On 04/06/2013 04:19 PM, Nathaniel Smith wrote:
Hi all,
If you build current numpy master with
NPY_RELAXED_STRIDES_CHECKING=1 python setup.py install
then Cython code using ndarrays starts blowing up, e.g.:
# foo.pyx
def add_one(array):
cdef double[::1] a = array
a[0] += 1.
return array
foo.add_one(np.ascontiguousarray(np.arange(10.)[::100]))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "foo.pyx", line 2, in foo.add_one (foo.c:1210)
cdef double[::1] a = array
ValueError: Buffer and memoryview are not contiguous in the same dimension.
The problem (as discussed before) is that Cython has an unnecessarily
strict definition of "contiguous", so NPY_RELAXED_STRIDES_CHECKING=1
pretty much breaks all existing compiled Cython modules.
Our plan is to make NPY_RELAXED_STRIDES_CHECKING=1 into the default
sooner or later, and Cython is a major blocker on this plan. It may
become the default as soon as the 1.8 pre-releases (with the
expectation that we'll probably have to switch back again before the
actual release, but still).
References:
Previous thread:
http://thread.gmane.org/gmane.comp.python.cython.devel/14634
Detailed discussion of the difference between numpy/cython's current
definition of "contiguity", and the correct definition:
http://thread.gmane.org/gmane.comp.python.cython.devel/14634/focus=14640
The PR implementing NPY_RELAXED_STRIDES_CHECKING:
https://github.com/numpy/numpy/pull/3162
Another test case:
https://github.com/numpy/numpy/issues/2956
We're hoping that Cython will also switch soon to the more accurate
check for contiguity. This shouldn't cause any backwards compatibility
problems -- it just means Cython code would make strictly fewer
copies, and error out due to lack of contiguity strictly less often,
even with older numpys. And it seems like a necessary step for getting
this untangled and minimizing user pain. What do you think?
I agree that we should follow NumPy here, but can't see myself having
time to do the change in near future. I don't know about Mark?
I think it is a fairly simple change though if any NumPyers would like
to do it, look at in Cython/Utility/MemoryView_C.c in the function
_pyx_memviewslice_is_contig
looks like it should just be to add a check for shape too.
I guess you have changed your implementation of PEP 3118 too slightly on
the NumPy side? Is this undefined in the PEP or are you now not in
strict adherence to it?
Dag Sverre
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