Acquisition

Acquisition is the technology that allows dynamic behavior to be shared between Zope objects via containment.

Acquisition's flavor permeates Zope. It can be used almost everywhere within Zope: in DTML, in Zope Page Templates, in Script (Python) objects, and even in Zope URLs?. Because of its ubiquity in Zope, a basic understanding of acquisition is important.

Acquisition vs. Inheritance

The chapter entitled Object Orientation describes a concept called inheritance. Using inheritance, an object can inherit some of the behaviors of a specific class, overriding or adding other behaviors as necessary. Behaviors of a class are nearly always defined by its methods, although attributes can be inherited as well.

In a typical object-oriented language, there are rules to the way a subclass inherits behavior from its superclasses. For example, in Python (a multiple-inheritance language), a class may have more than one superclass, and rules are used to determine which of a class' superclasses is used to define behavior in any given circumstance. We'll define a few Python classes here to demonstrate. You don't really need to know Python inside and out to understand these examples. Just know that a class statement defines a class and a def statement inside of a class statement defines a method. A class statement followed by one or more words inside (Parenthesis) causes that class to inherit behavior from the classes named in the parenthesis.:

class SuperA?:
    def amethod(self):
        print "I am the amethod method of the SuperA? class"

    def anothermethod(self):
        print "I am the anothermethod method of the SuperA? class"

class SuperB?:
    def amethod(self):
        print "I am the amethod method of the SuperB? class"

    def anothermethod(self):
        print "I am the anothermethod method of the SuperB? class"

    def athirdmethod(self):
        print "I am the anothermethod method of the SuperB? class"

class Sub(SuperA?, SuperB?):
    def amethod(self):
        print "I am the amethod method of the Sub class"

Anonymous User - Jan. 12, 2004 11:08 pm:
 In the SuperB? class, athirdmethod() should probably print that it is the athirdmethod method, rather than
 the anothermethod method (which it isn't). It might confuse people who are new or (like me) sleepy.

If we make an instance of the "Sub" class, and attempt to call one of its methods, there are rules in place to determine whether the behavior of the method will be defined by the Sub class itself, its SuperA? superclass, or its SuperB? superclass. The rules are fairly simple. If the Sub class has itself defined the named method, that method definition will be used. Otherwise, the inheritance hierarchy will be searched for a method definition.

The inheritance hierarchy is defined by the class' superclass definitions. In the case of the Sub class above, it has a simple inheritance hierarchy: it inherits first from the SuperA? superclass, then it inherits from the SuperB? superclass. This means that if you call a method on an instance of the Sub class, and that method is not defined as part of the Sub class' definition, it will first search for the method in the SuperA? class and if it doesn't find it there, it will search in the SuperB? class.

Anonymous User - Jan. 30, 2004 11:26 pm:
 This is probably nitpicking, but you should probably explain that the order in which the superclasses are
named in the subclass definition is what is important, not the fact that A comes before B in the alphabet
 (which an unsuspecting reader might assume).

Here is an example of calling methods on an instance of the above-defined Sub class using the Python interpreter:

>>> instance = Sub()
>>> instance.amethod()
I am the amethod method of the Sub class
>>> instance.anothermethod()
I am the anothermethod method of the SuperA? class
>>> instance.athirdmethod()
I am the anothermethod method of the SuperB? class

Anonymous User - Jan. 12, 2004 11:09 pm:
 If the athirdmethod() in SuperB? is adjusted per the suggestion that it output that it is 'athirdmethod'
 instead of anothermethod, this output should probably be updated to reflect that as well.

Note that when we called the anothermethod method on the Sub instance, we got the return value of SuperA?'s method definition for that method, even though both SuperA? and SuperB? defined that method. This is because the inheritance hierarchy specifies that the first superclass (SuperA?) is searched first.

The point of this example is that instances of objects use their inheritance hierarchy to determine their behavior. In non-Zope applications, this is the only way that object instances know about their set of behaviors. However, in Zope, objects make use of another facility to search for their behaviors: acquisition.

Acquisition is about Containment

The concept behind acquisition is simple:

• Objects are situated inside other objects. These objects act as their "containers". For example, the container of a DTML Method named "amethod" inside the DTML_Example folder is the DTML_Example folder.
• Objects may acquire behavior from their containers.

Inheritance stipulates that an object can learn about its behavior from its superclasses via an inheritance hierarchy. Acquisition, on the other hand, stipulates that an object can additionally learn about its behavior through its containment hierarchy. In Zope, an object's inheritance hierarchy is always searched for behavior before its acquisition hierarchy. If the method or attribute is not found in the object's inheritance hierarchy, the acquisition hierarchy is searched.

Say What?

Let's toss aside the formal explanations. Acquisition can be best explained with a simple example.

Place a DTML Method named acquisition_test in your Zope root folder. Give it the following body:

<html>
<body>
  <p>
   I am being called from within the <dtml-var title> Folder!
  </p>
</body>
</html>

Save it and then use the DTML Method "View" tab to see the result of the DTML method in your Workspace frame. You will see something not unlike the following:

I am being called from within the Zope Folder!

The title of the Zope root folder is Zope, so this makes sense. Now create a Folder inside your Zope root folder named AcquisitionTestFolder and a title of "TheAcquisitionTest?". We're going to invoke the acquisition_test method in the context of the AcquisitionTestFolder? folder. To do this, assuming your Zope is running on your local machine on port 8080, visit the URL http://localhost:8080/AcquisitionTestFolder/acquisition_test. You will see something not unlike the following:

I am being called from within the TheAcquisitionTest? Folder!

Note that even though an object named acquisition_test does not "live" inside the AcquisitionTestFolder? folder, Zope found the method and displayed a result anyway! Not only did Zope display a result, instead of inserting the title of the Zope root folder, it inserted the title of the AcquisitionTestFolder? folder! This is an example of acquisition in action. The concept is simple: if a named object is not found as an attribute of the object you're searching, its containers are searched until the object is found. In this way, acquisition can add behavior to objects. In this case, we added a behavior to the AcqusitionTestFolder? folder that it didn't have before (by way of giving it an acquisition_test method).

Providing Services

It can be said that acquisition allows objects to acquire services by way of containment. For example, our AcquisitionTestFolder? folder acquired the services of the acquisition_test method.

Not only do objects acquire services, they also provide them. For example, adding a Mail Host object to a Folder named AFolder provides other objects in that folder with the ability to send mail. But it also provides objects contained in subfolders of that folder with the capability to send mail. If you create subfolders of AFolder named AnotherFolder and AThirdFolder, you can be assured that objects placed in these folders will also be able to send mail in exactly the same way as objects placed in AFolder.

Acquisition "goes both ways". When you create an object in Zope, it has the capability to automatically acquire services. Additionally, it automatically provides services that other objects can acquire. This makes reuse of services very easy since you don't have to do anything special to make services available to other objects.

Getting Deeper with Multiple Levels

If you place a method in the root folder, and create a subfolder in the root folder, you can acquires the method's behaviors. So what happens if things get more complex? Perhaps you have a method that needs to be acquired from withinside a couple of folders. Is it acquired from its parent, or its parent's parent, or what?

The answer is that acquisition works on the entire object hierarchy. If for example you have a DTML Method in the root folder. Also in the root folder you have three nested Folders named "Users", "Barney" and "Songs". You may call this URL:

/Users/Barney/Songs/HappySong?

The HappySong? method is found in the root folder unless one of the other folders "Users", "Barney" or "Songs" happens to also have a method named "HappySong?", in which case that method is used. The HappySong? method is searched for first directly in the "Songs" folder. If it is not found, the acquisition hierarchy is searched starting at the first container in the hierarchy: "Barney". If it is not found in "Barney", the "Users" folder is searched. If it is not found in the "Users" folder, the root folder is searched. This search is called searching the acquisition path or alternately searching the containment hierarchy.

Aquisition is not limited to searching a containment hierarchy: it can also search a context hierarchy. Acquisition by context is terribly difficult to explain, and you should avoid it if possible. However, if you want more information about aquiring via a context and you are ready to have your brain explode, please see the presentation named Acquisition Algebra.

Summary

Acquisition allows behavior to be distributed throughout the system. When you add a new object to Zope, you don't need to specify all its behavior, only the part of its behavior that is unique to it. For the rest of its behavior it relies on other objects. This means that you can change an object's behavior by changing where it is located in the object hierarchy. This is a very powerful function which gives your Zope applications flexibility.

Acquisition is useful for providing objects with behavior that doesn't need to be specified by their own methods or methods found in their inheritance hierarchies. Acquisition is particularly useful for sharing information (such as headers and footers) between objects in different folders as well. You will see how you can make use of acquisition within different Zope technologies in upcoming chapters.

A more exhaustive technical explanation of the underpinnings of Zope's acquisition technology is available in the Zope Developer's Guide.