Suppose you have the following situation
#include <iostream>
class Animal {
public:
virtual void speak() = 0;
};
class Dog : public Animal {
void speak() { std::cout << "woff!" <<std::endl; }
};
class Cat : public Animal {
void speak() { std::cout << "meow!" <<std::endl; }
};
void makeSpeak(Animal &a) {
a.speak();
}
int main() {
Dog d;
Cat c;
makeSpeak(d);
makeSpeak(c);
}
As you can see, makeSpeak is a routine that accepts a generic Animal object. In this case, Animal is quite similar to a Java interface, as it contains only a pure virtual method. makeSpeak does not know the nature of the Animal it gets passed. It just sends it the signal “speak” and leaves the late binding to take care of which method to call: either Cat::speak() or Dog::speak(). This means that, as far as makeSpeak is concerned, the knowledge of which subclass is actually passed is irrelevant.
But what about Python? Let’s see the code for the same case in Python. Please note that I try to be as similar as possible to the C++ case for a moment:
class Animal(object):
def speak(self):
raise NotImplementedError()
class Dog(Animal):
def speak(self):
print "woff!"
class Cat(Animal):
def speak(self):
print "meow"
def makeSpeak(a):
a.speak()
d=Dog()
c=Cat()
makeSpeak(d)
makeSpeak(c)
Now, in this example you see the same strategy. You use inheritance to leverage the hierarchical concept of both Dogs and Cats being Animals.
But in Python, there’s no need for this hierarchy. This works equally well
class Dog:
def speak(self):
print "woff!"
class Cat:
def speak(self):
print "meow"
def makeSpeak(a):
a.speak()
d=Dog()
c=Cat()
makeSpeak(d)
makeSpeak(c)
In Python you can send the signal “speak” to any object you want. If the object is able to deal with it, it will be executed, otherwise it will raise an exception. Suppose you add a class Airplane to both codes, and submit an Airplane object to makeSpeak. In the C++ case, it won’t compile, as Airplane is not a derived class of Animal. In the Python case, it will raise an exception at runtime, which could even be an expected behavior.
On the other side, suppose you add a MouthOfTruth class with a method speak(). In the C++ case, either you will have to refactor your hierarchy, or you will have to define a different makeSpeak method to accept MouthOfTruth objects, or in java you could extract the behavior into a CanSpeakIface and implement the interface for each. There are many solutions...
What I’d like to point out is that I haven’t found a single reason yet to use inheritance in Python (apart of frameworks and trees of exceptions, but I guess that alternative strategies exist). you don’t need to implement a base-derived hierarchy to perform polymorphically. If you want to use inheritance to reuse implementation, you can accomplish the same through containment and delegation, with the added benefit that you can alter it at runtime, and you clearly define the interface of the contained, without risking unintended side effects.
So, in the end, the question stands: what's the point of inheritance in Python?
Edit: thanks for the very interesting answers. Indeed you can use it for code reuse, but I am always careful when reusing implementation. In general, I tend to do very shallow inheritance trees or no tree at all, and if a functionality is common I refactor it out as a common module routine and then call it from each object. I do see the advantage of having one single point of change (eg. instead of adding to Dog, Cat, Moose and so on, I just add to Animal, which is the basic advantage of inheritance), but you can achieve the same with a delegation chain (eg. a la JavaScript). I'm not claiming it's better though, just another way.
I also found a similar post on this regard.
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