Classes and Functions

Outline

• Reminders
• Object-Oriented Programming
• Methods
• Init

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# import packages
import numpy as np
import matplotlib.pyplot as plt
import pandas as pd
import seaborn as sns; sns.set()

Important Notes/Reminders

• Monday, Nov. 18th, no formal class, AWS visiting
• Part 2 - progress report due Fri., Nov. 22nd
• Friday, DMD, no class.

Object-oriented Programming

Python is an object-oriented programming language, which means that it provides features that support object-oriented programming, which has these defining characteristics:

• Programs include class and method definitions.
• Most of the computation is expressed in terms of operations on objects.
• Objects often represent things in the real world, and methods often correspond to the ways things in the real world interact.

Before, we would define the class and a function as such

class Time:
    """Represents the time of day.
    attributes: hour, minute, second
    """

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

Methods

This observation is the motivation for methods; a method is a function that is associated with a particular class. Methods are semantically the same as functions, but there are two syntactic differences:

• Methods are defined inside a class definition in order to make the relationship between the class and the method explicit.
• The syntax for invoking a method is different from the syntax for calling a function.

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

start = Time() # this is an instance of a Time object
start.hour = 9
start.minute = 45
start.second = 00
print_time(start)

09:45:00

Now, let's use method syntax to call this function

start.print_time()

09:45:00

Note: print_time is the name of the method , and start is the object the method is invoked on, which is called the subject.

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:
    def print_time(self):
        print('%.2d:%.2d:%.2d' % (self.hour, self.minute, self.second))
        
    def increment_time(self, seconds):
        return self
        # put the definition of increment_time from last notebook

The init method

The init method (short for “initialization”) is a special method that gets invoked when an object is instantiated. Its full name is __init__ (two underscore characters, followed by init, and then two more underscores). An init method for the Time class might look like this:

class Time:
    """Represents the time of day.
    attributes: hour, minute, second
    """
    def __init__(self, hour=0, minute=0, second=0):
        # assign self.hour as hour (passed in)
        self.hour = hour
        self.minute = minute
        self.second = second
    
    def print_time(self):
        print('%.2d:%.2d:%.2d' % (self.hour, self.minute, self.second))

start2 = Time(3,2,11)

start2.print_time()

03:02:11