TypedChoiceField.to_python() always returns a string

[Yoshio Hasegawa]

Coercion always fails when using IntegerChoices with a TypedChoiceField in a Django form.

When a value is cleaned in a TypedChoiceField, the inherited ChoiceField's to_python() method will return the value casted as a string. This means that when coercion with an IntegerChoices object is attempted, it will always fail as IntegerChoices will expect an int type when instantiated.

Example

class SomeIntegerChoice(models.IntegerChoices):
    VAL_2350 = (2350, "value 2350")
    VAL_4100 = (4100, "value 4100")
    VAL_8760 = (8760, "value 8760")

form.fields["Integer_choice"] = forms.TypedChoiceField(
    required=required,
    choices=SomeIntegerChoice.choices,
    coerce=SomeIntegerChoice,
)

This field will never pass validation since ChoiceField (inherited by TypedChoiceField) has a to_python() method that casts the provided value as a string:

# class ChoiceField:
# ...
def to_python(self, value):
    """Return a string."""
    if value in self.empty_values:
        return ""
    return str(value)

To explain further... ChoiceField.to_python() will be called when TypedChoiceField attempts to clean() a value. After the value is cleaned, it is coerced using the provided class definition via the coerce property.

Here is example code from Django to show how this happens:

# class TypedChoiceField:
# ...
def _coerce(self, value):
    """
    Validate that the value can be coerced to the right type (if not empty).
    """
    if value == self.empty_value or value in self.empty_values:
        return self.empty_value
    try:
        value = self.coerce(value)
    except (ValueError, TypeError, ValidationError):
        raise ValidationError(
            self.error_messages["invalid_choice"],
            code="invalid_choice",
            params={"value": value},
        )
    return value

def clean(self, value):
    value = super().clean(value)
    return self._coerce(value)

[Mariusz Felisiak]

As far as I'm aware, it's an issue in your code. For IntegerChoices you should pass coerce=int and everything works fine.

[Jacob Walls]

Replying to Mariusz Felisiak:

As far as I'm aware, it's an issue in your code. For IntegerChoices you should pass coerce=int and everything works fine.

Mariusz is right, coerce=int is enough to make the error go away, but the OP might have wanted to be able to use enum values in their code, e.g.:

>>> TypedChoiceField(choices=Suit, coerce=Suit).clean(4)
Suit.CLUB

Also, although it's handy that int can handle strings just as well as ints, decimal.Decimal would surely produce interesting differences:

>>> decimal.Decimal(4.0)
Decimal('4')
>>> decimal.Decimal(str(4.0))  # to_python does str()
Decimal('4.0')

---

I'm suggesting we take another look, because the state of play after #21397 is not quite ideal -- it leaves to_python() to return str, whereas it's documented to return the proper python type. Breaking this expectation led to 500s in the admin changelist in #36865/4cecf3039586ea738afafb9a28c946bff42c37c1 (crash report in #37263).

Tests pass with the following starting suggestion (maybe also worth looking into self.empty_values):

django/forms/fields.py

@@ -938 +938 @@
             )
         return value
 
-    def clean(self, value):
-        value = super().clean(value)
+    def to_python(self, value):
         return self._coerce(value)
 
 
@@ -1015 +1014 @@
                 )
         return new_value
 
-    def clean(self, value):
-        value = super().clean(value)
+    def to_python(self, value):
         return self._coerce(value)
 
     def validate(self, value):

tests/forms_tests/field_tests/test_typedchoicefield.py

@@ -89 +89 @@
         f = TypedChoiceField(
             choices=[(1, "1"), (2, "2")], coerce=coerce_func, required=True
         )
-        self.assertEqual(decimal.Decimal("1.2"), f.clean("2"))
+        self.assertEqual(decimal.Decimal("1.2"), f.to_python("2"))
         with self.assertRaisesMessage(ValidationError, "'This field is required.'"):
             f.clean("")
-        msg = "'Select a valid choice. 3 is not one of the available choices.'"
+        msg = "'Select a valid choice. 1.3 is not one of the available choices.'"
         with self.assertRaisesMessage(ValidationError, msg):
             f.clean("3")

tests/forms_tests/field_tests/test_typedmultiplechoicefield.py

@@ -80 +80 @@
         f = TypedMultipleChoiceField(
             choices=[(1, "1"), (2, "2")], coerce=coerce_func, required=True
         )
-        self.assertEqual([decimal.Decimal("1.2")], f.clean(["2"]))
+        self.assertEqual([decimal.Decimal("1.2")], f.to_python(["2"]))
         with self.assertRaisesMessage(ValidationError, "'This field is required.'"):
             f.clean([])
-        msg = "'Select a valid choice. 3 is not one of the available choices.'"
+        msg = "'Select a valid choice. 1.3 is not one of the available choices.'"
         with self.assertRaisesMessage(ValidationError, msg):
             f.clean(["3"])

Since the solve is the same as the original request here, I figured to reopen.