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

[mark florisson]

On 20 July 2011 21:44, Lisandro Dalcin <dalcinl at gmail.com> wrote:
> On 20 July 2011 16:27, mark florisson <markflorisson88 at gmail.com> wrote:
>> 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?
>>
>
> This is like asking to provide a C source in order to being able to
> #include "someheader.h" in other C sources. End users do not need
> implementation source code, they need declarations,interface,header
> files.
>

I personally don't really mind, but what about not allowing cpdef in
.pxds for enums, but only in .pyx files:

from mypxd cimport my_extern_enum
cpdef enum my_extern_enum

Of course, the usual

cpdef enum my_enum:
   ...

would still be allowed in .pyx files.

> --
> Lisandro Dalcin
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