Lecture 19 — Classes, Part 2

Overview

  • Review of classes

  • Revisiting our Yelp data: a Restaurant class.

  • Techniques that we will see:

    • Calling class methods from within the class

    • Class objects storing other objects, such as lists

    • Lists of class objects

Review of Classes

We will use our Point2d class solution from Lecture 18 to review the following:

  • Attributes:

    • These store the data associated with each class instance.

    • They are usually defined inside the class to create a common set of attributes across all class instances.

  • Initialization: function __init__ called when the object is created.

    • Should assign initial values to all attributes

  • Methods

    • Each includes the object, often referred to as self, as the first argument.

    • Some change the object, some create new objects

  • Special methods start and end with two underscores. Python interprets their use in a variety of distinct ways:

    • __str__ is the string conversion function

    • __add__, __sub__, etc. become operators

  • Each of these special methods builds on the “more primitive” methods

Larger Example — Restaurant Class

Recall Lab 5 on the Yelp data:

  • Read and parse input lines that look like:

    The Greek House|42.73|-73.69|27 3rd St+Troy, NY 12180|\
   http://www.yelp.com/biz/the-greek-house-troy|Greek|1|5|4|5|4|4|5|5|5|5|5|4

  • Find restaurants and print out information based on a user selection

  • Original implementation based on a list was awkward:

    • We had to remember the role of each index of the list — 0 was the name, 1 was the latitude, etc.

  • New implementation here is based on a class

Start to a Solution, the Main Code

Let’s look at lec19_restaurants_exercise.py, downloadable as part of the Lecture_19 zip file in the Course Materials section of Submitty:

  • This is the code that uses the Restaurant class.

    • We start by considering how the class will be used rather than how we write it.

  • Main function to initialize a restaurant is called convert_input_to_restaurant

    • Parses a restaurant line

    • Creates and returns a Restaurant object

  • Function build_restaurant_list

    • Opens the input file

    • Reads each line

    • Calls convert_input_to_restaurant, and appends the resulting restaurant to the back of a list

  • Main code:

    • Builds the restaurant list

    • Prints the first three restaurants in the list

    • Includes commented-out code that

      • Gets the name of a city

      • Finds the restaurant with the highest average rating

      We will complete this code soon.

Functionality Needed in the Restaurant Class

  • Some functionality is determined by reading the code we have already written

    • Includes both methods and attributes

  • Add other functionality by considering the methods that must be in the Restaurant class, including the parameters that must be passed to each method.

  • Add attributes last…

Turning to the Actual Restaurant Class

Look at Restaurant.py which was distributed with the Lecture_19 files.

  • The __init__ function specifies the attributes.

    • Other attributes could be added, such as the average rating, but instead these are computed as needed by methods.

    • Importantly, each class object stores a list of ratings, illustrating the fact that classes can store data structures such as lists, sets, and dictionaries.

  • The Restaurant class has more complicated attributes than our previous objects

    • Point2d object,

    • A list for the address entries

    • A list of scores

  • There is nothing special about working with these attributes other than they “feel” more complicated.

    • Just apply what you know in using them

    • Our lecture exercises will help

In-Class Example

Together we will add the following two methods to Restaurant to get our demonstration example to work:

  1. The is_in_city method

  2. The average_review method

Discussion

  • What is not in the Restaurant class?

    • No input or line parsing. Usually, we don’t want the class tied to the particular form of the input.

    • As an alternative, we could add a method for each of several different forms of input.

  • Often it is hard to make the decision about what should be inside and what should be outside the class.

    • One example the method we wrote to test if restaurant is in a particular city. As an alternative we could have written a different method that returns that name of the city and make the comparison outside the class.

  • We could add an Address class:

    • Reuse for objects other than restaurants

    • Not needed in this (relatively) short example.

    • More flexible than our use of a list of strings from an address line.

Summary

  • Review of the main components of a Python class:

    • Attributes

    • Methods

    • Special methods with names starting and ending with __

      • Initializer method is most important

  • Important uses of Python classes that we have seen today:

    • Classes containing other objects as attributes

    • Lists of class objects.

  • Design of Python classes

    • Start by outlining how they are to be used

    • Leads to design of methods

    • Specification of attributes and implementation of methods comes last