On May 14, 4:38 pm, "[EMAIL PROTECTED]"
<[EMAIL PROTECTED]> wrote:
> On 14 mai, 16:30, George Sakkis <[EMAIL PROTECTED]> wrote:
>
> > On May 14, 10:19 am, "Diez B. Roggisch" <[EMAIL PROTECTED]> wrote:
>
> > > > An instance method works on the instance
> > > > A Static method is basically a function nested within a class object
> > > > A class method is overkill?
>
> > > If anything, a static method is overkill. See it this way: *if* you for
> > > some
> > > reason put a method into an enclosing context - isn't it worth having a
> > > reference to that?
>
> > My feeling exactly; these days I almost always use class methods
> > instead of static. I vaguely remember seeing somewhere an example
> > where a static method was the only (elegant) solution; neither a class
> > method nor a plain function would do. I'll post it if I find it unless
> > someone beats me to it.
>
> No concrete example here but I surely remember having used
> staticmethods in one case where I needed class-based polymorphic
> dispatch and eventually implementation inheritance (to get a default
> implementation) for something that didn't required access to the
> class object.
Yes, that's common, but a class method could be used just as well
here. I couldn't find the original example but IIRC it was related to
monkeypatching. Say that you have a package P1 that has some useful
classes:
class A(object): pass
class B(A): pass
class C(A): pass
You also have an unrelated package P2 with some useful functions:
def f1(): return 0
def f2(): return 1
Naturally you want to combine these two. One common pattern when doing
so turns out to be like:
# e.g. x = random.choice([A(), B(), C()])
...
assert isinstance(x, A)
if type(x) in (A,B):
print f1()
elif type(x) is C:
print f2()
With a less trivial hierarchy and number of functions, it becomes
clear that it would help a lot if you could refactor it somehow so you
can just say "print x.f()" and it would do the right thing.
Monkeypatching it as a normal method ("A.f = f1; C.f = f2") doesn't
work because it expects self as the first argument, and similarly for
class methods and the cls argument. OTOH static methods do the trick:
A.f = staticmethod(f1)
C.f = staticmethod(f2)
print x.f()
Admittedly it's not a common scenario but it's good to know it's there
if you need it.
George
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