Exercise 42 Is-A, Has-A, Objects, and Classes

An important concept that you have to understand is the difference between a Class and an Object. The problem is, there is no real "difference" between a class and an object. They are actually the same thing at different points in time. I will demonstrate by a Zen koan:

What is the difference between a Fish and a Salmon?

Did that question sort of confuse you? Really sit down and think about it for a minute. I mean, a Fish and a Salmon are different but, wait, they are the same thing right? A Salmon is a kind of Fish, so I mean it's not different. But at the same time, becase a Salmon is a particular type of Fish and so it's actually different from all other Fish. That's what makes it a Salmon and not a Halibut. So a Salmon and a Fish are the same but different. Weird.

This question is confusing because most people do not think about real things this way, but they intuitively understand them. You do not need to think about the difference between a Fish and a Salmon because you know how they are related. You know a Salmon is a kind of Fish and that there are other kinds of Fish without having to understand that.

Let's take it one step further, let's say you have a bucket full of 3 Salmon and because you are a nice person, you have decided to name them Frank, Joe, and Mary. Now, think about this question:

What is the difference between Mary and a Salmon?

Again this is a weird question, but it's a bit easier than the Fish vs. Salmon question. You know that Mary is a Salmon, and so she's not really different. She's just a specific "instance" of a Salmon. Joe and Frank are also instances of Salmon. But, what do I mean when I say instance? I mean they were created from some other Salmon and now represent a real thing that has Salmon-like attributes.

Now for the mind bending idea: Fish is a Class, and Salmon is a Class, and Mary is an Object. Think about that for a second. Alright let's break it down real slow and see if you get it.

A Fish is a Class, meaning it's not a real thing, but rather a word we attach to instances of things with similar attributes. Got fins? Got gills? Lives in water? Alright it's probably a Fish.

Someone with a Ph.D. then comes along and says, "No my young friend, this Fish is actually Salmo salar, affectionately known as a Salmon." This professor has just clarified the Fish further and made a new Class called "Salmon" that has more specific attributes. Longer nose, reddish flesh, big, lives in the ocean or fresh water, tasty? Ok, probably a Salmon.

Finally, a cook comes along and tells the Ph.D., "No, you see this Salmon right here, I'll call her Mary and I'm going to make a tasty fillet out of her with a nice sauce." Now you have this instance of a Salmon (which also is an instance of a Fish) named Mary turned into something real that is filling your belly. It has become an Object.

There you have it: Mary is a kind of Salmon that is a kind of Fish. Object is a Class is a Class.
How This Looks In Code

This is a weird concept, but to be very honest you only have to worry about it when you make new classes, and when you use a class. I will show you two tricks to help you figure out whether something is a Class or Object.

First, you need to learn two catch phrases "is-a" and "has-a". You use the phrase is-a when you talk about objects and classes being related to each other by a class relationship. You use has-a when you talk about objects and classes that are related only because they reference each other.

Now, go through this piece of code and replace each ##?? comment with a replacement comment that says whether the next line represents an is-a or a has-a relationship, and what that relationship is. In the beginning of the code, I've laid out a few examples, so you just have to write the remaining ones.

Remember, is-a is the relationship between Fish and Salmon, while has-a is the relationship between Salmon and Gills.

## Animal is-a object (yes, sort of confusing) look at the extra credit
class Animal(object):
    pass

## ??
class Dog(Animal):

    def __init__(self, name):
        ## ??
        self.name = name

## ??
class Cat(Animal):

    def __init__(self, name):
        ## ??
        self.name = name

## ??
class Person(object):

    def __init__(self, name):
        ## ??
        self.name = name

        ## Person has-a pet of some kind
        self.pet = None

## ??
class Employee(Person):

    def __init__(self, name, salary):
        ## ?? hmm what is this strange magic?
        super(Employee, self).__init__(name)
        ## ??
        self.salary = salary

## ??
class Fish(object):
    pass

## ??
class Salmon(Fish):
    pass

## ??
class Halibut(Fish):
    pass


## rover is-a Dog
rover = Dog("Rover")

## ??
satan = Cat("Satan")

## ??
mary = Person("Mary")

## ??
mary.pet = satan

## ??
frank = Employee("Frank", 120000)

## ??
frank.pet = rover

## ??
flipper = Fish()

## ??
crouse = Salmon()

## ??
harry = Halibut()

About class Name(object)

Remember how I was yelling at you to always use class Name(object) and I couldn't tell you why? Now I can tell you, because you just learned about the difference between a class and an object. I couldn't tell you until now because you would have just been confused and couldn't learn to use the technology.

What happened is Python's original rendition of class was broken in many serious ways. By the time they admitted the fault it was too late, and they had to support it. In order to fix the problem, they needed some "new class" style so that the "old classes" would keep working but you could use the new more correct version.

This is where "class is-a object" comes in. They decided that they would use the word "object", lowercased, to be the "class" that you inherit from to make a class. Confusing right? A class inherits from the class named object to make a class but it's not an object really it's a class, but do not forget to inherit from object.

Exactly. The choice of one single word meant that I couldn't teach you about this until now. Now you can try to understand the concept of a class that is an object if you like.

However, I would suggest you do not. Just completely ignore the idea of old style vs. new style classes and assume that Python always requires (object) when you make a class. Save your brain power for something important.
Study Drills

    Research why Python added this strange object class, and what that means.
    Is it possible to use a Class like it's an Object?
    Fill out the animals, fish, and people in this exercise with functions that make them do things. See what happens when functions are in a "base class" like Animal vs. in say Dog.
    Find other people's code and work out all the is-a and has-a relationships.
    Make some new relationships that are lists and dicts so you can also have "has-many" relationships.
    Do you think there's a such thing as a "is-many" relationship? Read about "multiple inheritance", then avoid it if you can.

Common Student Questions

What are these ## ?? comments for?
    Those are "fill-in-the-blank" comments that you are supposed to fill in with the right "is-a", "has-a" concepts. Re-read this exercise and look at the other comments to see what I mean.
What is the point of self.pet = None?
    That makes sure that the self.pet attribute of that class is set to a default of None.
What does super(Employee, self).__init__(name) do?
    That's how you can run the __init__ method of a parent class reliably. Go search for "python super" and read the various advice on it being evil and good for you.

Copyright (C) 2010 Zed. A. Shaw

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