Congruence of 'map' and 'for' broken in 2.1??

[Alex Martelli]

<nanotech at europa.com> wrote in message
news:mailman.988402866.799.python-list at python.org...
    [snip]
>   >>> print reduce(lambda x, y: x+y, map(int,fh.xreadlines()))
>   Traceback (most recent call last):
>     File "<pyshell#91>", line 1, in ?
>       print reduce(lambda x, y: x+y, map(int,fh.xreadlines()))
>   TypeError: argument 2 to map() must be a sequence object
>
> Ack!! Why will map not fudge? What am I missing?? Is it pilot error??

I think map() needs to know the *length* of its sequence arguments,
and the object returned by .xreadlines() doesn't/can't supply it -- it's
more of an "undetermined-length sequence" (sensibly-- how would
length be determined without reading everything?).

Since we don't formally have interfaces/protocols in Python, but
rely on informal docs, such ambiguities recur -- it's never really
clear when the docs say "a sequence" whether they mean a length
is needed or not, just like "a function" may be any callable in
many cases but not all, etc.  PEP 245/246 might help...


Alex