non-member operator in Cython

Question

I'm currently doing a Cython wrapper for an existing C++ library. I have an overloaded non-member operator in C++ like

Data operator+(Data const& a, Data const& b)

And in the pxd file describing the header, I wrote

cdef extern from 'blabla.h':
    Data operator+(const Data&, const Data&)

Now how can I use this operator+ in another pyx file?

`include "your_pxd.pxd"` in your .pyx doesn't work for that? — Cristián Antuña, Apr 20 '15 at 20:02
@CristiánAntuña well I have to rename this operator to something else and call it as a normal function. I want to use `+` instead — Hot.PxL, Apr 21 '15 at 01:57

DavidW · Accepted Answer · 2015-04-21T08:14:34.767

For very simple cases, like in your example you can lie to Cython and tell it that the operator is a member function:

cdef extern from 'blabla.h':
  cdef cppclass Data:
    # the rest of the data definitions
    Data operator+(const Data&)

It only uses this information to know that it can translate the code a+b (where a and b are data objects) to __pyx_v_a + __pyx_v_b and let the c++ compiler do the rest (which it knows how to because of the import from "blabla.h"). Therefore, the distinction between member and non-member is irrelevant.

However: one of the main reasons to use nonmember operators is to allow things like

Data operator+(int a, const Data& b);

You can make this work, but it's very slightly messy. In your pxd file do

cdef extern from 'blabla.h':
    Data operator+(int, const Data&) # i.e. do nothing special

In your pyx file

from my_pxd_file import * # works fine
## but this isn't accepted unfortunately:
from my_pxd_file import operator+

If you wanted to avoid too much namespace pollution from doing import *, you could probably create a pxd file that only contains operators and not the class definitions (I haven't tested this though)

In conclusion - two methods depending on how complicated your use-case is...

non-member operator in Cython

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