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

I think `cdef enum` is how you declare a const in cython but enum's are ints so it would not work in your case I imagine. — Padraic Cunningham, May 26 '14 at 16:35
you're welcome. I think you can use `DEF` to declare a const but it is unsigned. It would work in your case though I think. — Padraic Cunningham, May 28 '14 at 01:15
@PadraicCunningham: I think it's a literal substitution just like the C preprocessor. So no matter the type. — 0xC0000022L, Sep 16 '16 at 10:15
Beware about using `DEF` as it raises deprecation warnings in Cython 3 onwards. — Fusion, Jul 30 '23 at 15:20

score 28 · Accepted Answer · answered May 26 '14 at 17:23

28

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.

answered May 26 '14 at 17:23

Fred Foo

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Thanks! It is really helpful to think in terms of how Cython works in C. – Yuxiang Wang May 28 '14 at 01:09
2

what about C++? we could make Cython compile to C++, right? Then can use const as we do in c++? – avocado Dec 09 '15 at 02:12

How to use const in Cython

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