8. Acquisition

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

Acquisition’s flavor permeates Zope and can be used almost everywhere within Zope: 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.

Over the years Acquisition has been proven to be a very powerful but often too complex technology to use. While it is predictable in simple interactions, it gets increasingly complicated to understand its behavior in most real-world-sized projects.

In order to understand Zope, you will still need an understanding of Acquisition today. Basing your application logic on it is highly discouraged, though.

8.1. 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 that define 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 (parentheses) causes that class to inherit behavior from the classes named in the parentheses (you can play along at home if you like, using the Python interpreter).:

>>> 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 'athirdmethod' method of the SuperB class"
...
>>> class Sub(SuperA, SuperB):
...     def amethod(self):
...         print "I am the 'amethod' method of the Sub class"
...

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. The case of the Sub class above 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. If it doesn’t find it there, it will search in the SuperB class. Python performs this search of the base classes using an order derived from the order of declaration. Note that for complex cases (e.g., where the same method is defined in several ancestors of base classes), the lookup order is too complicated to explain within the scope of this book. Please see the online Python documentation for the “method resolution order”, http://www.python.org/download/releases/2.3/mro/

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

>>> 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 'athirdmethod' method of the SuperB class

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.

8.2. Acquisition Is about Containment

The concept behind acquisition is simple:

• Objects are situated inside other objects, and these objects act as their “containers”. For example, the container of a Page Template named “apage” inside a Folder “afolder” is the “afolder” 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, then the acquisition hierarchy is searched.

8.3. Say What?

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

Place a Page Template named ‘acquisition_test’ in your Zope root folder. Give it the following body:

<html>
<body>
  <p>
   I am being called from within the
   <span tal:replace="context/title" />
   Folder!
  </p>
</body>
</html>

Save it, and then use the Page Template “View” tab to see the result of the template 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’ page 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 page 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 adding an ‘acquisition_test’ page).

8.4. 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’ page.

Not only do objects acquire services, but 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 in order to make services available to other objects.

8.5. Getting Deeper with Multiple Levels

If you place a method in the root folder, and create a subfolder in the root folder, you can acquire the method’s behaviors. So what happens if things get more complex? Perhaps you have a method that needs to be acquired from within 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 Page Template, “HappySong”, in the root folder, and 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 page is found in the root folder, unless one of the other folders “Users”, “Barney” or “Songs” happens to also have a page named “HappySong”, in which case that page is used instead. The HappySong page 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.

Acquisition 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 at all possible.

In the example above, for instance, in order to find and publish the “HappySong” template at the end of the URL, acquisition searches the containment hierarchy of the “Songs” folder first. Because “Songs” is contained within “Barney”, and “Barney” within “Users”, the containment hierarchy for “Songs” consists of each folder “up” from “Users” to the root.

Once the “HappySongs” template is found, there are two hierarchies of interest:

• Because “HappySongs” is located directly within the root, its containment hierarchy consists of only itself and the root.
• Because “HappySongs” was found by traversing first through the “Users”, “Barney”, and “Songs” folders, its context hierarchy includes those objects.

Acquisition searches the context hierarchy only after failing to find the named object in the containment hierarchy.

As with understanding Python’s concept of multiple inheritance, explaining the exact strategy used to order that search is not within the scope of this book.

8.6. Summary

Acquisition allows behavior to be distributed hierarchically throughout the system. When you add a new object to Zope, you don’t need to specify all of its behavior, only the part of its behavior that is unique to that object. 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 that 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.