[Cython] memoryview slices can't be None?

mark florisson markflorisson88 at gmail.com
Fri Feb 3 00:09:09 CET 2012

On 2 February 2012 21:38, Dag Sverre Seljebotn
<d.s.seljebotn at astro.uio.no> wrote:
> On 02/02/2012 10:16 PM, mark florisson wrote:
>> On 2 February 2012 12:19, Dag Sverre Seljebotn
>> <d.s.seljebotn at astro.uio.no>  wrote:
>>> I just realized that
>>> cdef int[:] a = None
>>> raises an exception; even though I'd argue that 'a' is of the "reference"
>>> kind of type where Cython usually allow None (i.e., "cdef MyClass b =
>>> None"
>>> is allowed even if type(None) is NoneType). Is this a bug or not, and is
>>> it
>>> possible to do something about it?
>>> Dag Sverre
>>> _______________________________________________
>>> cython-devel mailing list
>>> cython-devel at python.org
>>> http://mail.python.org/mailman/listinfo/cython-devel
>> Yeah I disabled that quite early. It was supposed to be working but
>> gave a lot of trouble in cases (segfaults, mainly). At the time I was
>> trying to get rid of all the segfaults and get the basic functionality
>> working, so I disabled it. Personally, I have never liked how things
> Well, you can segfault quite easily with
> cdef MyClass a = None
> print a.field
> so it doesn't make sense to slices different from cdef classes IMO.
>> can be None unchecked. I personally prefer to write
>> cdef foo(obj=None):
>>     cdef int[:] a
>>     if obj is None:
>>         obj = ...
>>     a = obj
>> Often you forget to write 'not None' when declaring the parameter (and
>> apparently that it only allowed for 'def' functions).
>> As such, I never bothered to re-enable it. However, it does support
>> control flow with uninitialized slices, and will raise an error if it
>> is uninitialized. Do we want this behaviour (e.g. for consistency)?
> When in doubt, go for consistency. So +1 for that reason. I do believe that
> setting stuff to None is rather vital in Python.
> What I typically do is more like this:
> def f(double[:] input, double[:] out=None):
>    if out is None:
>        out = np.empty_like(input)
>    ...
> Having to use another variable name is a bit of a pain. (Come on -- do you
> use "a" in real code? What do you actually call "the other obj"? I sometimes
> end up with "out_" and so on, but it creates smelly code quite quickly.)

No, it was just a contrived example.

> It's easy to segfault with cdef classes anyway, so decent nonechecking
> should be implemented at some point, and then memoryviews would use the same
> mechanisms. Java has decent null-checking...

The problem with none checking is that it has to occur at every point.
With initialized slices the control flow knows when the slices are
initialized, or when they might not be (and it can raise a
compile-time or runtime error, instead of a segfault if you're lucky).
I'm fine with implementing the behaviour, I just always left it at the
bottom of my todo list.

> Dag Sverre
> _______________________________________________
> cython-devel mailing list
> cython-devel at python.org
> http://mail.python.org/mailman/listinfo/cython-devel

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