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)

jfs · Accepted Answer · 2011-02-23 11:39:34Z

18

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=*)

edited Feb 23, 2011 at 11:39

answered Feb 23, 2011 at 11:27

jfs

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5 Comments

Andrew Aylett Over a year ago

The problem with this approach is that while C++ restricts you to omitting the parameters at end of the list, Python allows you to specify any subset of parameters you like, using keywords. This means that in your pyx code, just because error is not None does not mean that address, port and en_msg have reasonable values.

amicitas Over a year ago

I supposed you could define the python function using a variable length argument list, def init(*args) and then go from there. Then the keyword order is no longer a problem. This all seems a bit ugly though. Cython apparently has some kind of support for variable args, but I don't really understand the example in the FAQ: Variable Args.

jfs Over a year ago

@Andrew Aylett: Correct. It is upto you how strict you'd like to be: add explicit checks or just mention the call convention in the docstring. For example you could compare with the UNSET object and get TypeError for invalid function calls gist.github.com/6a01dc02e35fb04cf97d#file_pycode.pyx

jfs Over a year ago

@amicitas: va_list is for C functions that accept variable args; note cdef not def in the example.

Eney Over a year ago

This answer is over a decade old, is there a better option with newer cython versions?

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