“Cannot convert Python object argument to type ''” - error while wrapping c++-classes with Cython

Question

I\'m trying to run a cython example which is a bit more complex than the one which can be found in many tutorials (e.g. this guide ).

Here are a minimal example (please d

Answer 1

You should use PyRectangle in the signatures of def-functions and PyRectangle.c_rect when passing rectangles to C++-functions.

That means your code should be:

cdef class PyGroup2:
    ...
    def __cinit__(self, PyRectangle rect0, PyRectangle rect1):
        self.c_group2 = Group2(rect0.c_rect, rect1.c_rect)

Read on for a more detailed explanation why.

---

All arguments passed to def-functions are Python-objects (i.e. of type object in Cython-parlance), after all those functions will be called from pure Python, which only knows Python-objects.

However, you can add some syntactic sugar and use "late-binding" in the signature of a def-function, for example, instead of

def do_something(n):
  ...

use

def do_something(int n):
  ...

Under the hood, Cython will transform this code to something like:

def do_something(n_):
   cdef int n = n_ # conversion to C-int
   ...

This automatic conversion is possible for builtin-types like int or double, because there is functionality in Python-C-API for these conversions (i.e. PyLong_AsLong, PyFloat_AsDouble). Cython also handles the error checking, so you should not undertake these conversion manually.

However, for user-defined types/classes like your Rectangle-class such automatic conversion is not possible - Cython can only automatically convert to cdef-classes/extensions, i.e. PyRectangle, thus PyRectangle should be used in the signature:

cdef class PyGroup2:
    ...
    def __cinit__(self, PyRectangle rect0, PyRectangle rect1):
        ...

After Cython took care of conversion from object to PyRectangle, the last step from PyRectangle to Rectangle must be taken manually by utilizing the c_rect - pointer:

...
def __cinit__(self, PyRectangle rect0, PyRectangle rect1):
    self.c_group2 = Group2(rect0.c_rect, rect1.c_rect)

---

The rules are similar for cpdef-function, because they can be called from pure Python. The "early binding" works only for types which Cython can automatically coverted from/to Python objects.

Unsurprisingly, the only function which can habe C++-classes in their signatures are the cdef-functions.