How can a #defined C value be exposed to Python in a Cython module?

0

Issue

I’d like to make the integer constants (ACTIVE_TAG, etc) defined here:

//island management, m_activationState1
#define ACTIVE_TAG 1
#define ISLAND_SLEEPING 2
#define WANTS_DEACTIVATION 3
#define DISABLE_DEACTIVATION 4
#define DISABLE_SIMULATION 5

available as normal attributes of a Cython defined module I’m working on, so that Python application code can access them (to pass them in to wrapped APIs which are defined in terms of them).

I’ve looked at defining these with cdef as integers or enums, but neither of these approaches actually binds the value to an an attribute in the Cython module. What other options are there?

Solution

Here’s one way, which, while seemingly tedious, could feasibly be automated for any given .h file as input:

Step 1. Get all the constants you want into a file, say bulletdefs.h, which has the #defines but with leading underscores, e.g:

#define _ACTIVE_TAG 1
#define _ISLAND_SLEEPING 2
#define _WANTS_DEACTIVATION 3
#define _DISABLE_DEACTIVATION 4
#define _DISABLE_SIMULATION 5

Step 2. Insert a section into the pyx file for your module, say bullet.pyx:

cdef extern from "bulletdefs.h":
    cdef int _ACTIVE_TAG
    cdef int _ISLAND_SLEEPING
    cdef int _WANTS_DEACTIVATION
    cdef int _DISABLE_DEACTIVATION
    cdef int _DISABLE_SIMULATION

ACTIVE_TAG = _ACTIVE_TAG
ISLAND_SLEEPING = _ISLAND_SLEEPING
WANTS_DEACTIVATION = _WANTS_DEACTIVATION
DISABLE_DEACTIVATION = _DISABLE_DEACTIVATION
DISABLE_SIMULATION = _DISABLE_SIMULATION

Then, when you compile your module, you should get the intended effect:

Python 2.7.1+ (r271:86832, Apr 11 2011, 18:05:24) 
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import bullet
>>> bullet.ACTIVE_TAG
1
>>> bullet.DISABLE_SIMULATION
5
>>>

Answered By – Vinay Sajip

This Answer collected from stackoverflow, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0

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