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.

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.

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.

Let’s look at lec19_restaurant_exercise.py, downloadable as part of the lecture19_files.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.

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…

Look at Restaurant.py which was distributed with lecture19_files.zip.

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.

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

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

Review of the main components of a Python class:
- Attributes
- Methods
- Special methods with names starting and ending with __
  - Initializer method is the 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