15.4. Class Attributes¶
A class attribute is a variable that belongs to the class rather than to an object. We create class attributes inside the class but outside all methods, just like we create global variables in a .py file but outside all functions. Here’s an example of a class attribute named count , which keeps track of how many CreateCounter objects have been created:
>>> class CreateCounter:
>>> count = 0 # This is a class attribute.
>>> def __init__(self):
>>> CreateCounter.count += 1
>>> print('Objects created:', CreateCounter.count)
>>> a = CreateCounter()
>>> b = CreateCounter()
>>> c = CreateCounter()
>>> print('Objects created:', CreateCounter.count)
>>> # Prints 0.
>>> # Prints 3.
Objects created: 0
Objects created: 3
The CreateCounter class has a single class attribute named count . All CreateCounter objects share this attribute rather than having their own separate count attributes. This is why the CreateCounter.count += 1 line in the constructor function can keep count of every CreateCounter object created. When you run this program, the output will look like this:
Objects created: 0 Objects created: 3
We rarely use class attributes. Even this “count how many CreateCounter objects have been created” example can be done more simply by using a global variable instead of a class attribute.