Class MultipleChoiceField

django/forms/fields.py:946–990

class MultipleChoiceField(ChoiceField):
    hidden_widget = MultipleHiddenInput
    widget = SelectMultiple
    default_error_messages = {
        "invalid_choice": _(
            "Select a valid choice. %(value)s is not one of the available choices."
        ),
        "invalid_list": _("Enter a list of values."),
    }

    def to_python(self, value):
        if not value:
            return []
        elif not isinstance(value, (list, tuple)):
            raise ValidationError(
                self.error_messages["invalid_list"], code="invalid_list"
            )
        return [str(val) for val in value]

    def validate(self, value):
        """Validate that the input is a list or tuple."""
        if self.required and not value:
            raise ValidationError(self.error_messages["required"], code="required")
        # Validate that each value in the value list is in self.choices.
        # Avoid redundant validation, and keep elements ordered.
        for val in OrderedSet(value):
            if not self.valid_value(val):
                raise ValidationError(
                    self.error_messages["invalid_choice"],
                    code="invalid_choice",
                    params={"value": val},
                )

    def has_changed(self, initial, data):
        if self.disabled:
            return False
        if initial is None:
            initial = []
        if data is None:
            data = []
        if len(initial) != len(data):
            return True
        initial_set = {str(value) for value in initial}
        data_set = {str(value) for value in data}
        return data_set != initial_set


class TypedMultipleChoiceField(MultipleChoiceField):