t+1 - Latest Comments in defaultdict.fromkeys does not play nice.

Re: defaultdict.fromkeys does not play nice.

Matt Wilson — Wed, 04 Jun 2008 10:32:46 -0000

Hey Georg,

Yeah, I agree that the defaultdict.fromkeys behavior is not a bug, but it does seem a little surprising. Anyhow, I posted to comp.lang.python here about this topic and got some pretty good explanation.

Re: defaultdict.fromkeys does not play nice.

Georg Brandl — Wed, 04 Jun 2008 03:14:29 -0000

With dictionaries, the second argument for fromkeys() is the value (not a value factory) that all keys from the first argument will be set to. Since defaultdict.fromkeys() has no special handling, this works the same with defaultdict. That's not a bug to me.

Of course defaultdict.fromkeys could be extended to take a factory function argument, but as you have seen you can set the factory yourself too.

All in all, I don't see the value of using fromkeys() for a defaultdict.

Re: defaultdict.fromkeys does not play nice.

Gary Bernhardt — Tue, 03 Jun 2008 14:42:49 -0000

Ahh... good point. Clearly I didn't think it through. :)

I considered taking a stab at fixing this, but the effort involved for a first-time Python contributor is prohibitive. So laziness wins again.

Re: defaultdict.fromkeys does not play nice.

Matt Wilson — Tue, 03 Jun 2008 13:33:59 -0000

Hey Gary, thanks for the comment.

Setting the default_factory after I make an instance doesn't go back and rerun the keys through the default factory. So, in that sense, fromkeys is kinda useless to me. That's why I'd prefer that it just raised that NotImplementedError.

>>> b = defaultdict.fromkeys(['x', 'y'], list)
>>> b
defaultdict(None, {'y': <type 'list'="">, 'x': <type 'list'="">})
>>> b.default_factory = list
>>> b['z']
[]
>>> b['x']
<type 'list'=""></type></type></type>

Re: defaultdict.fromkeys does not play nice.

Gary Bernhardt — Tue, 03 Jun 2008 13:19:31 -0000

It looks like defaultdict.fromkeys just fails to pass the factory function along:

>>> defaultdict(list)
defaultdict(<type 'list'="">, {})
>>> defaultdict.fromkeys([], list)
defaultdict(None, {})

There's no mention of fromkeys in the collections module's source, so it's probably just inheriting it from dict. That makes sense - dict doesn't know about the default_factory. You can manually set the defaultdict's default_factory argument to fix it.</type>