cpease00 / python-class-variables-class-methods-lab-nyc-career-ds-062518

Python Class Variables and Class Methods Lab

Introduction

In this lab, we are going to put our skills to the test by creating class methods and class variables that will help our program remember instance objects and allow us to operate on these objects in interesting ways. We will be working with a Driver class, which we can define in our file, driver.py.

Objectives

• Use class variables to keep track of data pertaining to a class
• Define class methods that expose data pertaining to a class

Instructions

Okay, so, we have a fleet of drivers and we want to be able to make queries to get details about all of these drivers. Our Driver class should have two class variables; _all and _count. The _all class variable should be assigned to a list that keeps track of all instance objects for the Driver class. The _count class variable should keep track of the number of drivers in our fleet. Initially, we wont have any drivers, so it should be set to 0.

note: remember to load the autoreload extension from IPython

%load_ext autoreload
%autoreload 2

from driver import Driver

We want our drivers to have the following attributes; name, car make, and car model. Again, by convention these attributes should have a leading underscore and be snakecased where appropriate. We will also want to define instance methods using the appropriate decorator to read (get) all of these attributes.

Driver("Helga Pataki", "Toyota", "Camry")
Driver("Arnold Shortman", "Toyota", "Highlander")
Driver("Gerald Johanssen", "Toyota", "Camry")
Driver("Robert 'Big Bob' Pataki", "Honda", "Pilot")
Driver("Grandpa Phil", "Jeep", "Grand Cherokee")
Driver("Rhonda Wellington Lloyd", "Kia", "Sonata")
Driver("Phoebe Heyerdahl", "Honda", "Civic")

Great! Now, onto the more fun stuff. Let's create a few different instance methods that will help us answer questions like how many drivers do we currently have in our fleet? What percent of drivers drive a Toyota and of that, how many drive a Camry? Or more generally, which car make/models do our drivers drive?

To do this, our class will need to have the two class varibles we mentioned earlier, _all and _count, as well as the class methods listed below:

Note: although it is not necessary, feel free to use more class variables such as _car_makes or _car_models. Also, consider when is the best time to increment our _count class variable or add a new instance object to our _all list? It should be the last two lines in our __init__ method after we have instantiated our instance object and instance variables.

class Person:
    
    _all = []
    _count = 0
    
    def __init__(self, cls, name, age):
        self.name = name
        self.age = age
        # call class method to append `self` to _all
        # call class method to increment _count by 1
        

Driver.fleet_size() # returns the number of drivers in the fleet
# example: 7

Driver.driver_names() # returns a list of driver names as strings
# example: ['Helga Pataki', 'Arnold Shortman','Gerald Johanssen', 
# "Robert 'Big Bob' Pataki", 'Grandpa Phil', 'Rhonda Wellington Lloyd',
# 'Phoebe Heyerdahl']

Driver.fleet_makes() # returns a list of car makes in the fleet
# example: ['Toyota', 'Toyota', 'Toyota', 'Honda', 'Jeep', 'Kia', 'Honda']

Driver.fleet_models() # returns a list of car models in the fleet
# example: ['Camry', 'Highlander', 'Camry', 'Pilot', 'Grand Cherokee', 'Sonata', 'Civic']

Driver.fleet_makes_count() 
# returns a dictionary containing a histogram with the key of a car make 
# pointing to the number of cars of that make in the fleet
# example: {'Honda': 2, 'Jeep': 1, 'Kia': 1, 'Toyota': 3}

Driver.fleet_models_count() 
# returns a list of dictionaries as histograms with the key of a car model
# pointing to the number of cars of that model in the fleet
# example: {'Camry': 2, 'Civic': 1, 'Grand Cherokee': 1, 'Highlander': 1, 'Pilot': 1, 'Sonata': 1}

Driver.percent_of_fleet("Toyota") 
# returns the percentage of Toyotas in the fleet
# example: 45.857%

Hint: for the last method, percent_of_fleet, you will need to return a string that represents the percentage as a float with the percent sign at the end of the string. We can use the float() and str() functions to accomplish this as well as concating strings to add the % sign:

num = float((2/10)*100)
num_string = str(num)
percent = num_string + "%"
percent

Summary

In this lab we practiced using class methods and class variables to both store our class's instance objects and operate on them in order to provide answers to our questions about the fleet. We might have noticed that the Driver class is getting pretty inflated with these querying methods. Perhaps there is a way we can structure our code to make this a bit cleaner for us? Maybe we could have another class that has these query methods that we use in our other classes? Let's find out!