[Python-Dev] Re: Python-Dev Digest, Vol 8, Issue 91

[Alex Martelli]

On 2004 Mar 30, at 17:09, Edward Loper wrote:
    ...
> On a related note, now that Python has class methods, is there much 
> point in a "singleton" pattern?  In particular, why not just make a 
> class that only defines class methods, and uses the class namespace to 
> store variables (instead of an instance namespace)?

I think it's generally better to use a module when what you want is a 
module's functionality, and that's what you seem to be describing here. 
  What using a class _instance_ buys you is special methods (and 
possibly descriptors), e.g. sometimes you might want __getattr__ (or a 
property) on "something of which there is a single instance" (avoiding 
the "singleton" word by paraphrase -- IMHO, "singleton" means "a set 
with a single element", NOT "the single element of that set", which is 
different -- anyway...).  Generally, something like:

class foo(object):
    ''' whatever you want in here '''
foo = foo()

deals even with such obscure desiderata.  Left out of all this (and a 
key force in the Gof4 writeup of "Singleton") is _inheritance_ -- the 
ability for the user to _subclass_ the singleton's class while still 
preserving the "there can be only one" constraint (best known as "the 
Highlander rule"...).  IMHO, even if Guido hates it, my Borg 
non-pattern is still the best ("least bad") way to provide such VERY 
obscure feechurs.  However, in the simpler approach above,

class _myspecialfoo(foo.__class__):
    ''' tweak a few things here '''
foo.__class__ = _myspecialfoo

can also work.  All which being said -- in almost 5 years of coding 
almost only in Python, I think I found myself needing anything 
"singletonish" (beyond a plain module), in production code, just once, 
and even that once went away once the code that used to rely on it got 
restructured, refactored and cleaned up.  This personal experience is 
certainly a good part of what makes me believe that the importance of 
everything singletonish is vastly exaggerated in popular perception -- 
almost _every_ other classic DP is more important and interesting, yet 
Singleton keeps coming up nevertheless...!


Alex