Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Peter Otten <__peter__@web.de>
Newsgroups: comp.lang.python
Subject: Re: reversed(zip(...)) not working as intended
Date: Sun, 06 Mar 2016 19:53:36 +0100
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Xref: csiph.com comp.lang.python:104165

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()))

?

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().