FAQ How Inventory class works

From DANSE

Inventory has descriptors as its static members. Descriptors are special python objects that defines __get__ (and __set__) methods. (Note: they are not instances of pyre.inventory.Descriptor.Descriptor. class pyre.inventory.Descriptor.Descriptor is not really a Descriptor class meant by http://users.rcn.com/python/download/Descriptor.htm. In pyre, pyre.inventory.Trait.Trait is the real Descriptor class.) An instance of descriptor describe a property of his parent, but does not hold the value of this property. This is why you can inherit Inventory but its static members do not conflict in different instances of Inventory classes.

For example

 class Inventory(Component.Inventory):
 
     import pyre
 
     a = pyre.inventory.str('a', default="" )

Here pyre.inventory.str makes a Str instance. Str is a subclass of Trait. So the instance Inventory.a is a descriptor that says the instance of Inventory class will have a property called a. This property is a string, and it defaults to be empty.

When Inventory class is instantiated,

 inventory = Inventory(...)

and when we are asking for its property,

 inventory.a

The __set__ and __get__ functions of Trait class will get called and which, in turn, calls getTraitValue and setTraitValue of the Inventory class.

So you can see the class Trait and Inventory have to cooperate to implement this idea of Descriptor.

Notary

Inventory has its metaclass pyre.inventory.Notary.Notary. The metaclass's __init__ will be called when the object of the class (Note: the class object != the class instance) is built. In Notary's __init__, all traits of an Inventory class will be collected to two registries, one for properties, and one for facilities.