Export constants from header with Cython

问题:

I'm using Cython to wrap a C library. The C library's header file defines a number of constants, so in my cython module, I have something like:

cdef extern from "lib.h":
    cdef int CONST_A = 0
    cdef int CONST_B = 1

When I compile the extension, the constants are not available in Python. I tried doing something like this, but it did not seem to work:

cdef extern from "lib.h":
    cdef int CONST_A = 0
    cdef int CONST_B = 1

CONST_A = CONST_A
CONST_B = CONST_B

Any ideas on how to make these constants available in Python?

回答1:

You're right in that there seems to be a hole in Cython there.

I can declare cpdef int CONST_C = 3 and it compiles but isn't visible from Python. That would seem to be a bug -- if it accepts the syntax, it should do something reasonable with it.

One thing that did seem to work is this:

cpdef enum:
    CONST_A = 0
    CONST_B = 1

That may or may not be good enough for your case, but it seems it would work well for many use-cases -- maybe it's why the bug wasn't noticed?

回答2:

The solution I use which would be fairly easy to then switch to the "right" solution once supported is to re-declare the enum values one by one in the .pyx file prefixed by the enum's name, then they are quite easy to spot.

So in foo.pxd:

cdef enum Mood:
    happy
    sad

Then in foo.pyx at global scope:

Mood_happy = happy
Mood_sad = sad

And in bar.py:

from foo import Mood_happy, Mood_sad

# do something with Mood_happy and Mood_sad as ints

回答3:

Too late comment but how about the following code ? This idea is derived from How can a #defined C value be exposed to Python in a Cython module?

This solution works well in my library. "CONST_A" and "CONST_B" are alias for cdef/cpdef functions. In of Cython(.pyx), they can be used as a macro. To expose macro to Python, CONST_A = _CONST_A and CONST_B = _CONST_B are needed.

a.pyx

cdef extern from "lib.h":
    cdef int _CONST_A "CONST_A"
    cdef int _CONST_B "CONST_B"

def use_const():
    if CONST_A:
      do_something()
    if CONST_B:
      do_something()

CONST_A = _CONST_A
CONST_B = _CONST_B

After compile, you can use them in python code.

use_a.py

import a
print(a.CONST_A)
print(a.CONST_B)