Almost everwhere the descriptor protocol is mentioned, it specifies
__get__(obj, typ=None). Why is a default value needed for the second
argument? In which case does Python call a descriptor without a second
argument?
Thanks,
Shalabh
2  1730 
Shalabh Chaturvedi <sh*****@cafepy .com> writes: Almost everwhere the descriptor protocol is mentioned, it specifies
__get__(obj, typ=None).
Really? I thought it was "__get__(ob j, cls=None)" <wink>.
Why is a default value needed for the second argument? In which case
does Python call a descriptor without a second argument?
It doesn't *look* like it does, ever.
/However/ the wrapper for tp_descr_get (typeobject.c:w rap_descr_get)
accepts 1 or 2 arguments translating an absent or None second argument
to NULL, so if you want to behave like C-implemented descriptors,
you'd better accept 1 or 2 arguments. IOW, it's just part of the
"descriptor protocol".
As to *why* it's like this... erm, not sure about that.
Cheers,
mwh
--
Our Constitution never promised us a good or efficient government,
just a representative one. And that's what we got.
-- http://www.advogato.org/person/mrorg...html?start=109
Michael Hudson wrote: Shalabh Chaturvedi <sh*****@cafepy .com> writes:
.... Why is a default value needed for the second argument? In which case
does Python call a descriptor without a second argument?
It doesn't *look* like it does, ever.
/However/ the wrapper for tp_descr_get (typeobject.c:w rap_descr_get)
accepts 1 or 2 arguments translating an absent or None second argument
to NULL, so if you want to behave like C-implemented descriptors,
you'd better accept 1 or 2 arguments. IOW, it's just part of the
"descriptor protocol".
As to *why* it's like this... erm, not sure about that.
Possibly so that the interface is easier to use directly:
for prop in obj.getProperti es():
try:
prop.__get__( obj )
except Exception:
argh()
Most of the time the class argument is unused (for simple property-like
code, anyway), so why not let people omit it when calling directly.
Seems like a good design choice to me.
Enjoy,
Mike
_______________ _______________ _________
Mike C. Fletcher
Designer, VR Plumber, Coder http://members.rogers.com/mcfletch/ This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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