CS104: Chapter 15

CS 104 Students and me

Big Question

Q: You say creating a class is how to create a new type. Why would we even want to do this?!

A: First, to group data together that belongs together. (Second, to group operations on the data with the data. But, more on that, in due course.)

Grouping Data

• The only data structures we have are lists and tuples.

• We can store multiple related data items together in a list or tuple, and then keep a list of these lists/tuples.

• E.g., cars = [ (“Honda”, “Odyssey”, 2001, “silver”, “VIN1523y7380”), (“Toyota”, “Prius”, 2007, “blue”, “VIN99383829X95”), …]

Accessing Data

• If we need to compute with the i-th car’s color, we do:car = cars[i] # car is a tuple if car[3] == “blue”: # do something…

• Or, was it car[4]? car[5]? If we have 55 items to keep for a car how do we remember which number is in which place in the tuple of a car? How can we make the code readable?

A better way

• Would be nice to make a container to hold multiple data items, and each attribute in the structure has a nice name.

• Kind of like a row in Excel, where you give a name to each column.

• Then, you could access the cars’ colors, by car = cars[i]if car.color == “blue”: # do something…

So…

• We need a way to group multiple attributes of something into a single container, and then be able to access, with nice names, the attributes in the container.

• Enter: the class.

Terminology

• A class defines the template or recipe for what an object looks like – i.e., what attributes it has.

• Defining a class does not make any variables (called objects). It just defines what an object would hold if you did create one.

• You instantiate a class to make an object, aka a variable. An object is also called an instance of a class.

Example: Car classclass Car: # “class” is like “def”, but for

# classes, not functions. def __init__(self): “””Constructor for a Car.””” self._make = “” self._model = “” self._yr = 1900 self._color = “”

# main code: instantiate some car objectscar1 = Car() # call __init__, the constructor.car1._make = “Honda”car1._model = “Odyssey” # etccar1._yr = 2001car1._color = “silver”

Using a car instance

• Now that we’ve wrapped up multiple attributes into one container (i.e., “object”), and given each a nice readable name, we can deal with each container as one thing.– get/set values in it– pass it as a parameter– return it from a function– sort the objects, without mixing up whose values

go with whose…

Technical Details

• If student is an object (of class Student) with attribute, id, you can access it with student.id.

• Similar to module.variable or module.func()

• module != class. Module is a file containing variable, function (and class) definitions. Class is a new type. But both are containers.

Q and As

• Can you talk more about the object diagrams?• Can you walk through the example in which

you are making/defining the class point on page 3 -- specifically where “p is an alias for blank”?

• Is there a limit to the amount of attributes a class can have?

Q and As (2)

• We talk about how objects are mutable, but if we can define the attributes of a created object can we define them to be immutable?

• Is a “class definition” like a “function definition” with just a different kind of pattern? (Does it work similar to a function definition?)

Q and A (3)

• Why doesn't the class “Point” clarify what its attributes are? How do you know what the attributes of a class are?