On 10/07/2013 02:24 PM, Steven D'Aprano wrote:
On Fri, Oct 04, 2013 at 11:06:15PM +0200, Victor Stinner wrote:
if type(self) is not dict:
# This only applies to subclasses, not dict itself.
try:
transform = type(self).__transform__
except AttributeError:
pass
else:
key = transform(key)
# now use the key as usual
Am I barking up the wrong tree? Would this slow down dict access too
much?
Considering that __transform__ would usually not exist, and triggered
exceptions are costly, I think it would.
From the docs[1]:
(10)
If a subclass of dict defines a method __missing__, if the key k is not present, the a[k] operation calls that
method with the key k as argument. The a[k] operation then returns or raises whatever is returned or raised by the
__missing__(k) call if the key is not present. No other operations or methods invoke __missing__(). If __missing__ is
not defined, KeyError is raised. __missing__ must be a method; it cannot be an instance variable. For an example, see
collections.defaultdict. New in version 2.5.
So something more like:
transform = getattr(self, '__transform__', None)
if transform is not None:
key = transform(key)
...
A key difference (pun unavoidable ;) between __missing__ and __transform__ is that __missing__ is only called when a key
is not found, while __transform__ needs to be called /every/ time a key is looked up:
d[k]
d.get(k)
d.has_key(k)
d.fromkeys(...)
d.setdefault(...)
k in d
--
~Ethan~
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