Cython “Not allowed in a constant expression”, bou

I am relatively new to Cython and have encountered an error that my research has failed me on (I am using Python3 in spyder and my Sython version is 0.26)

I tried this:

import cython

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

and it works fine. But then I tried this:

import cython

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

and I receive the error

[1/1] Cythonizing test.pyx

Error compiling Cython file:
------------------------------------------------------------
...
import 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.

UPDATE

I realized I have used import Cython instead of cimport cython. I tried again with

cimport cython

But get the same error.

UPDATE 2

On a similar note the code

cdef int N = 3
cdef double[:] lout = array.array('d', N)

throws the error

TypeError: 'int' object is not iterable

I assume for the same reason that C cannot deal with (possibly) dynamic array allocation. Instead we must use

cdef double[:] lout = numpy.empty(N, 'd')

and I assume there is a line that converts N into a static type somewhere before putting it into the C-array

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)