On 3/15/18 12:35 PM, Peter Otten wrote:
Ned Batchelder wrote:
On 3/15/18 9:57 AM, Vlastimil Brom wrote:
2018-03-15 12:54 GMT+01:00 Arkadiusz Bulski <arek.bul...@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
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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)
>>>
__getslice__() takes precedence, and the OP subclasses list:
$ python
Python 2.7.6 (default, Nov 23 2017, 15:49:48)
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
class A(list):
... def __getitem__(self, index): return index
...
a = A()
a[:]
[]
a[::]
slice(None, None, None)
A.__getslice__ = lambda self, *args: args
a[:]
(0, 9223372036854775807)
Another good reason not to subclass list.
--Ned.
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