Description
Typed memoryviews don't work with read-only buffers. A banal example:
cpdef getmax(double[:] x):
cdef double max = -float('inf')
for val in x:
if val > max:
max = val
return maxExample session:
>>> import numpy as np
>>> values = np.random.random(8)
>>> values
array([ 0.4574, 0.8468, 0.744 , 0.3764, 0.7581, 0.8123, 0.8783, 0.9341])
>>> import getmax
>>> getmax.getmax(values)
0.9341213061235054
>>> values.setflags(write=False)
>>> assert values.flags.writeable == False
>>> getmax.getmax(values)
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-8-669165de7389> in <module>()
----> 1 getmax.getmax(values)
/tmp/getmax.pyx in getmax.getmax (getmax.c:1617)()
----> 1 cpdef getmax(double[:] x):
2 cdef:
3 double max = -float('inf')
4
5 for val in x:
/tmp/getmax.so in View.MemoryView.memoryview_cwrapper (getmax.c:7403)()
/tmp/getmax.so in View.MemoryView.memoryview.__cinit__ (getmax.c:3678)()
ValueError: buffer source array is read-onlyThere is no evident reason why above simple function wouldn't work with a read-only buffer. About two years ago, this issue was raised on the mailing list along with a RFC patch:
https://mail.python.org/pipermail/cython-devel/2015-February/004316.html
Some discussion about a possible use for a const keyword:
https://groups.google.com/forum/#!topic/cython-users/32CMgaLrNhc