ui factory - what should it do?

[Jan Hudec]

On Wed, Feb 22, 2006 at 11:43:08 -0500, Aaron Bentley wrote:
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> Martin Pool wrote:
> > In that particular case it's clearly working for only the duration of
> > __init__.  (Which is perhaps not an ideal idiom; other job-type classes
> > have an explicit "do it" method, and perhaps we should standardize
> > that.)
> 
> While it's a bit weird the first time you see it, it seems quite a handy
> way of doing it, and it's the way I did the TreeTransform mergers (e.g.
> Merge3Merger).
> 
> Because if I were to convert fetcher to have a do-it method, I'd put the
> entire body of the __init__ method there.  And then I'd have to restore
> the members to safe values afterward, so no unsafe member functions
> could be called.
> 
> There is definitely a need for an idiom represent a procedure with
> multiple phases and shared data, where that shared data becomes obsolete
> at the end of the procedure, and this way didn't seem so bad, on reflection.

One can actually make a class behave exactly like a function in python. It
just takes a little fiddling with the metaclass.

Basically cls(arguments) expands to cls.__class__.__call__(arguments). If
cls.__class__ == type, that that method does roughly:
    instance = cls.__new__(arguments)
    instance.__init__(arguments)
    return instance
    
But cls.__class__ does not have to be type:

class CallableMeta(type):
    def __call__(self, *pa, **kwa):
	inst = type.__call__(self)
	return inst.run(*pa, **kwa)

class Callable(object):
    __metaclass__ = CallableMeta

    def run(arguments):
	do_whatever_you_want_here
	return any_appropriate_result

Actually, there is even simpler way -- you can simply put the code in
__new__. You just need to call super(...).__new__() to get the instance
(__new__ is a classmethod). But then __init__ will be called on whatever you
return -- which is OK for None, True, False, numbers and tuples, but not
other types.

I expect seasoned pythonists to reject this on basis of being dirty, but
TIMTOWDI even in languages that clame there is not ;-).

-- 
						 Jan 'Bulb' Hudec <bulb at ucw.cz>
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