On Mon, 21 Apr 2014 09:06:14 -0500, Skip Montanaro wrote:
[...]
> Now, dir(inst_b) will list both 'x' and 'y' as attributes (along with
> the various under under attributes). Without examining the source, is it
> possible to define some kind of "selective" dir, with a API like
>
> def selective_dir(inst, class_): pass
>
> which will list only those attributes of inst which were first defined
> in (some method defined by) class_? The output of calls with different
> class_ args would yield different lists:
>
> selective_dir(inst_b, B) -> ['y']
>
> selective_dir(inst_b, A) -> ['x']
In general, no. There's no way of telling what method added an attribute
after the event has taken place: given that instance.__dict__ has a key
"x", how do you know how it got there?
You may be able to do this cooperatively: have both A and B define a
__dir__ method which lists only the attributes they contribute, then call
dir(A) or dir(B) as necessary.
Or, if you find yourself in the position of having an instance of both A
and B, say, a and b, you can compare dir(a) and dir(b). Anything in the
later but not in the former probably was added by B not A.
I say "probably" because one might have things like this:
class A:
def __init__(self):
if type(self) is not A:
self.y = "Surprise!"
self.x = "something"
and of course don't forget that attributes can be added by external
entities too:
instance = A()
instance.z = "bet you forgot about this"
> I'm thinking some sort of gymnastics with inspect might do the trick,
> but after a quick skim of that module's functions nothing leapt out at
> me. OTOH, working through the code objects for the methods looks
> potentially promising:
>
>>>> B.__init__.im_func.func_code.co_names
> ('A', '__init__', 'y')
>>>> A.__init__.im_func.func_code.co_names
> ('x',)
You may have a bit of difficulty with classes that write directly to the
__dict__, or use setattr, or eval.
--
Steven D'Aprano
http://import-that.dreamwidth.org/
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