11.4. Adding Methods¶
The key advantage of using a class like Point rather than
something like a simple list [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 simple list 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. Methods are accessed using dot notation.
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
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 self.x
from the object itself. 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 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. It will perform a more complex task.
Notice that the caller of distanceFromOrigin does not explicitly
supply an argument to match the self parameter. This is true of
all method calls. The definition will always have one additional
parameter as compared to the invocation.