[Cython] [cython-users] Re: callback function pointer problem

[mark florisson]

On 30 September 2011 22:14, Dag Sverre Seljebotn
<d.s.seljebotn at astro.uio.no> wrote:
> Are you saying that when coercing a struct to an object, one would copy
> scalar fields by value but reference array fields? -1, that would be
> confusing. Either the whole struct through a view, or copy it all.

Hmm, yeah it might be confusing. To me it makes sense though, as
scalars are immutable but arrays are not, although by that logic the
struct should be a view. So perhaps a list makes more sense. Making
arrays in structs lists means you likely want to do the same for the
normal case, i.e. arrays as variables. I'm not sure you'd want normal
arrays to object conversion to be implicit though, I'd rather see an
explicit cast for that.

> It bothers me that structs are passed by value in Cython, but it seems
> impossible to change that now. (i.e, once upon a time one could have
> required the use of a copy method to do a struct assignment and give a
> syntax error otherwise, which would have worked nicer with Python
> semantics).
>
>
> --
> Sent from my Android phone with K-9 Mail. Please excuse my brevity.
>
> mark florisson <markflorisson88 at gmail.com> wrote:
>>
>> On 29 September 2011 22:48, Robert Bradshaw <robertwb at math.washington.edu>
>> wrote: > On Thu, Sep 29, 2011 at 5:29 AM, mark florisson >
>> <markflorisson88 at gmail.com> wrote: >> On 29 September 2011 13:13, Miguel
>> Angel <skuda21 at gmail.com> wrote: >>>> >>>> Structs already coerce to python
>> dicts. In the memoryview branch it >>>> also does the reverse, i.e. coerce a
>> dict to a struct (you have to be >>>> careful with strings, making sure you
>> hold a reference to the string >>>> object as long as you use the pointer in
>> the struct). You have to be >>>> sure to declare every struct attribute
>> though. >>> >>> Could i see an example usage of this in any repository or
>> the documentation? I >>> can't find any. >>> >>> Regards, >>> Miguel Angel.
>> >>> >> >> It's not in the documentation yet, I updated that in a branch that
>> >> still needs to be merged (which hopefully will be in 0.16). The >>
>> functionality from struct -> object is there though: >> >> cdef extern from
>> *: >>    ctypedef struct MyStruct: >>        int attrib >> >> cdef MyStruct
>> s = {'attrib':10} # Note, this is NOT object->struct >> conversion, it only
>> eats compile-time expressions here > > or even > >    s =
>> MyStruct(attrib=10) That looks very nice, a lot more intuitive than the dict
>> assignment. > It would be natural to support array -> list conversion for
>> slices the > baseclass is convertable, e.g. > >    cdef double* data = ... >
>>    print data[:10] > > Converting an int[10] to a Python object without
>> slicing is a bit odd as > >    cdef int a[10] >    [compute a] >    cdef
>> object o = a >    a[0] = something else >    o[1] = another thing > > is
>> non-initiative, as arrays can't be assigned in C and "assignment" > of lists
>> in Python are by reference, not by copy. "o = a[:]" is plenty > clear that
>> there's a copy, so that's better. The other way is a bit > messier, but > >
>>    a[:n] = [python list] > > could certainly be supported via an implicit
>> loop. Another > consideration is that with primitive data types we're
>> venturing into > memory view territory, which might be a more useful
>> representation > than Python lists. > > - Robert > Indeed. Currently you can
>> only cast pointers to memoryview slices, but there's no reason to not
>> support arrays as well (without also needing to first cast the array to a
>> pointer). So if arrays have shape information, you could immediately coerce
>> to memoryview (cython.array) objects without needing a cast. This would also
>> work for arrays in structs, as they can only ever have a static size (known
>> at C compile time). Shall we support that? I won't be hard as the
>> infrastructure for casting pointers is already there.
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