How to use a class property to store function variables?

GZ zyzhu2000 at gmail.com
Wed Apr 28 10:04:12 CEST 2010


On Apr 28, 1:20 am, Chris Rebert <c... at rebertia.com> wrote:
> On Tue, Apr 27, 2010 at 11:02 PM, GZ <zyzhu2... at gmail.com> wrote:
> > On Apr 27, 9:20 pm, alex23 <wuwe... at gmail.com> wrote:
> >> GZ <zyzhu2... at gmail.com> wrote:
> >> > I do not think it will help me. I am not trying to define a function
> >> > fn() in the class, but rather I want to make it a "function reference"
> >> > so that I can initialize it any way I like later.
>
> >> It always helps to try an idea out before dismissing it out of hand.
> >> Experimentation in the interpreter is cheap and easy.
>
> >> >>> class A(object):
>
> >> ...   fn = staticmethod(lambda x: x*x)
> >> ...>>> A.fn(10)
> >> 100
> >> >>> A.fn = staticmethod(lambda x: x**x)
> >> >>> A.fn(3)
> >> 27
> >> >>> def third(x): return x/3
> >> ...
> >> >>> A.fn = staticmethod(third)
> >> >>> A.fn(9)
>
> >> 3
>
> >> However, I'm assuming you're wanting to do something like this:
>
> >> >>> class B(object):
>
> >> ...   def act(self):
> >> ...     print self.fn()
>
> >> That is, providing a hook in .act() that you can redefine on demand.
> >> If so, note that you only need to decorate functions as staticmethods
> >> if you're assigning them to the class. If you intend on overriding on
> >> _instances_, you don't:
>
> >> >>> B.fn = staticmethod(lambda: 'one') # assign on class
> >> >>> b = B() # instantiate
> >> >>> b.act() # act on instance
> >> one
> >> >>> B.fn = staticmethod(lambda: 'two') # assign on class
> >> >>> b.act() # existing instance calls new version on class
> >> two
> >> >>> b.fn = staticmethod(lambda: 'three') # assign on instance
> >> >>> b.act()
>
> >> Traceback (most recent call last):
> >>   File "<stdin>", line 1, in <module>
> >>   File "<stdin>", line 3, in act
> >> TypeError: 'staticmethod' object is not callable>>> b.fn = lambda: 'three' # look Ma, no staticmethod!
> >> >>> b.act()
>
> >> three
>
> >> Incidentally, this is known as the Strategy pattern, and you can see a
> >> simple example of it in Python here:http://en.wikipedia.org/wiki/Strategy_pattern#Python
>
> >> Hope this helps.
>
> > Another question: I am not sure how staticmethod works internally. And
> > the python doc does not seem to say. What does it do?
>
> It involves the relatively arcane magic of "descriptors".
> Seehttp://docs.python.org/reference/datamodel.html#implementing-descriptors
> or for a more complete but advanced explanation, the "Static methods
> and class methods" section ofhttp://www.python.org/download/releases/2.2.3/descrintro/
>
> Understanding exactly how staticmethod() and friends work is not too
> essential in practice though.
>
> Cheers,
> Chris
> --http://blog.rebertia.com- Hide quoted text -
>
> - Show quoted text -

Got it. I appreciate your help.



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