How to use the colander.null function in colander

def test_deserialize_null(self):
        import colander
        typ = self._makeOne()
        node = DummySchemaNode(None)
        result = typ.deserialize(node, colander.null)
        self.assertEqual(result, colander.null)

def deserialize(field, pstruct):
        if pstruct is colander.null:
            return colander.null
        return [item.strip() for item in pstruct.split(",") if item]

    def deserialize(self, cstruct=colander.null):
        """Preprocess received data to carefully merge defaults.
        """
        if cstruct is not colander.null:
            defaults = cstruct.get("defaults")
            requests = cstruct.get("requests")
            if isinstance(defaults, dict) and isinstance(requests, list):
                for request in requests:
                    if isinstance(request, dict):
                        merge_dicts(request, defaults)
        return super().deserialize(cstruct)

job_impressions=colander.null,
                   job_media_sheets=colander.null,
                   job_priority=colander.null,
                   job_hold_until=colander.null,
                   multiple_document_handling=colander.null,
                   copies=colander.null,
                   finishings=colander.null,
                   page_ranges=colander.null,
                   sides=colander.null,
                   number_up=colander.null,
                   orientation_requested=colander.null,
                   printer_resolution=colander.null,
                   print_quality=colander.null,
                   auth_info=colander.null,
                   job_billing=colander.null,
                   job_sheets=colander.null,
                   media=colander.null):
        kw = {'operation_attributes_tag':
              {'printer_uri': printer_uri,
               'job_name': job_name,
               'ipp_attribute_fidelity': ipp_attribute_fidelity,
               'job_k_octets': job_k_octets,
               'job_impressions': job_impressions,
               'job_media_sheets': job_media_sheets},
              'job_attributes_tag':
              {'job_priority': job_priority,
               'job_hold_until': job_hold_until,
               'job_sheets': job_sheets,
               'multiple_document_handling': multiple_document_handling,
               'copies': copies,
               'finishings': finishings,
               'page_ranges': page_ranges,

a value, but all accept `colander.null`)

        all values for server_default should result in 'drop'
        for Colander missing

        autoincrement results in drop
        """
        if isinstance(column.default, ColumnDefault):
            if column.default.is_callable:
                kwargs["missing"] = drop
            elif column.default.is_clause_element:  # SQL expression
                kwargs["missing"] = drop
            elif column.default.is_scalar:
                kwargs["missing"] = column.default.arg
        elif column.nullable:
            kwargs["missing"] = colander.null
        elif isinstance(column.server_default, FetchedValue):
            kwargs["missing"] = drop  # value generated by SQLA backend
        elif (hasattr(column.table, "_autoincrement_column") and id(column.table._autoincrement_column) == id(column)):
            # this column is the autoincrement column, so we can drop
            # it if it's missing and let the database generate it
            kwargs["missing"] = drop

        kwargs.update(type_overrides_kwargs)
        kwargs.update(typedecorator_overrides)
        kwargs.update(declarative_overrides)
        kwargs.update(overrides)

        return colander.SchemaNode(type_, *children, **kwargs)

    @pyipptool_coroutine
    def send_document(self,
                      job_uri=colander.null,
                      printer_uri=colander.null,
                      job_id=colander.null,
                      requesting_user_name=None,
                      document_name=colander.null,
                      compression=colander.null,
                      document_format='application/pdf',
                      document_natural_language=colander.null,
                      last_document=True,
                      document_content=None,
                      ):
        """
        :param document_content: Binary Content or Named File
        """
        delete = False
        filename, delete = _get_filename_for_content(document_content)
        kw = {'operation_attributes_tag':
              {'job_uri': job_uri,
               'printer_uri': printer_uri,
               'job_id': job_id,
               'requesting_user_name': requesting_user_name,

def get_col_default_value(self, col, obj):
        value = None
        col_type = self.get_column_type(col)
        if obj and hasattr(col, 'instance_name'):
            value = getattr(obj, col.instance_name)
        elif obj and hasattr(col, 'name'):
            try:
                value = getattr(obj, col.name, colander.null)
            except UnicodeEncodeError:
                value = colander.null
        if value is None:
            value = colander.null
        elif col_type == ChoiceType:
            value = value[0]
        if col_type == sa_types.Boolean:
            value = bool(value)
        return value

def serialize(self, node, appstruct):
        if appstruct is null:  # colander.null
            return null

        appstruct = zip(appstruct['time'].astype(object), appstruct['value'])

        return super(DatetimeValue1dArray, self).serialize(node, appstruct)

def deserialize(self, node, cstruct):
        if not cstruct:
            return colander.null

        try:
            r = urlparse.urlparse(cstruct)
            # require absolute urls
            if not r.scheme or not r.netloc or not r.hostname:
                raise Exception()
            # poke at stuff to make sure the parts are valid:
            r.port, r.username, r.password
            return r
        except Exception, e:
            raise colander.Invalid(node,
                                   _('"${val}" is not a URL (${err})',
                                     mapping={'val':cstruct, 'err': e})
                                   )

def widget(self, kw: dict):
        widget = DateTimeInputWidget()
        schema = widget._pstruct_schema
        schema['date_submit'].missing = null  # Fix readonly template bug
        schema['time_submit'].missing = null
        return widget