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Foundations of Python Programming: Functions First

Section 13.5 Adding Other Methods to a Class

The key advantage of using a class like Point rather than something like a simple tuple (7, 6) now becomes apparent. We can add methods to the Point class that are sensible operations for points. Had we chosen to use a tuple to represent the point, we would not have this capability. Creating a class like Point brings an exceptional amount of “organizational power” to our programs, and to our thinking. We can group together the sensible operations, and the kinds of data they apply to, and each instance of the class can have its own state.
A method behaves like a function but it is invoked on a specific instance. For example, with a list bound to variable L, L.append(7) calls the function append, with the list itself as the first parameter and 7 as the second parameter. Methods are accessed using dot notation. This is why L.append(7) has 2 parameters even though you may think it only has one: the list stored in the variable L is the first parameter value and 7 is the second.
Let’s add two simple methods to allow a point to give us information about its state. The getX method, when invoked, will return the value of the x coordinate.
The implementation of this method is straight forward since we already know how to write functions that return values. One thing to notice is that even though the getX method does not need any other parameter information to do its work, there is still one formal parameter, self. As we stated earlier, all methods defined in a class that operate on objects of that class will have self as their first parameter. Again, this serves as a reference to the object itself which in turn gives access to the state data inside the object.
Note that the getX method simply returns the value of the instance variable x from the object self. In other words, the implementation of the method is to go to the state of the object itself and get the value of x. Likewise, the getY method looks almost the same.
Let’s add another method, distanceFromOrigin, to see better how methods work. This method will again not need any additional information to do its work, beyond the data stored in the instance variables. It will perform a more complex task.
Notice that the call of distanceFromOrigin does not explicitly supply an argument to match the self parameter. This is true of all method calls. The definition will always seem to have one additional parameter as compared to the invocation.
Check Your Understanding
  1. Create a class called Animal that accepts two numbers as inputs and assigns them respectively to two instance variables: arms and legs. Create an instance method called limbs that, when called, returns the total number of limbs the animal has. To the variable name spider, assign an instance of Animal that has 4 arms and 4 legs. Call the limbs method on the spider instance and save the result to the variable name spidlimbs.
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