Fixed #13181 -- Added support for callable choices to forms.ChoiceField

Thanks vanschelven and expleo for the initial patch.

django/forms/fields.py

@@ -798,6 +798,15 @@ class NullBooleanField(BooleanField):
         return initial != data
 
 
+class CallableChoiceIterator(object):
+    def __init__(self, choices_func):
+        self.choices_func = choices_func
+
+    def __iter__(self):
+        for e in self.choices_func():
+            yield e
+
+
 class ChoiceField(Field):
     widget = Select
     default_error_messages = {
@@ -822,7 +831,12 @@ class ChoiceField(Field):
         # Setting choices also sets the choices on the widget.
         # choices can be any iterable, but we call list() on it because
         # it will be consumed more than once.
-        self._choices = self.widget.choices = list(value)
+        if callable(value):
+            value = CallableChoiceIterator(value)
+        else:
+            value = list(value)
+
+        self._choices = self.widget.choices = value
 
     choices = property(_get_choices, _set_choices)
 

docs/ref/forms/fields.txt

@@ -387,10 +387,16 @@ For each field, we describe the default widget used if you don't specify
 
     .. attribute:: choices
 
-        An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this
-        field. This argument accepts the same formats as the ``choices`` argument
-        to a model field. See the :ref:`model field reference documentation on
-        choices <field-choices>` for more details.
+        Either an iterable (e.g., a list or tuple) of 2-tuples to use as
+        choices for this field, or a callable that returns such an iterable.
+        This argument accepts the same formats as the ``choices`` argument to a
+        model field. See the :ref:`model field reference documentation on
+        choices <field-choices>` for more details. If the argument is a
+        callable, it is evaluated each time the field's form is initialized.
+
+        .. versionchanged:: 1.8
+
+            The ability to pass a callable to ``choices`` was added.
 
 ``TypedChoiceField``
 ~~~~~~~~~~~~~~~~~~~~

docs/releases/1.8.txt

@@ -240,6 +240,9 @@ Forms
   will also update ``UploadedFile.content_type`` with the image's content type
   as determined by Pillow.
 
+* You can now pass a callable that returns an iterable of choices when
+  instantiating a :class:`~django.forms.ChoiceField`.
+
 Generic Views
 ^^^^^^^^^^^^^
 

tests/forms_tests/tests/test_fields.py

@@ -961,6 +961,28 @@ class FieldsTests(SimpleTestCase):
         self.assertEqual('5', f.clean('5'))
         self.assertRaisesMessage(ValidationError, "'Select a valid choice. 6 is not one of the available choices.'", f.clean, '6')
 
+    def test_choicefield_callable(self):
+        choices = lambda: [('J', 'John'), ('P', 'Paul')]
+        f = ChoiceField(choices=choices)
+        self.assertEqual('J', f.clean('J'))
+
+    def test_choicefield_callable_may_evaluate_to_different_values(self):
+        choices = []
+
+        def choices_as_callable():
+            return choices
+
+        class ChoiceFieldForm(Form):
+            choicefield = ChoiceField(choices=choices_as_callable)
+
+        choices = [('J', 'John')]
+        form = ChoiceFieldForm()
+        self.assertEqual([('J', 'John')], list(form.fields['choicefield'].choices))
+
+        choices = [('P', 'Paul')]
+        form = ChoiceFieldForm()
+        self.assertEqual([('P', 'Paul')], list(form.fields['choicefield'].choices))
+
     # TypedChoiceField ############################################################
     # TypedChoiceField is just like ChoiceField, except that coerced types will
     # be returned: