[1.11.x] Fixed #28555 -- Made CharField convert whitespace-only values to the empty_value when strip is enabled.

Backport of 48c394a6fc2594891f766293afec8f86d63e1015 from master

django/forms/fields.py

@@ -233,11 +233,12 @@ class CharField(Field):
 
     def to_python(self, value):
         "Returns a Unicode object."
+        if value not in self.empty_values:
+            value = force_text(value)
+            if self.strip:
+                value = value.strip()
         if value in self.empty_values:
             return self.empty_value
-        value = force_text(value)
-        if self.strip:
-            value = value.strip()
         return value
 
     def widget_attrs(self, widget):

docs/releases/1.11.6.txt

@@ -9,4 +9,5 @@ Django 1.11.6 fixes several bugs in 1.11.5.
 Bugfixes
 ========
 
-* ...
+* Made the ``CharField`` form field convert whitespace-only values to the
+  ``empty_value`` when ``strip`` is enabled (:ticket:`28555`).

tests/forms_tests/field_tests/test_charfield.py

@@ -121,6 +121,29 @@ class CharFieldTest(FormFieldAssertionsMixin, SimpleTestCase):
         self.assertEqual(f.clean(' 1'), ' 1')
         self.assertEqual(f.clean('1 '), '1 ')
 
+    def test_strip_before_checking_empty(self):
+        """
+        A whitespace-only value, ' ', is stripped to an empty string and then
+        converted to the empty value, None.
+        """
+        f = CharField(required=False, empty_value=None)
+        self.assertIsNone(f.clean(' '))
+
+    def test_clean_non_string(self):
+        """CharField.clean() calls str(value) before stripping it."""
+        class StringWrapper:
+            def __init__(self, v):
+                self.v = v
+
+            def __str__(self):
+                return self.v
+
+        value = StringWrapper(' ')
+        f1 = CharField(required=False, empty_value=None)
+        self.assertIsNone(f1.clean(value))
+        f2 = CharField(strip=False)
+        self.assertEqual(f2.clean(value), ' ')
+
     def test_charfield_disabled(self):
         f = CharField(disabled=True)
         self.assertWidgetRendersTo(f, '<input type="text" name="f" id="id_f" disabled required />')