OPC Item Properties

Every OPC Item may have so-called properties that contain further information about the item. Example properties would be the access rights or a description of the item. These OPC properties are modeled as a specific Python object, called OPCProperty, which contains the following information:

Name:

The name uniquely identifies an OPC property. Names must be of the type QName.

Value:

Properties most often have a value, for instance in case of a property ``accessRights'', it stores the strings readable or writable.

Description:

In order to easily understand the meaning of a property, it can store a description.

ItemPath/ItemName:

The address of the property, consisting of the ItemPath and ItemName.

ResultID/ErrorText:

In case a property is erroneous, for instance if it cannot be read or does not exist, the error can be stored in a ResultID and a descriptive error text.

Listing 6 shows in line 1 to 5 how to create and access a PyOPC property object.

language=C

The OPC XML-DA standard specifies various common properties, which should be preferred over custom properties, if possible. A full list of these available properties is given in [OPCXMLDA]. The alternative are custom properties that will often be in the namespace of PyOPC. Therefore the framework offers a simple shortcut in creating properties: if the property name is a string instead of a QName, PyOPC searches in a table for a matching OPC property. If one is found, the OPC XML-DA namespace is used, moreover the description is filled out automatically. If the property is unknown, the PyOPC namespace will automatically be used. This behavior is reflected in line 7 and 8 in listing 6.

Properties will be associated with items, therefore an ItemContainer object provides the following methods to add, delete and list properties:

• addProperty(self, property) / addProperties(self,properties) adds one property or a list of properties to the ItemContainer object.
• getProperty(self, name) retrieves a property according to its name
• delProperty(self, name) / popProperty (self, name) deletes a property. popProperty returns the property before deletion.
• listProperties(self) returns a list of all item properties.