[Feature Request] dict.setdefault()

Mon Apr 11 19:25:54 EDT 2011

On 04/11/2011 05:44 PM, Chris Angelico wrote:
> On Tue, Apr 12, 2011 at 8:41 AM, MRAB<python at mrabarnett.plus.com> wrote:
>> I'm not sure that "setdefault" should take **kw args for this because
>> of its existing argument structure (key + optional value).
>>>> A new method like "updatedefault" may be better, IMHO. It would act
>> like "update" except that it wouldn't overwrite existing values.
>> Wouldn't x.updatedefault(y) be pretty much y.update(x) ?

As I understand, the difference would be the following pseudocode:
 def update(self, d):
 for k,v in dict(d).iteritems():
 self[k] = v
 def updatedefault(self, d={}, **kwargs):
 for k,v in chain(
 dict(d).iteritems(),
 kwargs.iteritems()
 ):
 # MRAB's comment about "wouldn't overwrite existing"
 if k not in self:
 self[k] = v
My concern with the initial request is that dict.setdefault() 
already returns the (existent or defaulted) value, so you can do 
things like
 d.setdefault(my_key, []).append(item)
If you allow it to take multiple kwargs, what would the return 
value be (positionality of kwargs is lost, so returning a tuple 
wouldn't be readily possible)?
Finally, if it were added, I'd call it something like merge()
-tkc