autoborg &c (was )e: Unified Type/class (Singleton)

Pedro Rodriguez pedro_rodriguez at club-internet.fr
Mon Jan 28 15:33:30 CET 2002


"Alex Martelli" <aleax at aleax.it> wrote:

> "Pedro Rodriguez" <pedro_rodriguez at club-internet.fr> wrote in message
> news:pan.2002.01.28.11.53.09.55635.1770 at club-internet.fr...
>     ...
>> Borg vs. Singleton : I was just kiding, sort of... I wonder if it would
>> be good to provide a diagram when dealing with new concepts. Example :
> 
> For people who "thing visually", maybe.  

A diagram also helps to capture an intention.

> ... I prefer to think verbally
> (sharing state vs sharing identity).
> 

Verbally ? Never would have thought that of you ;) 
Still, phrases are always required, a diagram won't explain all
(I have something like 'the map is not the field' in mind).

> Back to the mechanics (metaclasses or otherwise) that one can use to
> implement creational design patterns (and other kinds).  When I have a
> class object C, "instantiation" is a call on C, say:
>     instance = C(*args, **kwds)
> so it involves C.__call__, which typically comes from C.__class__.
                 ^^^^^^^^^^ 

< example sniped for brievety >

Ok. Except for a probable typo on 'C.__call__' which may exist or not but 
is not involved in this process (your examples suggests C.__new__).


> It's easy to check this (Python's GREAT this way...):
> 
> class X(object):
>   def __new__(self, *args, **kwds):
>       result = object.__new__(self)
>       print 'X.__new__(%r,%s,%s)->%s'%(self, args, kwds, result)
>       return result
>   def __init__(self, *args, **kwds):
>       print 'X.__init__(%r,%s,%s)'%(self, args, kwds)
> 
> x=X('ba','be',bi='bo')
> 
> D:\>python ni.py
> X.__new__(<class '__main__.X'>,('ba', 'be'),{'bi': 'bo'})-><__main__.X
> object at
>  0x00789C50>
> X.__init__(<__main__.X object at 0x00789C50>,('ba', 'be'),{'bi': 'bo'})
> 
> 
> Nothing mysterious here, right?  Just notice the difference between
> 'self' in the two cases.  Now, clearly, if you want to influence what
> happens when C is called, your options include overriding __new__ and/or
> __init__ in C, and overriding __call__ in C's type (metaclass).
> 
> If you work in C.__init__, then programmers who subclass C must call
> C.__init__ explicitly (if they define their own __init__, as most often
> happens).  Similarly for __new__, although I guess it's much rarer for
> __new__ to be overridden in subclasses.  In any case, your control is
> limited, since the "template method" type.__call__ remains untouched.
>

It may be good to remind that __new__ is a static method whose signature
looks like 'def __new__(cls, ...)', and also, I felt confused when reading :

 
> If you choose to work by subclassing type (having C use your own
> metaclass), you can get at the "template method" itself and insert or
> tweak whatever you need to.  For example:
> 
> class autoborg(type):
>   metastate = {}
>   def __call__(self, *args, **kwds):
>       result = self.__new__(self, *args, **kwds) 
>       result.__dict__ = self.metastate
>       if result: result.__init__(*args, **kwds) 
>       return result
> 

... because, I would have written :
       result = type.__new__(self, *args, **kwds)
instead of :
       result = self.__new__(self, *args, **kwds)

I just realize now that 'parentClass.staticMethod()' is equivalent to
'self.staticMethod()' (same arguments used), whereas in classical classes
the meaning is different (bound/unbound method).

< sniping rest of fine explanations>
I guess I need a clearer head to sort out this __call__/__new__/__init__ 
chain. I wonder how much time it will take me to get my marks on those
matters, and use them when appropriate (headaches in sight ;)

> 
> By the way, I have a new counter for those who are horrified by the idea
> of naming Borg after a TV show -- "TV show, what TV show?  It's named in
> honor of Jorge Luis Borges, master of identity and separateness, just
> shortened for ease of use and uniformity of pronunciation across
> linguistic boundaries...". [And if you buy THIS, I have a nice bridge to
> sell you, and some very promising land in Florida, and...]
> 

I like it (not the Borges reference, whose writings I recommend, just like
those from Bioy Casares), it just these words 'just shortened'. If it
wasn't coming from you, I might have give it a thought ;)
Well... actually, I did it, just to show how naive I am ;)

Regards,
-- 

Pedro



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