Why weakref doesn't support built-in types in Python?

Question

In Python weakref document( http://docs.python.org/library/weakref.html ), it says that

Several built-in types such as list and dict do not directly

Answer 1

Most of the built-in types are not directly weak referenceable (e.g. str, int, float, list, dict, None), and there are a few that cannot even be made so by sub-classing (e.g. tuples in CPython).

Some details about the underlying implementation of weakrefs for several built-in types can be found in this March-2005 python-list post by Raymond Hettinger.

Answer 2

My educated guess is that dicts and lists are used internally to implement weakrefs, so you would have an egg-chicken situation here.