[issue10533] defaultdict constructor with a concrete value
New submission from Łukasz Langa luk...@langa.pl: Currently the constructor in defaultdict only accepts factories. It would be very handy to allow for concrete values as well. It's implementable either by checking if the argument is callable or by a new keyword argument. -- assignee: lukasz.langa components: Library (Lib) keywords: easy messages: 122382 nosy: holdenweb, lukasz.langa, michael.foord priority: low severity: normal status: open title: defaultdict constructor with a concrete value versions: Python 3.2 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Michael Foord mich...@voidspace.org.uk added the comment: I would love this functionality (I almost always initialise defaultdict with a lambda that just returns a concrete value). Unfortunately it seems like adding a keyword argument isn't possible because defaultdict takes arbitrary keyword args (and populates the dict with them). defaultdict(lambda: 1, foo=1, bar=2, baz=3) -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Eric Smith e...@trueblade.com added the comment: -1 from me. You can't use keywords, and if you make the value callable at a later date then suddenly you'll change the behavior of seemingly unrelated code. Is a lambda so bad? -- nosy: +eric.smith ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Łukasz Langa luk...@langa.pl added the comment: Both arguments are true and definitive. Last possibility would be to introduce a factory function for defaultdicts that would only accept concrete values: from collections import fallbackdict Then this factory could produce defaultdict instances like this: fallbackdict(0, some_key=1) -- defaultdict(lambda: 0, some_key=1) That would be safe for future type changes as well. If this version does not sound appealing, I can't see any other way of reliably introducing this shortcut. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Eric Smith e...@trueblade.com added the comment: How about: from collections import defaultdict class defaultdict_value(defaultdict): def __init__(self, value): defaultdict.__init__(self, lambda : value) x = defaultdict_value(3) print(x[1]) -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Steve Holden st...@holdenweb.com added the comment: On 11/25/2010 11:48 AM, Eric Smith wrote: Eric Smith e...@trueblade.com added the comment: How about: from collections import defaultdict class defaultdict_value(defaultdict): def __init__(self, value): defaultdict.__init__(self, lambda : value) x = defaultdict_value(3) print(x[1]) -- +1 But I'd call it defaultdict_const(). regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 PyCon 2011 Atlanta March 9-17 http://us.pycon.org/ See Python Video! http://python.mirocommunity.org/ Holden Web LLC http://www.holdenweb.com/ -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Éric Araujo mer...@netwok.org added the comment: Like three-liners? whatsnew/2.5 gives us this one: class zerodict(dict): def __missing__(self, key): return 0 I don’t think it’s too painful to have to use defaultdict with a lambda. We can’t use a keyword argument and I’m -0.5 on changing behavior depending on the type of the first argument. -- nosy: +eric.araujo ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Changes by Raymond Hettinger rhettin...@users.sourceforge.net: -- assignee: lukasz.langa - rhettinger nosy: +rhettinger ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Raymond Hettinger rhettin...@users.sourceforge.net added the comment: It would be very handy to allow for concrete values as well. Do you have use cases for a concrete integer value that isn't zero? Since we can currently use defaultdict(int) or defaultdict(tuple), is the purpose just to create a more direct spelling of the same thing? The docs for defaultdict also show a general purpose way to generate any default constant (though that way isn't obvious if you haven't seen it in the docs). I'm reluctant to add yet another variant. We already have __missing__, defaultdict, Counter, dict.get, and dict.setdefault(). The docs for dictionaries need to make clear that Guido has already provided an idiom to be the one obvious way to do it: class MyConst(dict): def __missing__(self, key): return myconst That was really the whole point of adding __missing__ in the first place. The OP's proposal amounts to rejecting Guido's design which provides a very good general purpose solution. -- keywords: -easy type: - feature request ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Łukasz Langa luk...@langa.pl added the comment: A couple of points: 1. Eric's proposal is what I had in mind with the `fallbackdict' idea. 2. I'm also reluctant to add more variants to the standard library. Then again if it contained a `fallbackdict' I wouldn't probably ever use `defaultdict' again. How often do you need to provide a factory? 3. Naming the other variant `defaultdict_const', `defaultdict_value', `defaultdict_whatever' beats the purpose because it's actually more characters to type than `defaultdict(lambda:', especially when you count the longer import name. 4. I cannot come up with another typical integer value that would be useful, then again I've used , [] and set() numerous times. Adding zerodict, stringdict, listdict, setdict is obviously absurd. 5. The discussion started on Twitter amongst a couple of core devs and __missing__ didn't appear to be the one obvious way to anyone. 6. Of course I'm in no position to reject Guido's design on anything. Then again even `defaultdict(lambda:' is simply so much shorter than subclassing dict. To sum up: if you don't find the idea of adding `fallbackdict' (possibly with an different *short* name) worth it, then I'm +1 on correcting the docs in terms of __missing__ and leaving the implementation as is. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Steve Holden st...@holdenweb.com added the comment: On 11/25/2010 1:44 PM, Łukasz Langa wrote: To sum up: if you don't find the idea of adding `fallbackdict' (possibly with an different *short* name) worth it, then I'm +1 on correcting the docs in terms of __missing__ and leaving the implementation as is. +1 regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 PyCon 2011 Atlanta March 9-17 http://us.pycon.org/ See Python Video! http://python.mirocommunity.org/ Holden Web LLC http://www.holdenweb.com/ -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue10533] defaultdict constructor with a concrete value
Alex alex.gay...@gmail.com added the comment: I agree with Łukasz, it's more clutter than is worth for what amounts to: fallbackdict = lambda c, **kwargs: defaultdict(lambda c, **kwargs) I will note, however, that almost all my use cases are with factories, primarily list set or int, and it was only this week that I first needed something else! -- nosy: +alex ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue10533 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
