Functions, Classes, and Methods in Python

In Python, the signature of a function (which is part of its definition) looks very similar to the code used to call that function.

def lineOverrun(text, line_length):
    return max(0, len(text) - line_length)

line = "The quick brown log rolled down the lazy stairs."
extra = lineOverrun(line, 80)

Note that the number of arguments specified in the signature is the same as the number of arguments we pass when we call the function. When the function runs, the variable specified as the  first argument  in the function definition takes on the value of the variable given as the  first argument  in the call to the function. Same is true for the second argument. So the arguments exactly parallel each other in the call and the signature.

An object is an individual, distinct “thing” in the world of a program. Each object is an instance of a class — that is, it's a single entity with a distinct identity, but it is conceptually related to all other objects of that same class.

So one of the fundamental tasks of an object-oriented design process is to design and define classes, so that we can then solve problems with instances (objects) of those classes. In other words, before we can use a particular hammer to solve a problem, we must first define what a hammer is, conceptually.

A class describes a “conceptual bubble” — each instance of a class contains a number of data values, and is therefore in charge of them. The technical name for these data values is member variables.

Each instance of a class also carries along with it a selection of specialized capabilities — things that it can do, when asked by something on the outside, based on its knowledge and understanding of its member data. We express these capabilities by defining methods for the class. A method is just a specialized type of function that is part of a class.

Let's take a look at an example. Here we're defining the idea of a hammer that has a particular weight and a particular amount of wear. Every time we hit something with the hammer, the wear increases.

class Hammer:
    def __init__(self, initial_weight):
        self.weight = initial_weight
        self.wear = 0.0
    
    def hit(self, nail_height, speed):
        force = self.weight*speed
        self.wear = self.wear + force
        return nail_height / force

Note that, after Python has executed this code, no actual hammers exist in the program! All we've done is define a class; we've said what a hammer would be like if we ever chose to create one and use it.

We've defined two methods for the class Hammer: __init__() and hit().

• __init__() is a special method called a constructor. A constructor tells the client (the code that uses a class) how to create a new object that's an instance of that particular class. Constructors always have the name __init__(); it's a special name set aside by Python to designate constructors.

  One of the primary jobs of a constructor is to define and initialize the member variables. Note that each member variable is specified by writing self. and then the name of the variable. We've seen the . operator before: it's how we access something that's a part of, or inside, something else.

  So we can see that this syntax means that self.weight is a variable, weight, that is part of something else, namely self. So what is self? It's the special name given to the particular object that a given method is being called with. When a method is run on an object, within the instructions in that method, the object refers to itself with the name self. Good name, huh?

  So self.weight means “the variable named weight that belongs to the object currently under consideration”. In the constructor, that object is the object that's being created.

• hit() is just a regular method, a capability of objects of the class Hammer. Note that it has two distinct effects: it both modifies the value of a member variable (and therefore modifies the state of the Hammer object itself), and also returns a different value that it has computed. It can access its member variables by using the self. syntax.

  Methods are just functions, so the definition of a method is just a set of instructions that Python will follow when that method is called. A method is always called on a particular instance of the class for which the method is defined. So, in this case, we would only call hit() with an instance of the Hammer class. That particular object is what would be called self when running the method.

Though methods are really just functions defined as part of a class, we call them using a slightly different syntax. Here I've expanded the code above to include some client code that uses the class to create an object and then work with it.

class Hammer:
    def __init__(self, initial_weight):
        self.weight = initial_weight
        self.wear = 0.0
    
    def hit(self, nail_height, speed):
        force = self.weight*speed
        self.wear = self.wear + force
        return nail_height / force
    
    def getWear(self):
        return self.wear

sledge = Hammer(5.0)
nail = 2.0
print "Nail height:", nail
for hit_number in range(5):
    nail = sledge.hit(nail, 3.0)
    print "After a hit:", nail
print "Total wear:", sledge.getWear()

In this code, we first define the class Hammer, along with all its properties (the member variables) and capabilities (the methods). Note that we've defined an additional method, getWear(), in order to find out the wear on a given hammer once we've used it. (Stricly speaking, this method is unnecessary; we'll discuss this more in class. It's just here to provide an additional example of a method.)

After the class definition is some client code that uses this definition to create a Hammer object and work with it. Let's go line by line:

• First we construct a new instance of the class Hammer, and call it sledge. There are several interesting things to note about the process of constructing this object:

  • Even though we define the constructor using the special name __init__(), we call it using the name of the class. When Python sees client code use the name of a class as a function (that is, with open- and close-parentheses after it), it knows you're asking to construct an instance of that class, and so it calls the class's constructor.

  • In the definition of the constructor, there are two arguments. But we only had to pass one argument (5.0) when we called the constructor.

    This is because the first argument in the definition of a method is always self, and Python always fills that in automatically. For a constructor, self is the name given to the brand-new object that the constructor is responsible for creating.

    The arguments in the signature after self are assigned values respectively by the arguments explicitly passed into the constructor by the client. So, since initial_weight is the first argument in the signature after self, it gets the value 5.0.

• Next we define a nail, which we think of as just a floating-point value indicating its height above the board that we're trying to nail it into, and print out its height.
• Then we start repeatedly hitting the nail with the hammer.

• Within this loop, we call the hit() method of our hammer. Note again that the syntax differs in a variety of ways from the usual way we call functions:

  • To call a method on a particular object, we use the dot operator. The general form of such a call is object.method(arguments). We call the object on which the method is called the “target” of that method call.

  • When the method gets called, Python automatically makes the variable self refer to the target of that call.

    In the code above, the target of the call to the hit() method is the object sledge. So in the body of that method, when evaluating the call, self refers to the same Hammer object that sledge refers to.

  • Similar to the above, the arguments in the signature after self are assigned values respectively by the arguments explicitly passed into the method by the client. So nail_height = nail and speed = 3.0.
• After the loop is done, we call getWear() on sledge to find out how much wear it has accumulated from us banging it against that nail. Once again, when Python evaluates the body of getWear(), self = sledge, and there are no further arguments to be assigned.

There are a lot of equivalent terms or phrases used to express the same ideas when discussing topics related to objects and classes.

Class

A conceptual category or type of thing, as distinct from any specific individual thing that belongs to that class. The definition of a class describes the properties and capabilities that any particular instance of that class will have.

Object

An individual thing that belongs to some class. We call this an “instance” of that class. Equivalent phrases to describe a particular object include:

• “An instance of the class Thingie”
• “an object of class Thingie”
• “a Thingie object”
• “a Thingie”

Member Variable

A variable stored as a part of an object. The class to which that object belongs is what defines the member variables that the object has. Member variables are also known as “data members” or “instance variables”, or collectively as “member data”.

No one on the outside should need to know or care about the internal details of the object's state; it is the responsibility solely of the object itself to keep track of its state and make sense of it. This is the essential purpose of object-oriented design: separation of concerns. Only the object needs to work with the member variables, and the logic of this work is encoded in the class's definition.

Has

When we say that a particular object “has” a particular data member, we mean that that data member is defined as a part of that object, with a name that is only accessible using the dot operator on that object.

We may also say that a class “has” a particular data member, when all objects of that class have that data member. We can see this in the definition of the class because we will have written self.bar to indicate the bar data member that this class (and therefore every object of this class) has.

Synonyms for saying that an object or class has something are to say that it “owns” or “contains” that thing.

State

In computing in general, the state of something at a particular moment is the value it has at that moment, or the values that each of its constituent parts has. So the state of a program is the collective term for the value that each variable in the program has at one particular moment. The state of an object is the collective term for the value that each of its member variables has.

Client

The client is the code that uses a class (or objects of that class) to solve some problem or engage in some action.

Method

A method is a function that is defined as part of a class; conceptually, it is a capability of every object of that class. The primary purpose of a method is for client code to be able to ask a specific object to perform some action. We'll refer to the “target” of a method to indicate the particular object that was asked to perform that method.

The particular actions taken by the target when it performs some method may depend on the state of the target. An object, as a “conceptual bubble”, is in charge of understanding what its state means, and interpreting that state accordingly to decide what to do when performing the method.

This means that the client must specify a particular object in order to run a method: different objects have their own separate state, and so the computations involved in a method may be different for one object than for another. The client specifies the target of a method by putting it before the . operator.

When the client specifies a target and asks the target to run some method, there are a variety of shorthand terminologies: we may say that it runs the method “on” the target, or “with” the target, or even “from” the target.

(There are a couple of exceptions to the “every-method-must-have-a-target” rule, but we won't really get into them.)

.

The . operator is the way we access things that are defined as a part of something else. So foo.bar means “the bar that is part of foo” or “the bar that is inside foo”.

The way you say . when reading code out loud is “dot”.

So far in this course we have seen two different things come before the dot: modules and objects. (Remember, a module is what you get when you say import such_and_so.)

We have also seen two different things come after the dot: variables and functions. When a function is defined as part of a class (and therefore is accessed by putting a dot after the name of an object of that class), we call it a “method”.