Cython compilation error "Not allowed in a constant expression"

Question

The following

cimport cython

@cython.boundscheck(False)
def boundtest():
     cdef int r=4
     cdef double l[3]

works fine. But when I try this:

cimport cython

@cython.boundscheck(False)
def boundtest():
     cdef int r=4
     cdef double l[r]

I receive the following error:

[1/1] Cythonizing test.pyx

Error compiling Cython file:
------------------------------------------------------------
...
cimport cython

@cython.boundscheck(False)
def boundtest():
     cdef int r=4
     cdef double l[r]
                   ^
------------------------------------------------------------

test.pyx:13:20: Not allowed in a constant expression

The decorator was added due to finding this related stackexchange post and reading the Cython book by Kurt W. Smith. As far as I can tell this should work to tell Cython not to worry about out of bounds errors that may result from having a dynamic indexing variable but for some reason it does not. I have also tried changing boundscheck in the compiler options and globally to no avail.

If it weren't for the Cython documentation claiming to be up to date I would think boundscheck has been depreciated.

MSeifert · Accepted Answer · 2017-09-19T20:32:16.663

16

The failure has nothing to do with cython.boundscheck.

Boundchecking is just checking if you try to access an element of an array that isn't there. For example if you have a size 4 array and try to access element 5 - with boundscheck(True) it will give you an Exception, with boundscheck(False) it will result in undefined behavior (probably causing a segmentation fault).

The reason for the compilation failure is another: You can't create a static array with a dynamic length! The number of elements need to be known at compile time, that's just something c enforces (I guess).

However you could define r to be known at compile time:

DEF r=4

cimport cython

@cython.boundscheck(False)
def boundtest():
    cdef double l[r]

You could however simply create a NumPy array and store it in a memoryview variable:

cimport cython
import numpy as np

@cython.boundscheck(False)
def boundtest():
    cdef int r=4
    cdef double[:] l = np.empty(r, dtype=np.double)

edited Sep 19 '17 at 20:32

answered Sep 19 '17 at 17:51

MSeifert

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It works, thank you very much! I wish the OP in the linked post had been a bit more explicit about changing his code to use a memoryview variable. – Takoda Sep 19 '17 at 18:03
@Takoda Cython combines Python and C so it requires a bit of knowledge about both to get it right. Probably it was just "assumed" that you don't create dynamic length arrays using `cdef double l[r]` but only with known constants like `cdef double l[4]`. – MSeifert Sep 19 '17 at 18:17
1

Weird that Cython doesn't support C99 VLAs, but good for them. VLAs are dangerous. – Alex Huszagh Feb 21 '18 at 21:31

Cython compilation error "Not allowed in a constant expression"

1 Answers1

Linked