If we don't explicitly set the x and y positions on our Point object, either using move or by accessing them directly, we have a broken point with no real position. What will happen when we try to access it?
Well, let's just try it and see. Try it and see is an extremely useful tool for Python study. Open up your interactive interpreter and type away. The following interactive session shows what happens if we try to access a missing attribute. If you saved the previous example as a file or are using the examples distributed with the book, you can load it into the Python interpreter with the python -i more_arguments.py command:
>>> point = Point()
>>> point.x = 5
>>> print(point.x)
5
>>> print(point.y)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Point' object has no attribute 'y'
Well, at least it threw a useful exception. We'll cover exceptions in detail in Chapter 4, Expecting the Unexpected. You've probably seen them before (especially the ubiquitous SyntaxError, which means you typed something incorrectly!). At this point, simply be aware that it means something went wrong.
The output is useful for debugging. In the interactive interpreter, it tells us the error occurred at line 1, which is only partially true (in an interactive session, only one line is executed at a time). If we were running a script in a file, it would tell us the exact line number, making it easy to find the offending code. In addition, it tells us that the error is an AttributeError, and gives a helpful message telling us what that error means.
We can catch and recover from this error, but in this case, it feels like we should have specified some sort of default value. Perhaps every new object should be reset() by default, or maybe it would be nice if we could force the user to tell us what those positions should be when they create the object.
Most object-oriented programming languages have the concept of a constructor, a special method that creates and initializes the object when it is created. Python is a little different; it has a constructor and an initializer. The constructor function is rarely used, unless you're doing something very exotic. So, we'll start our discussion with the much more common initialization method.
The Python initialization method is the same as any other method, except it has a special name, __init__. The leading and trailing double underscores mean this is a special method that the Python interpreter will treat as a special case.
Never name a method of your own with leading and trailing double underscores. It may mean nothing to Python today, but there's always the possibility that the designers of Python will add a function that has a special purpose with that name in the future, and when they do, your code will break.
Let's add an initialization function on our Point class that requires the user to supply x and y coordinates when the Point object is instantiated:
class Point:
def __init__(self, x, y):
self.move(x, y)
def move(self, x, y):
self.x = x
self.y = y
def reset(self):
self.move(0, 0)
# Constructing a Point
point = Point(3, 5)
print(point.x, point.y)
Now, our point can never go without a y coordinate! If we try to construct a point without including the proper initialization parameters, it will fail with a not enough arguments error similar to the one we received earlier when we forgot the self argument.
If we don't want to make the two arguments required, we can use the same syntax Python functions use to provide default arguments. The keyword argument syntax appends an equals sign after each variable name. If the calling object does not provide this argument, then the default argument is used instead. The variables will still be available to the function, but they will have the values specified in the argument list. Here's an example:
class Point:
def __init__(self, x=0, y=0):
self.move(x, y)
Most of the time, we put our initialization statements in an __init__ function. But as mentioned earlier, Python has a constructor in addition to its initialization function. You may never need to use the other Python constructor (in well over a decade of professional Python coding, I can only think of two cases where I've used it, and in one of them, I probably shouldn't have!), but it helps to know it exists, so we'll cover it briefly.
The constructor function is called __new__ as opposed to __init__, and accepts exactly one argument; the class that is being constructed (it is called before the object is constructed, so there is no self argument). It also has to return the newly created object. This has interesting possibilities when it comes to the complicated art of metaprogramming, but is not very useful in day-to-day Python. In practice, you will rarely, if ever, need to use __new__. The __init__ method will almost always be sufficient.
