Why doesn't it work for the built-in classes?
Is using a subclass the best approach to fix it, or will I run into some hidden problems?
a = {}
a.p = 1 # raises AttributeError
class B(dict):
pass
b = B()
b.p = 1 # works
EDIT: my original comment that it doesn't work for b was incorrect (I made a mistake).
The builtin classes do not have the ability to have arbitrary attributes. This is done for reasons of performance, especially memory usage, you want the built-in classes like list and dict to be as small as possible so you can have many of them.
Therefore the built-in classes do not have the __dict__ dictionary that is needed for arbitrary attributes to work.
You can achieve the same for your classes. If they are written in C you simply do not implement the __dict__ support. If they are written in Python you use slots.
If you want to subclass dict you can always use UserDict (here the documentation).
And it works with what you're trying to do:
from collections import UserDict
a = UserDict()
a.p = 10 # works fine
__slots__; alsodictis implemented in C, and you're inheriting it.