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". See http://docs.python.org/reference/datamodel.html#implementing-descriptors or for a more complete but advanced explanation, the "Static methods and class methods" section of http://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 -- http://mail.python.org/mailman/listinfo/python-list
