Classes

Let’s return to the type() function for a moment. It returns the data type for the argument inside the parentheses, and we can print this information to the console.

The code below prints the data types for several different values.

#!/bin/python3

print(type(12))
print(type("Hello, World!"))
print(type(['dog', 'cat', 'fish', 3.14]))

<class 'int'>
<class 'str'>
<class 'list'>

Note how each data type gets reported after the word class.

As we saw before, objects have both properties and methods. All string values (objects) share a set of properties and methods, while list objects hold a different set. When two objects share a set of properties and methods, we say they belong to the same class.

Classes allow us to organize the same type of data into the same group. For example, all whole numbers are part of the int class, and any data enclosed in quote marks belongs to the str class.

Every object belongs to a particular class, and this determines what the object can do. The list ['dog', 'cat', 'fish'] CANNOT use any of the methods defined for the str class, but it CAN use all of the available list methods.

Python defines many classes for us and makes them available for our use. In each of the previous chapters, we’ve studied some of the ready-made classes, like int, str, float, bool, list, and dict.

Objects Have a Data Type

When we introduce a new value in our code, Python automatically identifies its data type and groups it into a class.

Example

i_can_code = True

print(type(i_can_code))

Console Output

<class 'bool'>

Every value has a data type, and each data type belongs to a class.

Python treats every data value as an object.

Since Python treats every value as an object, all objects have a data type, and all objects belong to a class.

On the previous page, we used the dog object. It contains both int and str properties (age and name), and it uses methods that don’t belong with any other data type. How should Python group it?

User Defined Classes

Recall that Python gives us plenty of ready-made functions to help us code, like print(), max(), and sum(). We can also import more functions from modules like we did with the string module. However, these functions cannot do all the jobs we need. Our real strength as programmers comes when we define our own functions. We can think about classes in a similar way.

Sometimes, none of the given Python classes work for what we need to do. On the previous page, the idea was to use an object that behaved differently from strings, lists, etc. dog introduced a new data type, but Python does NOT come with a pre-defined class for dog.

Just like creating our own functions, Python allows us to define our own custom classes. For the examples on the previous page, we defined a new class and gave it the properties and methods it needed to describe the dog object.

On the next few pages, we will explore how to define a new class, create objects from it, and learn why we want to do this.