Using MethodType to convert builtin function to instancemethod.

bad.py

from demo import incx

class Point(object):
    """A 1-diml Point"""
    def __init__(self, x):
        self.x = x
    incx = incx

p = Point(0)
p.incx()

print p.x

demo.pyx

# Compile this using:
#   python setup.py build_ext --inplace
def incx(self):
    """Increment x."""
    self.x += 1

good.py

from demo import incx

class Point(object):
    """A 1-diml Point"""
    def __init__(self, x):
        self.x = x

from types import MethodType
Point.incx = MethodType(incx, None, Point)

p = Point(0)
p.incx()

print p.x

setup.py

# python setup.py build_ext --inplace
from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

setup(
    cmdclass = {'build_ext': build_ext},
    ext_modules = [Extension("demo", ["demo.pyx"])]
)