13.5. Create a Class¶

13.5.1. What Classes Do¶

We often create many objects of the same type, like a series of whole numbers or several string values. Classes allow us to do this in an efficient way.

Think of classes like cookie cutters. They provide the structure for making multiple examples of the same data type, and they make sure each object can execute the same methods. We reuse a class every time we need to generate a new object.

Let’s revisit the dog object we used earlier to see how it relates to a class.

Classes let us create multiple, similar objects.

Each object contains the same property names, and each one can execute the same set of methods. The three objects all belong to the same class, even though their property values differ.

13.5.2. Design a New Class¶

Before we create a new class, we should begin by describing the objects that belong in that class. Take time to think about the properties and methods we want each object to have. Next, write these ideas down! Doing this will make designing and coding the class easier.

Now let’s convert our thoughts into code.

13.5.3. Define the New Class¶

To define a new class, begin with the class keyword, followed by the name of the class. The general syntax is:

1
2

class ClassName:
   # Class code...

Note that class definitions do NOT include parentheses, ().

Just like with variable and function names, Python coders follow a set of rules and recommendations when defining a new class.

1. Class names begin with a capital letter, followed by lowercase letters. For example, Cat.
2. For class names that contain more than one word, begin each new word with a capital letter. Do not use underscores to separate the words. For example, CoolCat. (FYI: This style is called UpperCamelCase).
3. If the class name contains an abbreviation, use capital letters for that abbreviation. For example, OSUBuckeyes.

1
2
3

class Cat:
   def __init__(self):
      # Assignment statements...

13.5.4. Use Parameters with __init__¶

Once we create cat_1 and cat_2, we can easily change the values for the name and age properties.

cat_1.name = "Garfield"

However, it would be better if we could assign these values when the objects are first created. Instead of giving every new Cat object the same name and age, we want to let these values vary from object to object.

We do this by adding parameters to the __init__ method:

1
2
3
4

class Cat:
   def __init__(self, a_name, an_age):
      self.name = a_name
      self.age = an_age

1. Modify the code in the editor above to match this format. Leave the third property assignment alone for now.
2. Run the program again. The statement cat_1 = Cat() should now throw an error. By adding parameters to __init__, Python expects values to be included when we call the class, but we did not provide any.
3. Add arguments for a name and an age in the statement. For example, cat_1 = Cat('Nala', 4). Do the same for cat_2, but use different values.
4. Run the program and fix any remaining bugs. Try changing the arguments you send to the class when creating a new object. How do those changes affect the output?
5. Next, follow a similar process for the third property. Define a parameter and include an argument when you call the class.

13.5.5. Check Your Understanding¶

The questions below refer to a class called Car.

1
2
3
4
5
6
7

class Car:
   def __init__(make, model, year, color, mpg):
      self.make = make
      self.model = model
      self.year = year
      self.color = color
      self.mpg = mpg