On Apr 28, 1:20 am, Chris Rebert <c...@rebertia.com> wrote: > On Tue, Apr 27, 2010 at 11:02 PM, GZ <zyzhu2...@gmail.com> wrote: > > On Apr 27, 9:20 pm, alex23 <wuwe...@gmail.com> wrote: > >> GZ <zyzhu2...@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. -- http://mail.python.org/mailman/listinfo/python-list
