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Multiple Dispatching

Метки: python

The above design is certainly satisfactory. Adding new types to the system consists of adding or modifying distinct classes without causing code changes to be propagated throughout the system. In addition, RTTI is not “misused” as it was in RecycleA.py. However, it’s possible to go one step further and take a purist viewpoint about RTTI and say that it should be eliminated altogether from the operation of sorting the trash into bins.

To accomplish this, you must first take the perspective that all type-dependent activities-such as detecting the type of a piece of trash and putting it into the appropriate bin-should be controlled through polymorphism and dynamic binding.

The previous examples first sorted by type, then acted on sequences of elements that were all of a particular type. But whenever you find yourself picking out particular types, stop and think. The whole idea of polymorphism (dynamically- bound method calls) is to handle type-specific information for you. So why are you hunting for types?

The answer is something you probably don’t think about: Python performs only single dispatching. That is, if you are performing an operation on more than one object whose type is unknown, Python will invoke the dynamic binding mechanism on only one of those types. This doesn’t solve the problem, so you end up detecting some types manually and effectively producing your own dynamic binding behavior.

The solution is called multiple dispatching, which means setting up a configuration such that a single method call produces more than one dynamic method call and thus determines more than one type in the process. To get this effect, you need to work with more than one type hierarchy: you’ll need a type hierarchy for each dispatch. The following example works with two hierarchies: the existing Trash family and a hierarchy of the types of trash bins that the trash will be placed into. This second hierarchy isn’t always obvious and in this case it needed to be created in order to produce multiple dispatching (in this case there will be only two dispatches, which is referred to as double dispatching).