__del__ not working with cyclic reference? (and memory-leaked?)

[Steven Taschuk]

Quoth Jane Austine:
  [...]
> (I should not use __del__ for singleton patterns, but
> any other alternatives?)

You could register an atexit procedure; see the atexit module in
the standard library.

You *can* use __del__ for a singleton as long as there is no
cyclic reference, or if the interpreter deals with the cycle
anyway.  Under 2.2.2 and 2.3b1 this seems to work:

    # foo.py
    class MySingleton(object):
        def __del__(self):
            print 'foo'
    mysingleton = MySingleton()

The difference is that here the instance is referred to by the
module dict rather than the MySingleton class dict.  (I think the
reason this makes a difference is that interpreter shutdown
includes a zapping of module dicts, breaking this cycle:

    >>> import foo
    >>> d = foo.__dict__
    >>> d['mysingleton'].__class__.__dict__['__del__'].func_globals is d
    True

Note that if your __del__ refers to global variables, they might
have been zapped by the time it gets called.)

-- 
Steven Taschuk                                                   w_w
staschuk at telusplanet.net                                      ,-= U
                                                               1 1