# Feature Proposal: Sequence .join method

David Murmann david.murmann at rwth-aachen.de
Fri Sep 30 19:06:40 CEST 2005

```Michael Spencer wrote:
> Terry Reedy wrote:
>> "David Murmann" <david.murmann at rwth-aachen.de> wrote in message
>> news:3q3pt9Fd7pklU1 at news.dfncis.de...
>>
>>>> def join(sep, seq):
>>>>    return reduce(lambda x, y: x + sep + y, seq, type(sep)())
>>>
>>> damn, i wanted too much. Proper implementation:
>>>
>>> def join(sep, seq):
>>>    if len(seq):
>>>        return reduce(lambda x, y: x + sep + y, seq)
>>>    return type(sep)()
>>>
>>> but still short enough
>>
>>
>> For general use, this is both too general and not general enough.
>>
>> If len(seq) exists then seq is probably reiterable, in which case it
>> may be possible to determine the output length and preallocate to make
>> the process O(n) instead of O(n**2).  I believe str.join does this.  A
>> user written join for lists could also.  A tuple function could make a
>> list first and then tuple(it) at the end.
>>
>> If seq is a general (non-empty) iterable, len(seq) may raise an
>> exception even though the reduce would work fine.
>>
>> Terry J. Reedy
>>
>>
>>
> For the general iterable case, you could have something like this:
>
>  >>> def interleave(sep, iterable):
>  ...     it = iter(iterable)
>  ...     next = it.next()
>  ...     try:
>  ...         while 1:
>  ...             item = next
>  ...             next = it.next()
>  ...             yield item
>  ...             yield sep
>  ...     except StopIteration:
>  ...         yield item
>  ...
>  >>> list(interleave(100,range(10)))
>  [0, 100, 1, 100, 2, 100, 3, 100, 4, 100, 5, 100, 6, 100, 7, 100, 8,
> 100, 9]

Well, as en.karpachov at ospaz.ru pointed out, there is already
itertools.chain which almost does this. In my opinion it could be useful
to add an optional keyword argument to it (like "connector" or "link"),
which is iterated between the other arguments.

> but I can't think of a use for it ;-)

Of course, i have a use case, but i don't know whether this is useful
enough to be added to the standard library. (Yet this would be a much
smaller change than changing all sequences ;)

thanks for all replies,
David.

```