method that can be called from a class and also from an instance
steve+comp.lang.python at pearwood.info
Fri Nov 23 14:06:37 CET 2012
On Fri, 23 Nov 2012 09:52:25 +0100, Peter Otten wrote:
> Steven D'Aprano wrote:
> Am I reading that right that you don't invoke method() as
No. I give an example and explicitly state:
You can use this class without instantiating:
Example.method('else') # returns 'something else'
> Then I'd probably use class attributes to store the
> default state and shade them by instance attributes as needed.
> class A:
> state = "default"
> def __init__(self, state=None):
> if state is not None:
> self.state = state
> def method(self): return self.state
That doesn't allow me to call A.method().
On the other hand, if method were a class method, then I could say
A.method() or A(state).method, but in both cases I would get the default.
So that isn't suitable.
> The same idea might work for the OP, too (but I'm not sure it's a good
> class B:
This needs to be a new-style class unless you're using Python 3.
> def inst_f(self):
> return "instance"
> def f(class_):
> return "class"
> def __init__(self):
> self.f = self.inst_f
> assert B.f() == "class"
> assert B().f() == "instance"
Without studying that in detail, it looks like that would be an
alternative solution to the same problem. The downsides are:
- you have two distinct but almost identical implementations of
method "f", one called "f" and one called "inst_f";
- it works by shadowing method "f" in the instance, which may
strike many people as too tricky for production software.
Me personally, I think the first objection is critical. Having to write
the same method twice, with subtle differences, is inviting bugs.
More information about the Python-list