Fixed #19997 -- Added custom EMPTY_VALUES to form fields

Thanks Loic Bistuer for the report and the patch.

tests/forms_tests/tests/forms.py

@@ -1797,3 +1797,23 @@ class FormsTestCase(TestCase):
         form = NameForm(data={'name' : ['fname', 'lname']})
         self.assertTrue(form.is_valid())
         self.assertEqual(form.cleaned_data, {'name' : 'fname lname'})
+
+    def test_custom_empty_values(self):
+        """
+        Test that form fields can customize what is considered as an empty value
+        for themselves (#19997).
+        """
+        class CustomJSONField(CharField):
+            empty_values = [None, '']
+            def to_python(self, value):
+                # Fake json.loads
+                if value == '{}':
+                    return {}
+                return super(CustomJSONField, self).to_python(value)
+
+        class JSONForm(forms.Form):
+            json = CustomJSONField()
+
+        form = JSONForm(data={'json': '{}'});
+        form.full_clean()
+        self.assertEqual(form.cleaned_data, {'json' : {}})