<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def clean(self, value):
 """Run validators and return the clean value."""

        if value is None:
            value = self.default

        try:
            value = self.to_python(value)
            self.validate(value)
        except ValidationError as error:
            raise ValidationError("invalid value for {}: {}".format(
                self.name,
                error.args[0]
            ))
        return value