How to use the ovs.db.data.Datum.from_json function in ovs

def do_parse_data(type_string, *data_strings):
    type_json = unbox_json(ovs.json.from_string(type_string))
    type_ = ovs.db.types.Type.from_json(type_json)
    for datum_string in data_strings:
        datum_json = unbox_json(ovs.json.from_string(datum_string))
        datum = data.Datum.from_json(type_, datum_json)
        print(ovs.json.to_string(datum.to_json()))

def _check_value(self, ovsrec_row, column_key_value):
        column, key, value_json = column_key_value
        column_schema = ovsrec_row._table.columns[column]
        value = ovs.db.data.Datum.from_json(
            column_schema.type, value_json).to_python(ovs.db.idl._uuid_to_row)
        datum = getattr(ovsrec_row, column)
        if key is None:
            if datum == value:
                return True
        else:
            if datum[key] != value:
                return True
        return False

def from_json(cls, idl, table, uuid, row_json):
        data = {}
        for column_name, datum_json in six.iteritems(row_json):
            column = table.columns.get(column_name)
            if not column:
                # XXX rate-limit
                vlog.warn("unknown column %s in table %s"
                          % (column_name, table.name))
                continue
            try:
                datum = ovs.db.data.Datum.from_json(column.type, datum_json)
            except error.Error as e:
                # XXX rate-limit
                vlog.warn("error parsing column %s in table %s: %s"
                          % (column_name, table.name, e))
                continue
            data[column_name] = datum
        return cls(idl, table, uuid, data)

vsctl_fatal('%s does not contain a column whose name matches "%s"'
                        % (ovsrec_row._table.name, column))

        column_schema = ovsrec_row._table.columns[column]
        if key is not None:
            value_json = ['map', [[key, value_json]]]
            if column_schema.type.value.type == ovs.db.types.VoidType:
                vsctl_fatal('cannot specify key to set for non-map column %s' %
                            column)
            datum = ovs.db.data.Datum.from_json(column_schema.type, value_json,
                                                self.symtab)
            values = getattr(ovsrec_row, column, {})
            values.update(datum.to_python(ovs.db.idl._uuid_to_row))
            setattr(ovsrec_row, column, values)
        else:
            datum = ovs.db.data.Datum.from_json(column_schema.type, value_json,
                                                self.symtab)
            setattr(ovsrec_row, column,
                    datum.to_python(ovs.db.idl._uuid_to_row))

def _check_value(self, ovsrec_row, column_value):
        """
        :type column_value: tuple of column and value_json
        """
        column, value_json = column_value
        column_schema = ovsrec_row._table.columns[column]
        value = ovs.db.data.Datum.from_json(
            column_schema.type, value_json).to_python(ovs.db.idl._uuid_to_row)
        datum = getattr(ovsrec_row, column)
        if column_schema.type.is_map():
            for k, v in value.items():
                if k in datum and datum[k] == v:
                    return True
        elif datum == value:
            return True

        return False

def from_json(json):
        if type(json) in [str, unicode]:
            return BaseType(AtomicType.from_json(json))

        parser = ovs.db.parser.Parser(json, "ovsdb type")
        atomic_type = AtomicType.from_json(parser.get("type", [str, unicode]))

        base = BaseType(atomic_type)

        enum = parser.get_optional("enum", [])
        if enum is not None:
            base.enum = ovs.db.data.Datum.from_json(
                    BaseType.get_enum_type(base.type), enum)
        elif base.type == IntegerType:
            base.min = parser.get_optional("minInteger", [int, long])
            base.max = parser.get_optional("maxInteger", [int, long])
            if (base.min is not None and base.max is not None
                    and base.min > base.max):
                raise error.Error("minInteger exceeds maxInteger", json)
        elif base.type == RealType:
            base.min = parser.get_optional("minReal", [int, long, float])
            base.max = parser.get_optional("maxReal", [int, long, float])
            if (base.min is not None and base.max is not None
                    and base.min > base.max):
                raise error.Error("minReal exceeds maxReal", json)
        elif base.type == StringType:
            base.min_length = BaseType.__parse_uint(parser, "minLength", 0)
            base.max_length = BaseType.__parse_uint(parser, "maxLength",

def __apply_diff(self, table, row, row_diff):
        old_row = {}
        for column_name, datum_diff_json in six.iteritems(row_diff):
            column = table.columns.get(column_name)
            if not column:
                # XXX rate-limit
                vlog.warn("unknown column %s updating table %s"
                          % (column_name, table.name))
                continue

            try:
                datum_diff = data.Datum.from_json(column.type, datum_diff_json)
            except error.Error as e:
                # XXX rate-limit
                vlog.warn("error parsing column %s in table %s: %s"
                          % (column_name, table.name, e))
                continue

            old_row[column_name] = row._data[column_name].copy()
            datum = row._data[column_name].diff(datum_diff)
            if datum != row._data[column_name]:
                row._data[column_name] = datum

        return old_row

def from_json(json):
        if isinstance(json, six.string_types):
            return BaseType(AtomicType.from_json(json))

        parser = ovs.db.parser.Parser(json, "ovsdb type")
        atomic_type = AtomicType.from_json(parser.get("type",
                                                      six.string_types))

        base = BaseType(atomic_type)

        enum = parser.get_optional("enum", [])
        if enum is not None:
            base.enum = ovs.db.data.Datum.from_json(
                    BaseType.get_enum_type(base.type), enum)
        elif base.type == IntegerType:
            base.min = parser.get_optional("minInteger", six.integer_types)
            base.max = parser.get_optional("maxInteger", six.integer_types)
            if (base.min is not None and base.max is not None
                    and base.min > base.max):
                raise error.Error("minInteger exceeds maxInteger", json)
        elif base.type == RealType:
            base.min = parser.get_optional("minReal", REAL_PYTHON_TYPES)
            base.max = parser.get_optional("maxReal", REAL_PYTHON_TYPES)
            if (base.min is not None and base.max is not None
                    and base.min > base.max):
                raise error.Error("minReal exceeds maxReal", json)
        elif base.type == StringType:
            base.min_length = BaseType.__parse_uint(parser, "minLength", 0)
            base.max_length = BaseType.__parse_uint(parser, "maxLength",