I spent an enlightening evening browsing through the source code for
weakref.proxy. The way that code works is to define every slot,
delegating to the proxied object to handle each call (wrapping and
unwrapping the proxied object as needed).

This is normally transparent to the user due to the fact that
__getattribute__ is one of the proxied methods (and at the C level, the
delegated slot invocations return NotImplemented or set the appropriate
exceptions). The only way it shows through is the fact that
operator.isNumber and operator.isMapping will always return True for the
proxy instance, and operator.isSequence will always return False - this
is due to the proxy type filling in the number and mapping slots, but
not the sequence slots.

However, this prompted me to try an experiment (Python 2.5.1), and the
results didn't fill me with confidence regarding the approach of
expecting 3rd party developers to explicitly delegate all of the special
methods:

>>> class Demo:
...   def __index__(self):
...     return 1
...
>>> a = Demo()
>>> b = weakref.proxy(a)
>>> operator.index(a)
1
>>> operator.index(b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'weakproxy' object cannot be interpreted as an index

Oops.