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8.2: Printing Objects

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  • In Chapter 16, we defined a class named Time and in Exercise 16.1.1, you wrote a function named print_time:

    class Time(object):
        """Represents the time of day."""
    def print_time(time):
        print '%.2d:%.2d:%.2d' % (time.hour, time.minute, time.second)

    To call this function, you have to pass a Time object as an argument:

    >>> start = Time()
    >>> start.hour = 9
    >>> start.minute = 45
    >>> start.second = 00
    >>> print_time(start)

    To make print_time a method, all we have to do is move the function definition inside the class definition. Notice the change in indentation.

    class Time(object):
        def print_time(time):
            print '%.2d:%.2d:%.2d' % (time.hour, time.minute, time.second)

    Now there are two ways to call print_time. The first (and less common) way is to use function syntax:

    >>> Time.print_time(start)

    In this use of dot notation, Time is the name of the class, and print_time is the name of the method. start is passed as a parameter.

    The second (and more concise) way is to use method syntax:

    >>> start.print_time()

    In this use of dot notation, print_time is the name of the method (again), and start is the object the method is invoked on, which is called the subject. Just as the subject of a sentence is what the sentence is about, the subject of a method invocation is what the method is about.

    Inside the method, the subject is assigned to the first parameter, so in this case start is assigned to time.

    By convention, the first parameter of a method is called self, so it would be more common to write print_time like this:

    class Time(object):
        def print_time(self):
            print '%.2d:%.2d:%.2d' % (self.hour, self.minute, self.second)

    The reason for this convention is an implicit metaphor:

    • The syntax for a function call, print_time(start), suggests that the function is the active agent. It says something like, “Hey print_time! Here’s an object for you to print.”
    • In object-oriented programming, the objects are the active agents. A method invocation like start.print_time() says “Hey start! Please print yourself.”

    This change in perspective might be more polite, but it is not obvious that it is useful. In the examples we have seen so far, it may not be. But sometimes shifting responsibility from the functions onto the objects makes it possible to write more versatile functions, and makes it easier to maintain and reuse code.

    Exercise \(\PageIndex{1}\)

    Rewrite time_to_int (from Section 16.4) as a method. It is probably not appropriate to rewrite int_to_time as a method; what object you would invoke it on?