python: dynamically adding attributes to a built-in class

Question

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).

Short lazy answer: `__slots__`; also `dict` is implemented in C, and you're inheriting it. — Daenyth, Jan 12 '12 at 22:21
what python version are you using? b.p = 1 works both in 2.7.2 and 3.2 — soulcheck, Jan 12 '12 at 22:24
Terribly sorry, it does work for b. Updated question to reflect this. — max, Jan 12 '12 at 22:38
You can make chnages to the built-in classes thenselves- you have to create subclasses of them. — jsbueno, Jan 12 '12 at 23:41
There is no need to inherit from `dict`. In fact Python doesn't appreciate inheriting from base classes at all. — Zaur Nasibov, Jan 13 '12 at 08:53

score 4 · Accepted Answer · answered Jan 13 '12 at 07:20

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.

Rik Poggi · Answer 2 · 2012-01-12T22:29:50.007

0

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

edited Jan 12 '12 at 22:29

answered Jan 12 '12 at 22:23

Rik Poggi

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python: dynamically adding attributes to a built-in class

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