Lab #4: Classes and Objects

This lab is divided into two parts. In Part 1, we will extend the Point class introduced in lecture and implement a Library class to work with some data from the Hawaii State Public Library. In Part 2, we will add some functionality to the Divvy classes we saw in the lectures.

Getting started

Open up a terminal and navigate (cd) to your cmsc12100-aut-20-username directory, where username is your CNetID. Run git pull upstream master to collect the lab materials and git pull to sync with your personal repository.

Part 1: Working with Objects

The goal of this part of the lab is to understand how objects work. In Lab 2, you wrote some functions of your own and got some practice calling them in order to create larger programs that were made possible through the composition of those functions. In this lab, you will take that idea a step further by using objects to organize and compose data.

Python is an object-oriented language, which means that everything in Python is really a structure called an object. So, for example, when we create a dictionary:

In [1] d = {"foo": 42, "bar": 37}

What we’re really doing is creating an instance of the dict class which we store in a variable called d. The “type of an object” is called its class. So, when we refer to “the dict class” we refer to the specification of a datatype that is used to contain character strings. In fact, we can also create a dictionary like this:

In [2] d = dict([("foo", 42), ("bar", 37)])

Or an empty dictionary like this:

In [3] d = dict()

In lecture, we’ve referred to some data types (like int and float as “primitive data types” that specify a domain of values (like integers, real numbers, boolean values, etc.). In Python, these data types are actually also objects, even if we don’t tend to think of them as such (in fact, some other programming languages, like Java, also handle primitive data types as non-object types). For example, if you create a float variable:

In [4] x = 0.33

Variable x is actually an instance of Python’s float class, which has a few handy methods, like as_integer_ratio, which returns the floating point number as a numerator/denominator tuple:

In [5] x = 0.25

In [6] x.as_integer_ratio()
Out[6](1, 4)

Play around with this type a bit. Notice anything interesting with certain floating point numbers?

Point class

In lecture, we introduced the Point class shown below.

class Point:

    def __init__(self, x, y):
        self.x = x
        self.y = y

    def distance_to_origin(self):

        return math.sqrt(self.x**2 + self.y**2)

This class has a constructor, two attributes (x and y), and one method called distance_to_origin which returns the distance betweeen the point and the origin.

We used the Point class (stored in a module called in IPython by importing the module, creating an instance of Point, and calling the distance_to_origin method on that instance.

In [1]: import point

In [2]: p0 = point.Point(-1, 1)

In [3]: p0.x
Out[3]: -1

In [4]: p0.y
Out[4]: 1

In [5]: p0.distance_to_origin()
Out[5]: 1.4142135623730951

In this section, you’ll add two methods to the Point class. Start by opening up in VSCode and start an IPython session for testing and experimentation.

Polar coordinates can be used as an alternative to the familiar Cartesian coordinate system. Polar coordinates are determined by a distance from a reference point (\(r\)) and an angle (\(\theta\)) from a reference direction. Given the Cartesian coordinates (x, y), we can compute Polar coordinates using the following formulas:

\[ \begin{align}\begin{aligned}r &= \sqrt{x^2 + y^2}\\\theta &= \tan^{-1}\bigg(\frac{y}{x}\bigg)\end{aligned}\end{align} \]

Complete the to_polar method in the Point class with the following header. For this problem, you can assume that \(x > 0\).

def to_polar(self):
    Compute the polar coordinates

    Returns: radial coordinate, angular coordinate
      in degrees (tuple)


You will need to use the sqrt function from the math module, as well as two others:

  • math.atan: returns the inverse tangent of a number in randians,

  • math.degrees: converts an angle from radians to degrees.

You can test your method by creating an instance of the Point class and calling your method on the instance.

In [6]: p1 = point.Point(1, 1)

In [7]: p1.to_polar()
Out[7]: (1.4142135623730951, 45.0)

In Lab #2, you wrote a function called dist to calculate the distance between two points (x1, y1) and (x2, y2) using the formula:

\[\sqrt{(x_1 - x_2)^2 + (y_1 - y_2)^2}\]

Your function probably looked something like this:

def dist(p1, p2):

    (x1, y1) = p1
    (x2, y2) = p2

    return math.sqrt((x1 - x2)*(x1 - x2) + (y1 - y2)*(y1 - y2))

And you might’ve used it in IPython like this:

In [8]: p3 = (1, 2)

In [9]: p4 = (1, 3)

In [10]: dist(p3, p4)
Out[10]: 1.0

Now you will implement the same computation in an object-oriented fashion. Complete the Point method called distance in with the header:

def distance(self, other):
    Calculate the distance between two points

        other: (Point) the other point

    Returns: distance (float)

This method returns the distance between the calling Point instance (self) and another Point instance (other). You can call your function from IPython like this:

In [11]: p5 = point.Point(0, 0)

In [12]: p6 = point.Point(1, 1)

In [13]: p5.distance(p6)
Out[13]: 1.4142135623730951

In [14]: p6.distance(p5)
Out[14]: 1.4142135623730951

Library class

In this exercise, you will be using data from the Statistics by type of media in Hawaii State Public Libraries for 2011. For each Hawaiian public library, the Hawaii State Public Library System keeps track of the following:

  • location of the branch (a Hawaiian island),

  • name of the branch,

  • number of references,

  • number books,

  • number of microform documents,

among some other things. For simplicity, we will assume that a library only contains references, books, and microform documents in its circulation.

In this exercise, you will implement a Library class to organize the data from this dataset along with some functions to work with the data. Start by opening up in VSCode and add the following to the Library class.

  • Write a constructor for the Library class. Your constructor should take six parameters (including self) and initialize five attributes. It’s important for our helper function that you follow this exact naming and ordering.

    • island

    • name

    • reference

    • book

    • microform

  • Write a method called total_circulation that returns the total number of items in circulation at the library. The circulation of a library is the number of references, books, and microform documents. This method will have only one parameter, self.

  • Write a method called has_microform_catalogue that returns True if a library has any microform documents and False otherwise. This method will also only have self has a parameter.

You can test your implementation by creating a Library object in IPython and calling your methods on your object.

In [1]: import library

In [2]: lib = library.Library("Oahu", "Kaneohe", 6906, 109880, 313)

In [3]: lib.island
Out[3]: 'Oahu'

In [4]:
Out[4]: 'Kaneohe'

In [5]: lib.reference
Out[5]: 6906

In [6]:
Out[6]: 109880

In [7]: lib.microform
Out[7]: 313

In [8]: lib.total_circulation()
Out[8]: 117099

In [9]: lib.has_microform_catalogue()
Out[9]: True

We have included a utility module called that creates an instance of your Library class for each Hawaiian public library included in the dataset. The Library objects are stored in a list called HI_LCS_2011, which you can utilize from IPython.

In [10]: import util

In [11]: len(util.HI_LCS_2011)
Out[11]: 51

In this task, you will complete the following functions to answer questions about the library data. Both functions take as input a list of Library objects (such as HI_LCS_2011). Be sure to use your Library class methods to complete the functions.

  • Write a function called branch_with_biggest_circulation that takes a list of Library objects as an argument and finds the name of the branch with the biggest number of items in circulation.

  • Write a function called percentage_with_microform that takes a list of Library objects as an argument and computes the percentage of branches that have a microform catalogue.

Finally, test your functions from IPython.

In [12]: library.branch_with_biggest_circulation(util.HI_LCS_2011)
Out[12]: 'Hawaii State'

In [13]: library.percentage_with_microform(util.HI_LCS_2011)
Out[13]: 7.8431372549019605

Unsurprisingly, Hawaii State has a large collection and few branches still have microform catalogues.

When Finished

When you are finished with the lab, please check in your work (assuming you are inside the lab directory):

git add
git add
git commit -m "Finished with lab4, part1"
git push

No, we’re not grading your work. We just want to make sure your repository is in a clean state and that your work is saved to your repository (and to our Git server)

Part 2: Class Composition

In this part of the lab, we will explore how to use multiple classes together using composition relationships between classes. To do so, we will use several classes that model information about the Divvy bike-sharing program.

Working with the Divvy Data

As you probably know, Divvy is Chicago’s enormously popular bike sharing system. In 2014, Divvy published (anonymized) data on all the Divvy bicycle trips taken in 2013 (this data was published as part of the 2013 Divvy Data Challenge). The dataset contains two files: one with information about each Divvy station, and one with information about each Divvy trip.

In the first part of the lab, we will be using four classes that model the Divvy dataset:

  • Location: A class representing a geographic location.

  • DivvyStation: A class representing an individual Divvy station.

  • DivvyTrip: A class representing an individual Divvy trip.

  • DivvyData: A class representing the entire dataset, which includes a list of stations and a list of trips.

An important aspect of object orientation is the ability to create relationships between different classes, to model real-world relationships. For example, a Divvy trip has an origin station and a destination station. Instead of trying to pack all the information about the stations in the DivvyTrip class, we instead have a separate DivvyStation class that is used to represent individual stations. The DivvyTrip class then only needs to have two attributes of type DivvyStation: one for the origin station and one for the destination station.

These relations are referred as composition relationships, because they allow us to define a class that is composed of other classes. A useful way to think of these kind of relationships is that, if you can describe the relationship as “has a” (e.g., “A DivvyStation has a Location”), it is probably a composition relationship.

All the composition relations between the Divvy classes are summarized in the following figure:


1,2. The DivvyData class represents the entire Divvy dataset, so it contains (1) a dictionary that maps station identifiers to DivvyStation objects, and (2) a list of DivvyTrip objects.

3. As discussed above, a DivvyTrip has two DivvyStation objects associated with it. This relationship is implemented simply by setting two attributes, from_station and to_station in the DivvyStation constructor.

4. Finally, each Divvy station has a location, which we represent using an instance of the Location class. Again, this is done simply setting an attribute, location to a Location object in the DivvyStation constructor.

The DivvyData class also has a bikeids attribute with a set of all the bike identifiers in the dataset. This is not a composition relationship, but it is something you may find useful in some of this lab’s tasks.

For more details on the Divvy classes, please read the Composition section of the textbook’s chapter on Classes and Objects.

Before you get started

Before you get started, you will need to download the Divvy dataset files. To do so, go into the lab4/data directory in your terminal, and run the following:


This will download the files; if the download is successful, you should see this at the end of the command’s output:

Extracting Divvy data...

We have provided the full dataset (divvy_2013_trips.csv) but also some smaller files that will take less time to load.

Next, you will want to have an IPython session open. Make sure you start IPython from the lab4 directory, and that you load the autoreload extension, and then import the divvy module:

In [1]: %load_ext autoreload

In [2]: %autoreload 2

In [3]: import divvy

Computing the total distance of each bike

We are going to start by adding a simple method to the DivvyData class, which is contained in the file in your lab4 directory. You’ll see this class already has methods to compute the total distance of all the trips (get_total_distance), and the total duration of all the trips (get_total_duration). Make sure you understand how these methods work before continuing!

You are going to add a new method to the DivvyData class that computes, for every bike in the Divvy dataset, the sum of the duration of all the trips taken by that bike:

def get_bike_times(self):
    Computes, for every bike in the Divvy dataset, the sum of the
    duration of all the trips taken by that bike.

    Returns a dictionary mapping bike identifiers (integer) to
    a duration in seconds (integer)

To implement this method, you will need to access the bikeid attribute of the DivvyTrip objects in the DivvyData class. This attribute contains the identifier of the bike used for that trip.

You can test your implementation from IPython by creating a DivvyData object with our “tiny” dataset, and then testing a few bikes individually. For example:

In [5]: data = divvy.DivvyData("data/divvy_2013_stations.csv",
   ...:                        "data/divvy_2013_trips_tiny.csv")

In [6]: dt = data.get_bike_times()

In [7]: dt[27]
Out[7]: 10105

In [8]: dt[44]
Out[8]: 1852

Later on, we’ll see a more thorough way to test your implementation. However, if your implementation works with the examples above, just move on to the next task for now.

Computing the number of times a bike has been moved

If you’ve lived in Chicago long enough, you may have spotted the Divvy vans that occasionally come to a Divvy station to place bikes on the station’s dock if the station is running low on bikes. Not just that, they’ll also take bikes away if the station has too many bikes and seems to be underutilized.

So, you will sometimes see trips like this in the dataset:

  • Trip #1234: Customer A took Bike 44 from station 10 to station 20

  • Trip #1235: Customer B took Bike 44 from station 20 to station 30

  • Trip #1236: Customer C took Bike 44 from station 50 to station 70

This means that the bike was moved by a Divvy van from station 30 to station 50!

You will add a method to the DivvyData class that finds any such movements for all the bikes in the dataset:

def get_bike_movements(self):
    Returns a dictionary mapping bike identifiers (integer)
    to a list of tuples, where each tuple represents that bike
    being moved from one station to another.

    Each tuple contains three values: the station the bike was
    moved from (DivvyStation object), the station the bike was
    moved to (DivvyStation object), and the difference in capacity
    between the two stations (more specifically, the capacity
    of the station the bike was moved to minus the capacity
    of the station the bike was moved from). Note that this
    will be an integer that can be either positive or negative.

    Note that the dictionary must also include entries for
    the bikes that have not been moved at all (those entries
    will just map to an empty list)

To implement this method, you will need to access the dpcapacity attribute of the DivvyStation objects. You will also want to use the bikeids attribute of DivvyData. Finally, take into account that the trips attribute in DivvyData has the trips sorted by their start time.

You can test your implementation from IPython using the data object we created earlier:

In [11]: bm = data.get_bike_movements()

In [13]: bm[409]
[(<DivvyStation 85: Michigan Ave & Oak St>,
  <DivvyStation 350: Ashland Ave & Chicago Ave>,

Notice how the DivvyStation objects are shown using the string representation returned by DivvyStation’s __repr__ method.

If you print the entire bm dictionary, you should actually see that none of the bikes have any movements (except bike 409). Remember we loaded a “tiny” subset of the full dataset, so this is not unexpected.

Testing your Divvy methods more thoroughly

As part of the file, we have included some code that uses these two methods to answer these questions:

  • What is the average amount of time a bike is used?

  • What is the most used bike in the Divvy system?

  • What is the average number of times a bike is moved?

  • Do vans tend to move bikes from high capacity stations to low capacity stations, from low to high, or neither?

Once you’ve implemented the two methods specified above, just run the file from the terminal as follows:

python3 data/divvy_2013_stations.csv data/divvy_2013_trips_medium.csv

Notice how we’re using our “medium”-sized dataset for this. If you implemented the methods correctly, the output should end with this:

The average total usage of a bike is 1d 8h 0m 39s
The most used bike is 444, used a total of 3d 17h 41m 10s

The average number of times a bike was moved was 16.17
On average, a bike is moved to a station with 1.05 more docks
(Standard deviation: 10.29)

If your code works with the medium data set, try running it with the full dataset:

python3 data/divvy_2013_stations.csv data/divvy_2013_trips.csv

Please note that this will take a few seconds to run. The output should end with the following:

The average total usage of a bike is 3d 18h 36m 38s
The most used bike is 199, used a total of 9d 11h 30m 17s

The average number of times a bike was moved was 122.30
On average, a bike is moved to a station with 0.12 more docks
(Standard deviation: 9.28)

These results are interesting, but don’t forget to check out the code that produces them! You can find it towards the bottom of the file.

When Finished

Run the following commands from the command-line inside of your lab directory:

git add
git commit -m "Finished with lab4, part2"
git push

Again, we’re not grading this.