staticmethod and __call__

Roeland Rengelink r.b.rigilink at chello.nl
Wed Dec 12 10:51:05 CET 2001


Bruce Eckel wrote:
> 
> Actually, I can simplify this in one sentence: if you can make
> __call__ a static method, then how do you create objects of that
> class?
> 

When I first investigated this, I hoped something like this would work:

>>> class A(object):
...    def __call__(*args, **kwargs):
...        print 'called'
...        return object.__new__(A, *args, **kwargs)
...    __call__ = staticmethod(__call__)

And it does, in the sense that A.__call__() returns a new instance

>>> b = A.__call__()
called
>>> b
<__main__.A object at 0x8161574>

but, A() doesn't do A.__call__

>>> A()
<__main__.A object at 0x815ed7c>

That is, implementing __call__ such that is does return (new) instances
is no problem. However, you can't convince Python to translate A() in
A.__call__(). I used to think this was to bad, but now think is actually
a good thing


By the way, having remembered __new__, here's your ItemGenerator again

>>> class ItemGenerator(object):
...     rgen = random.Random()
...     items = ['A', 'B', 'C']
...     def __new__(cls):  # __new__ is always a classmethod
...             return cls.rgen.choice(cls.items)
... 
>>> items = [ItemGenerator() for i in range(5)]
>>> items
['C', 'C', 'A', 'A', 'A']

Looking forward to your paper,

Roeland

-- 
r.b.rigilink at chello.nl

"Half of what I say is nonsense. Unfortunately I don't know which half"



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