restriction on sum: intentional bug?

[MRAB]

Stephen Hansen wrote:
> 
> 
> On Fri, Oct 16, 2009 at 8:39 AM, Alan G Isaac <alan.isaac at gmail.com 
> <mailto:alan.isaac at gmail.com>> wrote:
> 
>     I expected this to be fixed in Python 3:
> 
>                 sum(['ab','cd'],'')
> 
>     Traceback (most recent call last):
>      File "<stdin>", line 1, in <module>
>     TypeError: sum() can't sum strings [use ''.join(seq) instead]
> 
>     Of course it is not a good way to join strings,
>     but it should work, should it not?  Naturally,
> 
>                 '' + 'ab' + 'cd'
> 
>     'abcd'
> 
>     Why doesn't duck typing apply to `sum`?
> 
> 
> Because it would be so hideously slow and inefficient that it'd be way 
> too easy a way for people to program something they think should work 
> fine but really doesn't... alternatively, the function would have to do 
> two /completely/ different implementations based on what you're passing 
> in, and that violates duck typing too :)
> 
> Duck typing isn't an absolute. Sum's not broken... This is a 
> "practicality beats purity" thing; maybe sum should just add up 
> everything passed to it and as long as no errors happen, return the 
> result. But in practice,  if someone passes sequences of strings to it, 
> it'd create/destroy so many strings (which is way more expensive then 
> just creating and destroying successive integers), people would be 
> making really bad code-decisions even when it seems a perfectly valid 
> thing to do.
> 
It checks to see whether you're trying to sum strings, so it's already
treating them as a special case. Why can't it just use str.join
internally instead in that case instead of raising an exception?