[Cython] Fwd: [cython-users] can pointers be stored in Python's dict or list?

[Robert Bradshaw]

On Fri, Sep 14, 2012 at 2:29 PM, Dag Sverre Seljebotn
<d.s.seljebotn at astro.uio.no> wrote:
> Isn't there a case for converting to/from ctypes pointers rather than
> capsules? And if capsules, what would the secret word be? Hmm...

+1, ctypes would be even better. It's not in the standard library
'till 2.5, but I don't think that's a huge blocker as it can be
installed earlier.



> Robert Bradshaw <robertwb at gmail.com> wrote:
>>
>> Given the CObject vs Capsule differences in Py2 vs Py3, and the
>> clumsiness in using them (see below), any thoughts on automatically
>> converting void* to/from a Python object? There is the sticky issue of
>> pointer lifetime management, but we could assume the lifetime is
>> managed entirely in C in most cases.
>>
>> Note that <void*>object already has a specific meaning that we can't
>> hijack, but perhaps non-void* would be fine to do implicitly? Also, if
>> we built this into the language, could we provide type safety via the
>> name/description attribute? Something like this could be really handy
>> to have and isn't as easily done as a library (especially given the
>> 2/3 differences).
>>
>> - Robert
>>
>>
>> ---------- Forwarded message ----------
>> From: Robert Bradshaw <robertwb at gmail.com>
>> Date: Fri, Sep 14, 2012 at 1:11 PM
>> Subject: Re: [cython-users] can pointers be stored in Python's dict or
>> list?
>> To: Mark Summerfield <list at qtrac.plus.com>
>> Cc: cython-users at googlegroups.com
>>
>>
>> On Fri, Sep 14, 2012 at 12:54 PM, Mark Summerfield <list at qtrac.plus.com>
>> wrote:
>> > Hi Robert,
>> >
>> > On Fri, 14 Sep 2012 10:45:06 -0700
>> > Robert Bradshaw <robertwb at gmail.com> wrote:
>> >> On Fri, Sep 14, 2012 at 8:11 AM, Mark Summerfield <list at qtrac.plus.com>
>> >> wrote:
>> >> > Hi,
>> >> >
>> >> > (1)
>> >> >
>> >> > I am using Cython to provide a Python wrapper to a C library.
>> >> > The C library has a function which gives a handle (a pointer) which
>> >> > is
>> >> > then used to call other functions.
>> >> >
>> >> > I want to be able to store a dictionary of name-pointer pairs so that
>> >> > I
>> >> > can resuse handles.
>> >>
>> >
>> >> > This doesn't seem to work with dict or list because of the
>> >> >     Cannot convert 'Thing*' to Python object
>> >> > error.
>> >> >
>> >> > Right now I have a static array and a dictionary:
>> >> >
>> >> > cdef Thing* pointers[30]
>> >> > indexForThingName = {}
>> >> >
>> >> > but I'd really like to be able to store an arbitrary number of
>> >> > pointers, ideally in a dict.
>> >> >
>> >> > Is this possible?
>> >>
>> >> Sounds like you want
>> >> http://docs.python.org/release/3.1.5/c-api/capsule.html
>> >
>> > But that means I have to drop down to C. I want to stay in Python and
>> > Cython.
>>
>> You're already "in C" if you're dealing with pointers.
>>
>> > I was hoping that Cython offered some kind of
>> wrapper type,
>> > something that at the Python level would look like:
>> >
>> >     class Pointer(object):
>> >         def __init__(self, void *pointer):
>> >             cdef void *self.pointer = pointer # Doesn't work
>> >
>> > that could be put in a dict or list. Then I could do things like this:
>> >
>> >     Pointer p = <void*>c_func_that_returns_a_pointer()
>> >     mylist.append(p)
>> >     ...
>> >     Foo *q = <Foo*>mylist.pop()
>>
>> How about (untested)
>>
>>     from cpython.cobject cimport PyCObject_FromVoidPtr,
>> PyCObject_AsVoidPtr
>>
>>     cdef void* p = <void*>c_func_that_returns_a_pointer()
>>     mylist.append(PyCObject_FromVoidPtr(p, NULL)
>>     ...
>>     Foo *q = <Foo*>PyCObject_AsVoidPtr(mylist.pop())
>>
>> This should work for Python 2, use capsules (at the provided link) for
>> Python 3+. For a portable solution, you could cast the void* to a
>> intptr_t (or long long, if that's sufficient) which converts back and
>> forth from Python just fine (but is more hackish).
>>
>> > Oh well, I'll see if I can use libcpp's map...
>> >
>> >> > (2)
>> >> >
>> >> > Also, I tried doing
>> >> >
>> >> >     from libc.stdio cimport printf
>> >> >
>> >> > and then in my code:
>> >> >
>> >> >     printf("%p", pointer)
>> >> >
>> >> > but although it compiled without error, nothing was printed.
>> >>
>> >> Did it perhaps not flush? You also might want to try adding a newline.
>> >
>> > I changed it to printf("%p\n", pointer) and now I get
>> >     Cannot convert 'void *' to Python object
>>
>> Are you still cimporting printf?
>>
>> - Robert
>> ________________________________
>>
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