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19.6. Adding Other Methods to our 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 simple 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 turtle named tess, tess.right(90) asks the tess object to perform its right method and turn 90 degrees. 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.

class Point: """ Point class for representing and manipulating x,y coordinates. """ def __init__(self, initX, initY): """ Create a new point at the given coordinates. """ self.x = initX self.y = initY def getX(self): return self.x def getY(self): return self.y p = Point(7, 6) print(p.getX()) print(p.getY())

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.

class Point: """ Point class for representing and manipulating x,y coordinates. """ def __init__(self, initX, initY): """ Create a new point at the given coordinates. """ self.x = initX self.y = initY def getX(self): return self.x def getY(self): return self.y def distanceFromOrigin(self): return ((self.x ** 2) + (self.y ** 2)) ** 0.5 p = Point(7, 6) print(p.distanceFromOrigin())

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.

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