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devel / comp.lang.python / Re: Problem slicing a list with the C API

Re: Problem slicing a list with the C API

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From: ros...@gmail.com (Chris Angelico)
Newsgroups: comp.lang.python
Subject: Re: Problem slicing a list with the C API
Date: Sun, 13 Mar 2022 08:41:12 +1100
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by: Chris Angelico - Sat, 12 Mar 2022 21:41 UTC

On Sun, 13 Mar 2022 at 08:25, Jen Kris via Python-list
<python-list@python.org> wrote:
> PyObject* slice = PySlice_New(PyLong_FromLong(0), half_slice, 0);
> PyObject* subdata_a = PyObject_GetItem(pDictddata, slice);
>
> On the final line (subdata_a) I get a segfault. I know that the second parameter of PyObject_GetItem is a “key” and I suspect that’s where the problem comes from, but I don’t understand what a key is in this context.
>

The key is simply whatever would be in the square brackets in Python
code, so that part looks fine.

But dictionaries aren't usually subscripted with slices, so I'm a bit
confused as to what's going on here. What exactly is
dictdata/pDictdata?

Have you confirmed that pDictdata (a) isn't NULL, (b) is the object
you intend it to be, and (c) contains the objects you expect it to?
The segfault might not be from the slice object itself, it might be
from actually iterating over the thing being sliced and touching all
its elements. For instance, if dictdata is actually a list, that call
will be constructing a new list with references to the same elements,
so if one of them is broken (maybe NULL), it'll break badly.

ChrisA

Subject	Author
Re: Problem slicing a list with the C API	Chris Angelico