restriction on sum: intentional bug?

Carl Banks pavlovevidence at
Mon Oct 19 02:50:55 CEST 2009

On Oct 18, 4:07 pm, Ethan Furman <et... at> wrote:
> Dave Angel wrote:
> > Earlier, I would have agreed with you.  I assumed that this could be
> > done invisibly, with the only difference being performance.  But you
> > can't know whether join will do the trick without error till you know
> > that all the items are strings or Unicode strings.  And you can't check
> > that without going through the entire iterator.  At that point it's too
> > late to change your mind, as you can't back up an iterator.  So the user
> > who supplies a list with mixed strings and other stuff will get an
> > unexpected error, one that join generates.
> > To put it simply, I'd say that sum() should not dispatch to join()
> > unless it could be sure that no errors might result.
> How is this different than passing a list to sum with other incompatible
> types?
> Python 2.5.4 (r254:67916, Dec 23 2008, 15:10:54) [MSC v.1310 32 bit
> (Intel)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
>  >>> class Dummy(object):
> ...     pass
> ...
>  >>> test1 = [1, 2, 3.4, Dummy()]
>  >>> sum(test1)
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: unsupported operand type(s) for +: 'float' and 'Dummy'
>  >>> test2 = ['a', 'string', 'and', 'a', Dummy()]
>  >>> ''.join(test2)
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> TypeError: sequence item 4: expected string, Dummy found
> Looks like a TypeError either way, only the verbage changes.

This test doesn't mean very much since you didn't pass the the same
list to both calls.  The claim is that "".join() might do something
different than a non-special-cased sum() would have when called on the
same list, and indeed that is true.

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.

Carl Banks

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