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restriction on sum: intentional bug?

Gabriel Genellina gagsl-py2 at yahoo.com.ar
Mon Oct 19 22:42:04 EDT 2009 
En Sun, 18 Oct 2009 21:50:55 -0300, Carl Banks <pavlovevidence at gmail.com>  
escribió:

> Consider this thought experiment:
>
>
> class Something(object):
>     def __radd__(self,other):
>         return other + "q"
>
> x = ["a","b","c",Something()]
>
>
> If x were passed to "".join(), it would throw an exception; but if
> passed to a sum() without any special casing, it would successfully
> return "abcq".
>
> Thus there is divergence in the two behaviors, thus transparently
> calling "".join() to perform the summation is a Bad Thing Indeed, a
> much worse special-case behavior than throwing an exception.

Just for completeness, and in case anyone would like to try this O(n²)  
process, sum(x) may be rewritten as:

x = ["a","b","c",Something()]
print reduce(operator.add, x)

which does exactly the same thing, with the same quadratic behavior as  
sum(), but prints "abcq" as expected.

-- 
Gabriel Genellina
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