The following Python 2.7 code is not converted correctly to Python 3 by 2to3:

c = [(1, 10), (2, 20)]

# get a tuple with the first entries of every tuple in c,
# i.e. (1, 2)
x = zip(*c)[0]

print x


The result is


c = [(1, 10), (2, 20)]

# get a tuple with the first entries of every tuple in c,
# i.e. (1, 2)
x = zip(*c)[0]

print(x)



However running it with python3 gives the following exception

Traceback (most recent call last):
  File "result.py", line 7, in <module>
    x = zip(*c)[0]
TypeError: 'zip' object is not subscriptable



Tested with 2to3-2.7 and 2to3-3.4 on debian stable (jessie)

In addition to zip(), this problem also affects map() and filter().

The problem is that the match patterns in FixZip, FixMap, and FixFilter do not allow for more than one "trailer" node.  (And even if they did, their transform() methods aren't expecting it.)

For example, in the following expression, 'zip' is followed by two 'trailers', which are '(a,b)', and [0]:

zip(a,b)[0]

... but FixZip.PATTERN only expects a single trailer (the argument list), so the presence of a second trailer prevents the match: https://git.io/vMDP9


(Here's the relevant line of the grammar: https://git.io/vMDPJ)

I've written a patch that fixes this problem for zip, map, and filter, with tests.  See attached.

BTW, this problem was previously reported in 21628, but that issue was incorrectly closed as a duplicate, so I'm submitting my patch here.

Sorry for re-uploading the patch; I made some pep8 fixes.

Patch submitted as github PR #24:
https://github.com/python/cpython/pull/24

New changeset 93b4b47e3a720171d67f3b608de406aef462835c by Benjamin Peterson (Stuart Berg) in branch 'master':
bpo-28837: Fix lib2to3 handling of map/zip/filter calls when followed with a 'trailer', e.g. zip()[x] (#24)
https://github.com/python/cpython/commit/93b4b47e3a720171d67f3b608de406aef462835c

I think this at least needs to be backported to 3.6 (or can we close the issue now?)

New changeset 292b421d48ab91b0c68ea4040fc7371e7d6d610e by Mariatta in branch '3.6':
bpo-28837: Fix lib2to3 handling of map/zip/filter calls when followed with a 'trailer', e.g. zip()[x] (GH-24) (GH-2235)
https://github.com/python/cpython/commit/292b421d48ab91b0c68ea4040fc7371e7d6d610e

Backported to 3.6.