How to use const in Cython

Question

I have read in this link: https://github.com/cython/cython/wiki/FAQ#id35 that my Cython 0.20.1 should be able to support const.

However my following code does not compile:

cdef const double a = 2.5
print(a)

During compilation it says

test.pyx:5:5: Assignment to const 'a'
Traceback (most recent call last):
  File "setup1.py", line 11, in <module>
    ext_modules = cythonize(extensions)
  File "X:\WinPython3\python-3.3.5.amd64\lib\site-packages\Cython\Build\Dependencies.py", line 785, in cythonize
    cythonize_one(*args[1:])
  File "X:\WinPython3\python-3.3.5.amd64\lib\site-packages\Cython\Build\Dependencies.py", line 902, in cythonize_one
    raise CompileError(None, pyx_file)
Cython.Compiler.Errors.CompileError: test.pyx

Answer 1

const works for function arguments, but not for this construct because of how Cython compiles it to C.

cdef double a = 2.5

becomes

double __pyx_v_a;
/* lots of other declarations */

__pyx_v_a = 2.5;

and this wouldn't work with a const variable. In effect, you cannot use const in Cython in all the places were you can use it in C.