Handling default parameters in cython

Question

I am wrapping some c++ code using cython, and I am not sure what is the best best way to deal with parameters with default values.

In my c++ code I have function for which the parameters have default values. I would like to wrap these in such a way that these default values get used if the parameters are not given. Is there a way to do this?

At this point the only way that I can see to provide option parameters is to define them as part of the python code (in the def func satement in pycode.pyx below), but then I have defaults defined more than once which I don't want.

cppcode.h:

int init(const char *address=0, int port=0, int en_msg=false, int error=0);

pycode_c.pxd:

cdef extern from "cppcode.h":
int func(char *address, int port, int en_msg, int error)

pycode.pyx:

cimport pycode_c
def func(address, port, en_msg, error):
    return pycode_c.func(address, port, en_msg, error)

Answer 1

You could declare the function with different parameters ("cppcode.pxd"):

cdef extern from "cppcode.hpp":
     int init(char *address, int port, bint en_msg, int error)
     int init(char *address, int port, bint en_msg)
     int init(char *address, int port)
     int init(char *address)
     int init()

Where "cppcode.hpp":

int init(const char *address=0, int port=0, bool en_msg=false, int error=0);

It could be used in Cython code ("pycode.pyx"):

cimport cppcode

def init(address=None,port=None,en_msg=None,error=None):
    if error is not None:
        return cppcode.init(address, port, en_msg, error)
    elif en_msg is not None:
         return cppcode.init(address, port, en_msg)
    elif port is not None:
         return cppcode.init(address, port)
    elif address is not None:
         return cppcode.init(address)
    return cppcode.init()

And to try it in Python ("test_pycode.py"):

import pycode

pycode.init("address")

Output

address 0 false 0

Cython also has arg=* syntax (in *.pxd files) for optional parameters:

cdef foo(x=*)