trouble accessing a struct pointer from another object

Question

foo.h

typedef int bar;
typedef struct _foo
{
    bar* b;
} foo;
extern foo* foo_new();
extern bar* foo_bar_new(foo* f);

foo.pxd

cdef extern from "foo.h":
    ctypedef int bar;
    ctypedef struct foo:
        bar* b
    foo* foo_new();
    bar* foo_bar_new(foo* f);

foo.pyx

from foo cimport *

cdef class Foo:
    cdef foo* _ptr
    def __cinit__(self):
        self._ptr = foo_new()

cdef class Bar:
    cdef bar* _ptr
    def __cinit__(self, f):
        self._ptr = foo_bar_new(f._ptr)   # error
                                ^
foo.pyx:11:33: Cannot convert Python object to 'foo *'

cython foo.pyx throws an error at the marked line.

I'm not quite sure what I'm doing wrong. Changing _ptr in Foo to cpdef results in the same error.

== Update == I changed foo.pyx to the following and it works.

from foo cimport *

cdef class Foo:
    cdef foo* _ptr
    def __cinit__(self):
        self._ptr = foo_new()

    def new_bar(self):
        cdef bar* b
        b = foo_bar_new(self._ptr)
        tmp = Bar()
        tmp._set(b)
        return tmp

cdef class Bar:
    cdef bar* _ptr
    cdef _set(self, bar* bptr):
        self._ptr = bptr

    def get(self):
        return bar_get(self._ptr)

I can do f=Foo(); b=f.new_bar() but this doesn't seem ideal to me since I can't create with Bar(f) or similar. The problem is that I can't cdef __cinit__() since it's a special method. Can't cdef __init__() either. Any ideas on how to solve this problem?

UPDATE: I managed to get it to compile but it's not ideal. Updates in the original. — Trevor
– Trevor, Commented Dec 20, 2017 at 15:52
Possible duplicate of Initializing Cython objects with existing C Objects — danny
– danny, Commented Dec 21, 2017 at 14:22
In short, use factory functions that take a foo* argument and return a new Foo after assigning Foo._ptr to the passed in foo*. This is needed as __init__ can only accept python objects, not C types. — danny
– danny, Commented Dec 21, 2017 at 14:25
@danny I agree with you about factory functions being an option. However, my reading of the initial question is that OP was trying to pass Cython objects rather than pointers so I don't think your duplicate suggestion quite fits. It fits a bit better with the edit, but I think that's a workround more than what they're trying to do. — DavidW
– DavidW, Commented Dec 21, 2017 at 17:29
The OP specifically asks how to create Cython object from a pointer, therefore is a duplicate of existing answer. — danny
– danny, Commented Dec 27, 2017 at 10:21

DavidW · Accepted Answer · 2017-12-20 17:39:06Z

1

You need to ensure it knows what type f is, at compile time:

def __cinit__(self, Foo f):
    self._ptr = foo_bar_new(f._ptr)   # error

without doing that it has to assume that _ptr is a standard Python attribute lookup (done at runtime), and so will be a standard Python object. If you specify that f is a Foo then it's able to use the known definition of _ptr.

answered Dec 20, 2017 at 17:39

DavidW

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3 Comments

danny Over a year ago

The above will allow Bar(Foo()) to be used but not Bar(f) where f is of type foo*.

DavidW Over a year ago

Since __cinit__ is a def function in can only be called with Python types anyway. There isn't a particularly good method for constructing Cython objects directly from c types.

danny Over a year ago

'Particularly good' is relevant, though there is a method for doing that which have talked about before.

Trevor · Accepted Answer · 2017-12-29 22:20:23Z

0

I've been using a modification of the factory function model that seems to work ok.

cdef class Bar:
    cdef bar* _ptr
    def __cinit__(self):
        self._ptr = NULL

    cdef from_ptr(self, bar* ptr):
        self._ptr = ptr
        return self

cdef class Foo:
    cdef foo* _ptr
    def new_bar(self):
        cdef bar* b
        b = foo_bar_new(self._ptr)
        return Bar().from_ptr(b)

answered Dec 29, 2017 at 22:20

Trevor

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