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

Allen Li cyberdupo56 at gmail.com
Sat Aug 2 07:57:38 CEST 2014

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).
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