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.
Although classes create new objects, the objects themselves will NOT be exactly the same. For example, all string values have a length, but the value for the length depends on the number of characters in each string.
Objects of the same type have the same property names and methods. However, the values of the properties can differ.
Think of this like using the same cookie cutter to make sugar cookies vs. gingerbread cookies. Each cookie will have ingredients and flavor properties, but these will have completely different values depending on the recipe.
Let’s revisit the
dog object we used earlier to see how it relates to a
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.
Let’s say we want to create a set of cat objects. Take a moment to think about the properties the objects should have. We should be able to use these properties to describe each cat.
The values of the properties will allow us to tell the difference between animals, but for now just focus on coming up with a set of property names.
The editor below is NOT here to run code. Instead, use the space to add names for any properties you think belong with a cat object. Two examples have already been given.
Add at least two more property names to the editor, but feel free to go crazy!
If you are logged into your Trinket account, you can click the Remix button to save your ideas. We will use this editor again on a later page. Making your own copy will allow you to pick up where you left off. If you do not have a Trinket account, copy and paste your list of method names into something you can look at later.
Now let’s convert our thoughts into code.
In this design phase, we want to include as many property names as we can.
When we start coding, we should keep things simple and build something small that works. We won’t include all of our ideas right away.
Once we have working code, we can always update it later to make the class more powerful.
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:
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.
Class names begin with a capital letter, followed by lowercase letters. For example,
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).
If the class name contains an abbreviation, use capital letters for that abbreviation. For example,
188.8.131.52. Setting Property Values¶
Each new object contains a set of property values. To assign these values, our
class must include an initializer method. This method is defined like any
other function, but it always gets the special name
underscores, then the letters, then two more underscores).
Cat class, this looks like:
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class Cat: def __init__(self): # Assignment statements...
The initializer method automatically runs whenever we call a class, and it assigns values to each property name.
self parameter requires some explanation, and we will use an example to
help set up that discussion.
Use the instructions and editor below to complete the
To assign a value to a property, the syntax is:
self.property_name = property_value
On line 5,
self.name = 'Coach'assigns the value
'Coach'to the property called
On line 6, add the statement
self.age = 2to assign the integer
2to the property called
age. Be sure to indent the statement to match line 5. This puts
self.age = 2inside the
Pick one of the properties you listed earlier on this page. On line 7, assign a value to that property.
Paste the following code on lines 9 and 10. To keep the statements outside of the class, do NOT indent them.
cat_1 = Cat() print(cat_1.name, cat_1.age)
Line 9 creates a new object called
cat_1, and line 10 prints two of its properties. Run the program to check your work. Properly done, the output should be:
Modify line 10 to print all three property values.
Finally, create another object called
cat_2and print out its property values.
Lets step through the final code:
On line 9,
Cat()calls the class to create a new object.
Control moves to line 1, and the class statements execute.
__init__method runs, and it assigns values to each of the properties included in the code block. This is where
selfplays a role.
When we call a class,
selftakes on the name of the new object. When
cat_1 = Cat()executes,
selfgets assigned the value
self.name = 'Coach'evaluates as
cat_1.name = 'Coach'
self.age = 2evaluates as
cat_1.age = 2
__init__method finishes, control passes back to line 9, and the new object is assigned to the variable
In this Try It example, the
__init__method causes every new
Catobject to start with a
2, and the third property you defined.
The program creates two cat objects,
cat_2, that have the
same property values. Are the two objects the same? To answer this question,
add the statement
print(cat_1 == cat_2) to the bottom of the code and run
the program again. Is the output
cat_2 like twins. Even though they have identical
properties, they still represent separate animals. They might look and
behave exactly the same, but they are different objects!
13.5.4. Use Parameters with
Once we create
cat_2, we can easily change the values for
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
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class Cat: def __init__(self, a_name, an_age): self.name = a_name self.age = an_age
Modify the code in the editor above to match this format. Leave the third property assignment alone for now.
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.
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.
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?
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
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
If we call the class with
my_car = Car('Chevy', 'Astro', 1985, 'gray', 20),
what is output by
If we create another object called
other_car with the exact same
property values, what is the result of the expressions
my_car == other_car and
my_car.year == other_car.year?
What happens if we call the class with
my_car = Car('Tesla', 'Model S',
- The mpg property is assigned a value of 0.
- The mpg property is assigned the empty string.
- The mpg property is not added to the my_car object.
- The program crashes and displays an error message.