[Cython] How to define C-consts in python module scope

mark florisson markflorisson88 at gmail.com
Wed Jul 20 20:32:08 CEST 2011

On 20 July 2011 20:04, Lisandro Dalcin <dalcinl at gmail.com> wrote:
> On 20 July 2011 13:51, mark florisson <markflorisson88 at gmail.com> wrote:
>> On 20 July 2011 18:06, Lisandro Dalcin <dalcinl at gmail.com> wrote:
>>> On 19 July 2011 20:48, Robert Bradshaw <robertwb at math.washington.edu> wrote:
>>>> On Tue, Jul 19, 2011 at 3:02 PM, Lisandro Dalcin <dalcinl at gmail.com> wrote:
>>>>> On 19 July 2011 02:24, Vitja Makarov <vitja.makarov at gmail.com> wrote:
>>>>>> 2011/7/18 Robert Bradshaw <robertwb at math.washington.edu>:
>>>>>>> Trevor King and I discussed this quite a while back, but every time I
>>>>>>> got around to looking at his code (I don't think he ever created a
>>>>>>> formal pull request) something came up. The idea was that we could
>>>>>>> support cpdef structs and extern functions as well.
>>>>>> That's interesting, I think I shouldn't hurry with my pull request.
>>>>>> 2011/7/19 Robert Bradshaw <robertwb at math.washington.edu>:
>>>>>>> On Mon, Jul 18, 2011 at 4:34 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>>>>>>>> My suggestion is
>>>>>>>>  cdef exposed enum:
>>>>>>>>    ...
>>>>>>> I agree, public is an overloaded word. This meaning is analogous to
>>>>>>> its use for cdef class members. Perhaps we should use "api" for api
>>>>>>> generation, and public used for Python-level access, with cpdef being
>>>>>>> the preferred form for declaring Python-accessible types.
>>>>>> It seems to me that cpdef is more cythonic but exposed could be used
>>>>>> in this case:
>>>>>> cdef extern from "ev.h":
>>>>>>    exposed enum:
>>>>>>        EV_READ
>>>>>>        EV_WRITE
>>>>> And what about this?
>>>>> cdef extern from "ev.h":
>>>>>   cpdef enum:
>>>>>       EV_READ
>>>>>       EV_WRITE
>>>> Yep, exactly.
>>>>> BTW, how is this supposed to work with *.pxd files? I think the values
>>>>> will be exposed just in the matching implementation .pyx file, and not
>>>>> with cimport, right?
>>>> It would be an error unless there's an corresponding .pyx file (which
>>>> could be empty). The idea is that one could also define extern cpdef
>>>> functions and structs and wrappers would be provided.
>>> It would be an error? What do you mean? if you cpdef enum in foo.pxd,
>>> and have foo.pyx, then the enumerations should be exposed in the 'foo'
>>> module. But then if you "cimport foo" in bar.pyx, that should succeed
>>> but the enumerations should not be exposed in the "bar" module... Am I
>>> missing something?
>> I believe Robert confirmed what you said and added to that that it
>> would be an error to have a cpdef extern enum in a .pxd without a
>> corresponding .pyx file.
> But what an "error" means? Cython is going to fail compilation?
> Suppose I develop a package, and at setup.py install time my package
> installs somewhere a .pxd full of definitions in such a way that
> third-party code can cimport it. Then, while writing the third party
> code, you cimport the my package .pxd, and obviously there is no .pyx
> for my package, as you are writing YOUR code. What's going to happen
> in this case?

If you want to use cpdef in your .pxd, it means you want to expose
those to Python. This means they must be importable from Python, which
means you need an extension module that exposes the constants, which
means you need a .pyx. So you ship a (possibly empty) .pyx file along
with your .pxd. If you don't want to expose them to Python but only to
Cython, don't use cpdef. This compile time error would be issued
whenever a user does a cimport of a .pxd file that has a cpdef enum
with no corresponding .pyx file, i.e. the pxd is unusable, you
wouldn't ship it in the first place.

> --
> Lisandro Dalcin
> ---------------
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