Python Classes: Inheritance v. Composition

In this lesson we will talk about how to build classes based on existing classes. There are two approaches to doing this, inheritance and composition. Each has its strengths and weaknesses. We’ll look at each here.

Inheritance

In object-oriented programming (OOP), inheritance is a way to reuse code of existing objects. It’s good when you want to establish a subtype from an existing object. Objects are defined by classes, classes can inherit attributes and behavior from pre-existing classes. The resulting classes are known as derived classes or subclasses.

A subclass “inherits” all the attributes (methods, etc) of the parent class. We can create new attributes or methods to add to the behavior of the parent. We can change (“override”) some or all of the attributes or methods to change the behavior. We can also extend the behavior of the parent by using the original methods and adding a bit more.

We indicate that a new class should inherit from an existing class by placing the name of the existing class in the list of base classes. The class(es) named in the list of base classes must be in the current namespace when the class statement is evaluated. For compatibility across Python 2 and 3, any new classes we create wil always inherit from at least object. This basic class sits at the top of the Python data model, and is in the __builtin__ namespace.

This is a pseudocode model for the simplest subclass in Python:

class Subclass(Superclass):
    pass

Subclass now has exactly the same behavior as Superclass

Note

When we put object in the base class list, it means we are inheriting from object – getting the core functionality of all objects.

Overriding attributes

One of the core purposes of a subclass is to change the behavior of the parent class in some useful way. We call this overriding the inherited behavior. Overriding attributes of a parent class in Python is as simple as creating a new attribute with the same name:

class Circle(object):
    color = "red"

class NewCircle(Circle):
    color = "blue"

>>> nc = NewCircle
>>> print(nc.color)
blue

Any instances of the new class will have the blue color. Instances of the original class will have the red color.

Overriding methods

Overriding methods works in exactly the same way (remember, a method is an attribute in python).

class Circle(object):
...
    def grow(self, factor=2):
        """grows the circle's diameter by factor"""
        self.diameter = self.diameter * factor
...

class NewCircle(Circle):
...
    def grow(self, factor=2):
        """grows the area by factor..."""
        self.diameter = self.diameter * math.sqrt(2)

Instances of the new circle class will have the new behavior for the grow method. Instances of the existing class will continue to have the old behavior.

When overriding behavior for a subclass, remember that in good OO programming a subclass should be substantially similar to its parents. If you have a system which uses the parent class, you should be able to use the subclass in all the same places, and in all the same ways. This is known as the “Liskov Substitution Principle”. The authors of Think Python put it this way:

whenever you override a method, the interface of the new method should be
the same as the old.  It should take the same parameters, return the same
type, and obey the same preconditions and postconditions.

If you obey this rule, you will find that any function designed to work
with an instance of a superclass, like a Deck, will also work with
instances of subclasses like a Hand or PokerHand.  If you violate this
rule, your code will collapse like (sorry) a house of cards.

-- [ThinkPython 18.10]

Extending Methods

Wanting or needing to override __init__ is very common. After all, we are trying to modify how the parent class works. However, we often also want to do some or all of the things that the parent class does with __init__. We really want to extend the functionality of the parent class __init__. Think “do everything my parent does, plus this other stuff”.

class Circle(object):
    color = "red"
    def __init__(self, diameter):
        self.diameter = diameter
...
class CircleR(Circle):
    def __init__(self, radius):
        diameter = radius*2
        Circle.__init__(self, diameter)

You can do the same thing with the any methods of the parent class. There isn’t anything special about the __init__ method (except that it is called automatically).

class Circle(object):
...
    def get_area(self, diameter):
        return math.pi * (diameter/2.0)**2


class CircleR2(Circle):
...
    def get_area(self):
        return Circle.get_area(self, self.radius*2)

Attribute resolution order

We have discussed how Python looks up attributes of a class instance. It starts in the namespace of the instance, and then looks in the namespace of the class. What happens when your class is a subclass? If the name is not found in the namespace of our instance, or in the class, then the search continues in the parent class, and so on.

• Is it an instance attribute?
• Is it a class attribute?
• Is it a superclass attribute?
• Is it a super-superclass attribute?
• ...

The process of looking up attributes of a class in an inheritance hierarchy seems relatively straightforward. But Python also supports multiple inheritance (two or more names in the base class list). What happens then?

In Python 2.3 a new algorithm was added to Python to clarify this question. The clearest documentation of it can be found in the release notes for 2.3 and in a blog post on the History of Python blog.

For our purposes, it is enough to say that if you have any questions, you can use the Class.mro() method of any new-style class to get the ordered list of its parent classes:

In [37]: class A(object): x = 'A'
   ....:
In [38]: class B(object): x = 'B'
   ....:
In [39]: class C(A): pass
   ....:
In [40]: class D(C, B): pass
   ....:
In [41]: D.mro()
Out[41]: [__main__.D, __main__.C, __main__.A, __main__.B, object]
In [42]: D.x
Out[42]: 'A'
In [43]: class E(B, C): pass
   ....:
In [44]: E.mro()
Out[44]: [__main__.E, __main__.B, __main__.C, __main__.A, object]
In [45]: E.x
Out[45]: 'B'

The acronym MRO stands for Method Resolution Order. Clearly, though, it applies to all attributes of a class, not just to methods.

One final note, regarding the use of object in the base class list for a class. In Python 2, this is the way that we distinguish new-style classes from old-style classes. Old-style classes had a different way of dealing with attribute resolution. It faired poorly when used with multiple inheritance. New-style classes did better with this, especially after Python 2.3 But old-style classes were still around.

In Python 3, there is no such thing as old-style classes. All classes inherit from object whether specified or not. We provide the object base class to maintain compatibility between Python 2 and Python 3.

When to Subclass

Remember that we have stated previously that inheritance should be used primarily to promote code re-use. It’s really meant to be used when the thing you want to build is a variation on the parent class.

If you want to be able to use your new class in all the places and in all the ways that you can use the parent, then it should inherit from the parent. But this is not the only possible choice.

Composition

Let’s imagine that we have a class that needs to accumulate an arbitrary number of objects. A list can do that, so we should subclass list, right?

The thing is, that in addition to being able to accumulate objects, lists support a number of other operations. We can iterate over the objects they contain. We can sort and reverse them.

Does our new class need to do all those things? If the answer is no, then our new class might be better served by containing a list, rather than inheriting from it.

Composition is another Object Oriented programming approach. We use it when we want to use some aspect of another class without promising all of the features of that other class.

Think about our example. Maybe accumulating objects is all we want this new class to do. No other functionality from a list is required. We can build our class to contain a list:

In [46]: class Accumulator(object):
   ....:     def __init__(self):
   ....:         self._container = []
   ....:     def accumulate(self, obj):
   ....:         self._container.append(obj)
   ....:     def stuff(self):
   ....:         return self._container[:]
   ....:

Now, we can build an instance of our Accumulator class and start accumulating stuff:

In [47]: junk_drawer = Accumulator()
In [48]: junk_drawer.accumulate('spatula')
In [49]: junk_drawer.accumulate('cork screw')
In [50]: junk_drawer.accumulate('old rubber band')

And every so often, we can even ask to see what’s in the junk drawer (though like any good junk drawer you can’t actually take anything out):

In [51]: junk_drawer.stuff()
Out[51]: ['spatula', 'cork screw', 'old rubber band']

In [52]: junk_drawer.stuff().pop()
Out[52]: 'old rubber band'

In [53]: junk_drawer.stuff()
Out[53]: ['spatula', 'cork screw', 'old rubber band']

Type-Based Dispatch

One final word for this lesson about classes. We’ll occasionally see code that looks like this:

if isinstance(other, SomeClass):
    Do_something_with_other
else:
    Do_something_else

In general, it’s usually better to use “duck typing” (polymorphism). After all, if other has the right methods or attributes, then why would we care if it is an instance of SomeClass? But when it’s called for, you can use isinstance, or its cousin issubclass:

In [54]: isinstance(junk_drawer, Accumulator)
Out[54]: True

In [55]: isinstance(junk_drawer, object)
Out[55]: True

In [56]: issubclass(Accumulator, object)
Out[56]: True

In [57]: issubclass(object, Accumulator)
Out[57]: False

Wrap Up

In this lecture we learned about subclassing and composition, two approaches to OO programming. We learned how to make a subclass in Python. We learned about the method resolution order and how attributes are looked up when inheritance is in play. We also learned about the difference between old- and new-style classes and how to maintain compatibility in Python 3. Finally, we learned how to use composition to gain access to some of the powers of another class without needing to inherit it all.

As you work on your Data Structures assignments, consider how these new tools can help you.