Feature Proposal: Sequence .join method

[Jp Calderone]

On Fri, 30 Sep 2005 09:38:25 -0700, Michael Spencer <mahs at telcopartners.com> 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]
>  >>>
>
>but I can't think of a use for it ;-)

I have this version:

    def interlace(x, i):
        """interlace(x, i) -> i0, x, i1, x, ..., x, iN
        """
        i = iter(i)
        try:
            yield i.next()
        except StopIteration:
            return
        for e in i:
            yield x
            yield e

And I use it for things like interleave(", ", ["foo", bar, "baz"]), where bar is not a string, but can be handled along with strings by a lower-level chunk of code.

Jp