[Cython] memoryview slices can't be None?

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Thu Feb 2 22:38:15 CET 2012

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 

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...

Dag Sverre

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