List slicing on Python 2.7

[Ned Batchelder]

On 3/15/18 9:57 AM, Vlastimil Brom wrote:
> 2018-03-15 12:54 GMT+01:00 Arkadiusz Bulski <arek.bulski at gmail.com>:
>> I have a custom class (a lazy list-like container) that needs to support
>> slicing. The __getitem__ checks if index is of slice type, and does a list
>> comprehension over individual integer indexes. The code works fine on
>> Python 3 but fails on 2.7, both CPython and PyPy. The print inside
>> __getitem__ doesnt even get executed when its a slice. Does 2.7 have
>> different object model, where slices are handled by a different method than
>> __getitem__?
>>
>> The implementation and error log
>> https://github.com/construct/construct/blob/8839aac2b68c9e8240e9d9c041a196b0a7aa7d9b/construct/core.py#L4785-L4796
>> https://github.com/construct/construct/blob/8839aac2b68c9e8240e9d9c041a196b0a7aa7d9b/tests/test_core.py#L1148
>> https://travis-ci.org/construct/construct/jobs/353782126#L887
>>
>> --
>> ~ Arkadiusz Bulski
>> --
>> https://mail.python.org/mailman/listinfo/python-list
> Hi,
> it looks like, the older method __getslice__ is still used in python 2.7
> cf.:
> https://docs.python.org/2/reference/datamodel.html#object.__getslice__
>
> You might need to implement this method in your class as well for
> compatibility with python 2.
>
>

Python 2 will use __getitem__ for slices:

     $ python2.7
     Python 2.7.10 (default, May 30 2015, 12:06:13)
     [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)] on darwin
     Type "help", "copyright", "credits" or "license" for more information.
     >>> class Sliceable(object):
     ...     def __getitem__(self, thing):
     ...         print type(thing)
     ...         print thing
     ...
     >>> Sliceable()[1:5]
     <type 'slice'>
     slice(1, 5, None)
     >>> Sliceable()[:]
     <type 'slice'>
     slice(None, None, None)
     >>>

--Ned.