PYTHON setup.py build_ext --inplace
PYTHON -c "import foo"
PYTHON -c "import a"

######## setup.py ########

from Cython.Build import cythonize
from distutils.core import setup

setup(
  ext_modules = cythonize("*.pyx"),
)

######## foo.pxd ########

cdef int bar() except *

######## foo.pyx ########

cdef extern from "bar_impl.c":
    int bar() except *

######## bar_impl.c ########

static int bar() { return -1; }

######## a.pyx ########

cimport cython
from foo cimport bar

assert bar() == -1