reversed(zip(...)) not working as intended

[Sven R. Kunze]

On 06.03.2016 19:53, Peter Otten wrote:
> Sven R. Kunze wrote:
>
>> what's the reason that reversed(zip(...)) raises as a TypeError?
>>
>> Would allowing reversed to handle zip and related functions lead to
>> strange errors?
> In Python 3 zip() can deal with infinite iterables -- what would you expect
> from
>
> reversed(zip(count()))
>
> ?

Have I no idea. ;-)

But to me, "infinite" feels not like the most common use-case to most 
developers.

I just stumbled over it during rapid test case development:

for c in reversed(zip(ascii_lowercase, ascii_uppercase)):
     ...

Bam! That doesn't work although the code clearly describes what to do. :-(


>
> If all arguments of zip() are finite and of equal length you can write
>
> zip(reversed(a1), reversed(a2), ...)
>
> or if you find that really useful something like
>
>>>> class myzip(zip):
> ...     def __init__(self, *args):
> ...         self.args = args
> ...     def __reversed__(self):
> ...         return zip(*(reversed(a) for a in self.args))
> ...
>>>> list(reversed(myzip("abc", [1,2,3])))
> [('c', 3), ('b', 2), ('a', 1)]
>
> While this might look right at first sight it really opens a can of worms.
> First zip():
>
>>>> z = zip("abc", "def")
>>>> next(z)
> ('a', 'd')
>>>> list(z)
> [('b', 'e'), ('c', 'f')]
>
> Now myzip():
>
>>>> m = myzip("abc", "def")
>>>> next(m)
> ('a', 'd')
>>>> list(reversed(m))
> [('c', 'f'), ('b', 'e'), ('a', 'd')]
>
> Frankly, I have no idea what consistent behaviour should look like for a
> zip() that can be "reverse-iterated".
>
> PS: In Python 2 zip() would produce a list, so
>
>>>> list(reversed(zip("abc", "def")))
> [('c', 'f'), ('b', 'e'), ('a', 'd')]
>
> worked without requiring any code in zip().
>