Let's start with the most basic Python built-in, one that we've seen many times already, the one that we've extended in every class we have created: the object. Technically, we can instantiate an object without writing a subclass, as follows:
>>> o = object()
>>> o.x = 5
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute 'x'
Unfortunately, as you can see, it's not possible to set any attributes on an object that was instantiated directly. This isn't because the Python developers wanted to force us to write our own classes, or anything so sinister. They did this to save memory; a lot of memory. When Python allows an object to have arbitrary attributes, it takes a certain amount of system memory...