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

mark florisson markflorisson88 at gmail.com
Wed Jul 20 21:27:17 CEST 2011

On 20 July 2011 21:13, Lisandro Dalcin <dalcinl at gmail.com> wrote:
> On 20 July 2011 15:32, mark florisson <markflorisson88 at gmail.com> wrote:
>> 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.
> So supose I want to write a small wrapper for dlopen(), then I do:
> # dl.pxd
> cdef from extern from "dlfcn.h":
>    cpdef enum: RTLD_GLOBAL
>    void * dlopen()(const char *filename, int flag);
> # dl.pyx
> def open(filename, mode):
>    cdef void *handle = c_dlopen(fiename,handle)
>    return <long>handle
> Then the "dl" module namespace contains "dlopen" and "RTLD_GLOBAL"
> Now, suppose I code my setup.py to build the ext module and install
> dl.pxd as package data.
> Now a user runs "pip install dl" and installs my package. She wants to
> reuse my .pxd (installed somewhere) for calling dlopen() in its own
> Cython code:
> # user.pyx
> from dl cimport RTLD_GLOBAL, c_dlopen
> dlopen("somelibrary.so", RTLD_GLOBAL)
> So the user code is going to fail? Then I cannot take advantage of
> cpdef enum for developing my  "dl" module, I have to go back to
> manually exposing the enumerations

Why can't you ship both dl.pxd and dl.pyx as package data?

> --
> Lisandro Dalcin
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