On 2011-12-22, at 09:09 , Stefan Behnel wrote:
What if you need a default value for one but not the others, or different values for all three? Does this behavior really make sense?
What would be the use case? Would you want the lookup process to continue on the default argument if an intermediate attribute is not found? And if a subsequent lookup fails? Take the corresponding default value again and keep looking up on it? That feels rather unpredictable from a users' point of view.
A multiple-*args attrgetter fetches different arguments, for a "lookup process" you need a dotted path (attrgetter('foo.bar.baz')). For that one a default value returned after any lookup failure makes sense, but it's not what attrgetter('foo', 'bar', 'baz') does:
from collections import namedtuple from operator import attrgetter r = namedtuple('T', 'a b c d e')(1, 2, 3, 4, 5) attrgetter('a.__index__')(r) <method-wrapper '__index__' of int object at 0x10020b1e8> attrgetter('a', 'b', 'e')(r) (1, 2, 5)
And for the second case, you'd need to be able to specify a default value for each of a, b and e.
I think the idea is that you ask for an attribute, which (in the less common cases) happens to be a multi-step lookup, and if the attribute is not found, you want it to return a default value *for the attribute you requested*, i.e. not a different value for any of the intermediate attributes Right, we're talking about different things and as noted above I think you're figuring the behavior of attrgetter(arg0, arg1, …) wrong.
I don't think the 0.1% case where you want more than that is worth a substantially more complicated API. Thing is, I don't know about others but my main use of attrgetter *by far* is to fetch multiple attributes simultanously. When fetching a single attribute (even with multiple steps), it's often (if not always) shorter to use a lambda with an attribute lookup.