MemoryView questions

[Bradley Froehle]

Hi Folks,

I've got four questions relating to memoryviews in Cython 0.17.beta1, which hopefully you can help me with.

1. Can typed memory views accept multiple dimensions?

I've noticed that memory views do have an ndim attribute, however defining a function as:

    def f(double[:] x):

        pass

only works if x is one-dimensional.  Can I somehow allow x to have 1, 2, or 3 dimensions?  I'm fine adding an `assert 1 <= x.ndim <= 3` statement.

2. If I have a multidimensional array, why do I have to call `is_f_contig` to see if it is f-contiguous?  It seems like that should be a property, much like `ndim`.

    def f(double[:, :] x):

        print x.is_f_contig()

3. Supposing that I know in advance that my memory is contiguous, how can I get access to the underlying data pointer of the memoryview?

Certainly I could do:

    def f(double[::1, :] x):

        cfunction(&x[0,0])

... but what if I don't know the dimensionality of x?  (See Q 1).

Perhaps there should be a x.data attribute?

4. Can I not have inline memoryview functions in pxd files?

Consider:

### lib.pxd ###

cdef inline void f(double[:,:] x):

    pass

cdef inline f1(x):

    f(x)

### tst.pyx ###

cimport lib

Then compiling tst.pyx fails:

$ python setup.py build_ext --inplace

running build_ext

building 'tst' extension

creating build

creating build/temp.linux-x86_64-2.7

gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.7 -c tst.c -o build/temp.linux-x86_64-2.7/tst.o

tst.c: In function ‘__pyx_f_3lib_f1’:

tst.c:1232:3: warning: implicit declaration of function ‘__Pyx_PyObject_to_MemoryviewSlice_dsds_double’ [-Wimplicit-function-declaration]

tst.c:1232:13: error: incompatible types when assigning to type ‘__Pyx_memviewslice’ from type ‘int’

tst.c: At top level:

tst.c:13446:12: warning: ‘__Pyx_ValidateAndInit_memviewslice’ defined but not used [-Wunused-function]

error: command 'gcc' failed with exit status 1

Thanks!

-Brad

[Dave Hirschfeld]

Not sure about the answers to your questions in general, but I thought you
might be able to cheat by using fused types. Unfortunately this seems not
to work:

In [32]: %%cython
...: cimport cython
...:
...: from cpython cimport bool
...:
...: import numpy as np
...: cimport numpy as np
...:
...: ctypedef np.float64_t float64_t
...:
...: ctypedef fused myarray:
...: float64_t[:,:]
...: float64_t[:,:,:]
...: #
...:
...: cpdef myarray nop(myarray array):
...: return array
...:

In [33]: nop(rand(2))
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-33-bbba0b03d22e> in <module>()
----> 1 nop(rand(2))

f.pyd in f.__pyx_fused_cpdef (f.c:2503)()

TypeError: No matching signature found

In [34]: nop(rand(2,2))
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-34-fa44702cf241> in <module>()
----> 1 nop(rand(2,2))

f.pyd in f.__pyx_fused_cpdef (Cf.c:2503)()

TypeError: No matching signature found

In [35]: nop(rand(2,2,2))
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-35-c6b24e4d9bf8> in <module>()
----> 1 nop(rand(2,2,2))

f.pyd in f.__pyx_fused_cpdef f.c:2503)()

TypeError: No matching signature found

In [36]:


Is there any reason why this shouldn't work or is this a bug?

-Dave

[mark florisson]

Thanks, that's a bug. It seems the delimiter ', ' was wrong, since it
generates signature strings like 'double[:, :]'. It also didn't
dispatch on ndim, so that's fixed as well. There's a fix here
https://github.com/markflorisson88/cython/commit/f96a5622c68776d4a206f36d1f701c763bad7caf
which will be pushed for 0.17 if it doesn't break anything.