[Python-Dev] sum(...) limitation

Terry Reedy tjreedy at udel.edu
Sat Aug 2 08:35:32 CEST 2014

On 8/2/2014 1:57 AM, Allen Li wrote:
> On Fri, Aug 01, 2014 at 02:51:54PM -0700, Guido van Rossum wrote:
>> No. We just can't put all possible use cases in the docstring. :-)
>> On Fri, Aug 1, 2014 at 2:48 PM, Andrea Griffini <agriff at tin.it> wrote:
>>      help(sum) tells clearly that it should be used to sum numbers and not
>>      strings, and with strings actually fails.
>>      However sum([[1,2,3],[4],[],[5,6]], []) concatenates the lists.
>>      Is this to be considered a bug?
> Can you explain the rationale behind this design decision?  It seems
> terribly inconsistent.  Why are only strings explicitly restricted from
> being sum()ed?  sum() should either ban everything except numbers or
> accept everything that implements addition (duck typing).

O(n**2) behavior, ''.join(strings) alternative.

Terry Jan Reedy

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