[Python-Dev] Re: PEP 292 - Simpler String Substitutions

[Tim Peters]

[M.-A. Lemburg]
> ...
> Aside: ''.join() currently only works for true sequences - not iterators.

>>> def gen():
...     for s in "sure", "it", "does":
...         yield s
...
>>> ' '.join(gen())
'sure it does'
>>> u' '.join(gen())
u'sure it does'
>>>

Every function implemented with PySequence_Fast() works with any
iterable, although it's fastest if the input argument is a builtin
list or tuple.  For anything else (including list or tuple subclasses,
and other "true sequences"), it materializes a temp tuple, via the
iterator protocol.