Cython - copy constructors

cython

Solution

When playing with `cdef` classes, attribute access are compiled to C struct member access. As a consequence, to access to a `cdef` member of an object `A` you have to be sure of the type of `A`. In `__cinit__` you didn't tell Cython that other is an instance of `StructName`. Therefore Cython refuses to compile `other.__structname`. To fix the problem, just write

def __cinit__(self, StructName other = None):

Note: `None` is equivalent to `NULL` and therefore is accepted as a `StructName`.

If you want more polymorphism then you have to rely on type casts:

 def __cinit__(self, other = None):
     cdef StructName ostr
     if not other:
         self.__structname = c_libname.constructStructName()
     elif type(other) is StructName:
         ostr = <StructName> other
         self.__structname = ostr.__structname

Problem

I've got a C library that I'm trying to wrap in Cython. One of the classes I'm creating contains a pointer to a C structure. I'd like to write a copy constructor that would create a second Python object pointing to the same C structure, but I'm having trouble, as the pointer cannot be converted into a python object. Here's a sketch of what I'd like to have: ``` cdef class StructName: cdef c_libname.StructName* __structname def __cinit__(self, other = None): if not other: self.__structname = c_libname.constructStructName() elif type(other) is StructName: self.__structname = other.__structname ``` The real problem is that last line - it seems Cython can't access cdef fields from within a python method. I've tried writing an accessor method, with the same result. How can I create a copy constructor in this situation?

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