[Cython] Why does "__cinit__" insists on converting its arguments to Python objects?

[Robert Bradshaw]

Both __init__ and __cinit__ are passed the same arguments, and the
former has Python calling conventions. (Also, we use Python's
framework to allocate and construct the new object, so there's not a
huge amount of flexibility here and working around this would be quite
non-trivial).

On Thu, Jun 7, 2012 at 4:32 AM, Dieter Maurer <dieter at handshake.de> wrote:
> The following cython source leads to a "Cannot convert 'pointer' to Python object".
>
> ctypedef void * pointer
>
> cdef extern from "nonexistant.h":
>  cdef pointer to_pointer(object)
>
> cdef class C:
>  cdef pointer p
>
>  def __cinit__(self, pointer p): self.p = p
>
> c = C(to_pointer(None))
>
> Why does the constructor call tries an implicit conversion to a
> Python object even though it gets precisely the type indicated by
> its signature?
>
>
> I am working on a binding for "libxmlsec". The behaviour above leads
> to an unnatural mapping. Two examples:
>
> 1. "libxmlsec" has the concept of a key (used for digital signatures or
>   encryption), naturally mapped onto a "cdef class Key" encapsulating
>   the xmlsec key pointer.
>
>   "libxmlsec" provides many functions to create keys - naturally mapped
>   onto class methods used as alternative constructors.
>   Would "Cython" allow C level parameters for "__cinit__",
>   they could look like:
>
>     cdef xmlSecKeyPtr xkey = ... some "libxmlsec" key generating function ...
>     return Key(xkey)
>
>   With the restriction, this must look like:
>
>     cdef Key key
>     key.xkey = ... some "libxmlsec" key generating function ...
>     return key
>
>   Not yet too bad, unless the constructor requires C level arguments.
>
> 2. "libxmlsec" provides a whole bunch of transforms, handled in C code
>   via a set of so called "TransformId"s. Each "TransformId" is
>   generated by a function.
>
>   The natural way would like:
>
>      cdef class TransformId:
>        cdef xmlSecTransformId tid
>        def __cinit__(self, xmlSecTransformId tid): self.tid = tid
>
>      TransformInclC14N = TransformId(xmlSecTransformInclC14NGetKlass())
>      ... for all standard transforms ...
>
>   The restriction forces the introduction of a helper function:
>
>      cdef class TransformId:
>        cdef xmlSecTransformId tid
>
>      cdef _mkti(xmlSecTransformId tid):
>        cdef TransformId t = TransformId()
>        t.tid = tid
>        return t
>
>      TransformInclC14N = _mkti(xmlSecTransformInclC14NGetKlass())
>      ... for all standard transforms ...
>
>
>
>
>
> --
> Dieter
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