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

[Wolfgang Maier]

On 12.08.2014 00:15, Terry Reedy wrote:
> On 8/11/2014 5:35 PM, Ethan Furman wrote:
>> On 08/11/2014 02:25 PM, Nathaniel Smith wrote:
>
>>> class Nasty:
>>>      def __radd__(self, other):
>>>          return other + "foo"
>>>
>>> "".join(["some", "strings", "and", "one", Nasty()])
>>> sum(["some", "strings", "and", "one", Nasty()], "")
>
> I don't understand the point of this.
>
>> Quite frankly, I regard this as a point in sum's favor.  We have,
>> effectively, a string-subclass and join chokes on it.
>
> Nasty is a subclass of object, with no default value.  Make it a real
> str subclass and join works fine.
>
> class Nasty(str):
>      def __radd__(self, other):
>          return other + "foo"
>
> print("".join(["some", "strings", "and", "one", Nasty()]))
>  >>>
> somestringsandone
>

No, it's not, at least not as intended or the result would be

somestringsandonefoo

The point about Ethan's example is that join only works with str and 
subclasses thereof, but not with proxy classes wrapping a str object.