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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def update(self, **values):
 """ Performs an update on the model instance. You can pass in values to set on the model for updating, or you can call without values to execute an update against any modified fields. If no fields on the model have been modified since loading, no query will be performed. Model validation is performed normally. Setting a value to `None` is equivalent to running a CQL `DELETE` on that column. It is possible to do a blind update, that is, to update a field without having first selected the object out of the database. See :ref:`Blind Updates <blind_updates>` """

        for column_id, v in values.items():
            col = self._columns.get(column_id)

            # check for nonexistant columns
            if col is None:
                raise ValidationError(
                    "{0}.{1} has no column named: {2}".format(
                        self.__module__, self.__class__.__name__, column_id))

            # check for primary key update attempts
            if col.is_primary_key:
                current_value = getattr(self, column_id)
                if v != current_value:
                    raise ValidationError(
                        "Cannot apply update to primary key '{0}' for {1}.{2}".format(
                            column_id, self.__module__, self.__class__.__name__))

            setattr(self, column_id, v)

        # handle polymorphic models
        if self._is_polymorphic:
            if self._is_polymorphic_base:
                raise PolymorphicModelException('cannot update polymorphic base model')
            else:
                setattr(self, self._discriminator_column_name, self.__discriminator_value__)

        self.validate()
        self.__dmlquery__(self.__class__, self,
                          batch=self._batch,
                          ttl=self._ttl,
                          timestamp=self._timestamp,
                          consistency=self.__consistency__,
                          conditional=self._conditional,
                          timeout=self._timeout,
                          if_exists=self._if_exists).update()

        self._set_persisted()

        self._timestamp = None

        return self