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

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Fri Sep 30 23:14:57 CEST 2011

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.

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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