361.
Backport of d91cc25a2a from master.

django/forms/fields.py

@@ -191,17 +191,16 @@ class Field(six.with_metaclass(RenameFieldMethods, object)):
         """
         Return True if data differs from initial.
         """
-        # For purposes of seeing whether something has changed, None is
-        # the same as an empty string, if the data or initial value we get
-        # is None, replace it w/ ''.
-        initial_value = initial if initial is not None else ''
         try:
             data = self.to_python(data)
             if hasattr(self, '_coerce'):
-                data = self._coerce(data)
-                initial_value = self._coerce(initial_value)
+                return self._coerce(data) != self._coerce(initial)
         except ValidationError:
             return True
+        # For purposes of seeing whether something has changed, None is
+        # the same as an empty string, if the data or initial value we get
+        # is None, replace it with ''.
+        initial_value = initial if initial is not None else ''
         data_value = data if data is not None else ''
         return initial_value != data_value
 

docs/releases/1.9.1.txt

@@ -43,3 +43,6 @@ Bugfixes
 
 * Fixed a state bug when using an ``AlterModelManagers`` operation
   (:ticket:`25852`).
+
+* Fixed ``TypedChoiceField`` change detection with nullable fields
+  (:ticket:`25942`).

tests/forms_tests/tests/test_fields.py

@@ -1257,6 +1257,14 @@ class FieldsTests(SimpleTestCase):
         self.assertFalse(f.has_changed(1, '1'))
         self.assertFalse(f.has_changed('1', '1'))
 
+        f = TypedChoiceField(
+            choices=[('', '---------'), ('a', "a"), ('b', "b")], coerce=six.text_type,
+            required=False, initial=None, empty_value=None,
+        )
+        self.assertFalse(f.has_changed(None, ''))
+        self.assertTrue(f.has_changed('', 'a'))
+        self.assertFalse(f.has_changed('a', 'a'))
+
     def test_typedchoicefield_special_coerce(self):
         """
         Test a coerce function which results in a value not present in choices.