Op 2005-02-28, Dima Dorfman schreef <[EMAIL PROTECTED]>:
> On 2005-02-28, Antoon Pardon <[EMAIL PROTECTED]> wrote:
>> Op 2005-02-28, Diez B. Roggisch schreef <[EMAIL PROTECTED]>:
>>> I still don't see how that is supposed to work for "a lot of interesting
>>> things". Can you provide examples for one of these interesting things?
>>
>> Lazy evaluation where the value of something is calculated the first
>> time it is needed but accessed from some storage if it is needed again.
>
> I do this all the time. It's not very hard and doesn't require any
> extra language support, but I would like for there to be an
> authoritative list of type slots (autopromise_ops).
>
> import operator
>
> def promise(thunk):
> x = []
> def promised():
> if not x:
> x.append(thunk())
> return x[0]
> return promised
>
> autopromise_ops = [x for x in dir(operator) if x.startswith('__')]
> autopromise_ops += ['__getattribute__', '__call__', '__str__', '__repr__']
> autopromise_ops += ['__getattr__', '__setattr__', '__delattr__']
>
> def autopromise(thunk):
> p = promise(thunk)
> d = {}
> for op in autopromise_ops:
> def bindhack(op=op):
> return lambda self, *a, **kw: getattr(p(), op)(*a, **kw)
> d[op] = bindhack()
> return type('autopromise', (), d)()
>
> def test():
>
> lis = []
>
> def thunk():
> lis.append('ran thunk')
> return 'value'
>
> s = autopromise(thunk)
> p = s * 30
> assert p == 'value' * 30
> p = s * 10
> assert p == 'value' * 10
> assert lis == ['ran thunk'] # Just once
>
> print 'autopromise sanity test passed'
>
> An autopromise object is good almost everywhere the real one would be,
> and usually the only way to tell the difference is to call id or type
> on it. The main exception is when the thunk returns a builtin type
> (like a string or int) and you want to pass it to a builtin function
> that expects a particular type (this would also apply to Python
> functions that break duck typing on purpose, but those would just be
> getting the breakage they deserve).
Hmm, I'll have to take your word for it, because for the moment I
don't see what is going on. I'll have to study this some time.
--
Antoon Pardon
--
http://mail.python.org/mailman/listinfo/python-list
