[Python-ideas] The non-obvious nature of str.join (was Re: sum(...) limitation)

[Nathaniel Smith]

On Mon, Aug 11, 2014 at 10:10 PM, Ethan Furman <ethan at stoneleaf.us> wrote:
> On 08/11/2014 10:53 AM, Nathaniel Smith wrote:
>>
>>
>> But practically speaking, how would this work? In general str.join and
>> sum have different semantics. What happens if we descend deep into the
>> iterable and then discover a non-string (that might nonetheless still
>> have a + operator)?
>
>
> The same thing that happens now if you pass a list to join with a non-string
> entry:
>
> --> ' '.join(['some', 'list', 'of', 'words', 'and', 10, 'as', 'a',
> 'number'])
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: sequence item 5: expected string, int found

class Nasty:
    def __radd__(self, other):
        return other + "foo"

"".join(["some", "strings", "and", "one", Nasty()])
sum(["some", "strings", "and", "one", Nasty()], "")

-n

-- 
Nathaniel J. Smith
Postdoctoral researcher - Informatics - University of Edinburgh
http://vorpus.org