Learning Python - Mark Lutz [339]
Like simple variables, attributes of classes and instances are not declared ahead of time, but spring into existence the first time they are assigned values. When a method assigns to a self attribute, it creates or changes an attribute in an instance at the bottom of the class tree (i.e., one of the rectangles) because self automatically refers to the instance being processed.
In fact, because all the objects in class trees are just namespace objects, we can fetch or set any of their attributes by going through the appropriate names. Saying C1.setname is as valid as saying I1.setname, as long as the names C1 and I1 are in your code’s scopes.
As currently coded, our C1 class doesn’t attach a name attribute to an instance until the setname method is called. In fact, referencing I1.name before calling I1.setname would produce an undefined name error. If a class wants to guarantee that an attribute like name is always set in its instances, it more typically will fill out the attribute at construction time, like this:
class C1(C2, C3):
def __init__(self, who): # Set name when constructed
self.name = who # Self is either I1 or I2
I1 = C1('bob') # Sets I1.name to 'bob'
I2 = C1('mel') # Sets I2.name to 'mel'
print(I1.name) # Prints 'bob'
If it’s coded and inherited, Python automatically calls a method named __init__ each time an instance is generated from a class. The new instance is passed in to the self argument of __init__ as usual, and any values listed in parentheses in the class call go to arguments two and beyond. The effect here is to initialize instances when they are made, without requiring extra method calls.
The __init__ method is known as the constructor because of when it is run. It’s the most commonly used representative of a larger class of methods called operator overloading methods, which we’ll discuss in more detail in the chapters that follow. Such methods are inherited in class trees as usual and have double underscores at the start and end of their names to make them distinct. Python runs them automatically when instances that support them appear in the corresponding operations, and they are mostly an alternative to using simple method calls. They’re also optional: if omitted, the operations are not supported.
For example, to implement set intersection, a class might either provide a method named intersect, or overload the & expression operator to dispatch to the required logic by coding a method named __and__. Because the operator scheme makes instances look and feel more like built-in types, it allows some classes to provide a consistent and natural interface, and be compatible with code that expects a built-in type.
OOP Is About Code Reuse
And that, along with a few syntax details, is most of the OOP story in Python. Of course, there’s a bit more to it than just inheritance. For example, operator overloading is much more general than I’ve described so far—classes may also provide their own implementations of operations such as indexing, fetching attributes, printing, and more. By and large, though, OOP is about looking up attributes in trees.
So why would we be interested in building and searching trees of objects? Although it takes some experience to see how, when used well, classes support code reuse in ways that other Python program components cannot. With classes, we code by customizing existing software, instead of either changing existing code in-place or starting from scratch for each new project.
At a fundamental level, classes are really just packages of functions and other names, much like modules. However, the automatic attribute inheritance search that we get with classes supports customization of software above and beyond what we can do with modules and functions. Moreover, classes provide a natural structure for code that localizes logic and names, and so aids in debugging.
For instance, because methods are simply functions with a special first argument, we can mimic some of their behavior by manually passing objects to be processed to simple