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Note: a much easier way of going about this is at http://wiki.cython.org/tutorials/numpy .
Let's show it on an example, that takes Python numpy array of floats, converts it to C double array, sums the elements and modifies it in place and returns everything back to Python.
cdef extern from "numpy/arrayobject.h": ctypedef int intp ctypedef extern class numpy.ndarray [object PyArrayObject]: cdef char *data cdef int nd cdef intp *dimensions cdef intp *strides cdef int flags def mysum(ndarray a): cdef double *p = <double *>a.data cdef int dim = a.dimensions cdef double s=0 for i from 0 <= i < dim: s += p[i] p = 13. return s
cython matrix.pyx gcc -O3 -I/usr/include/python2.4/ -I/usr/lib/python2.4/site-packages/numpy/core/include/numpy/ -c -o matrix.o matrix.c gcc -shared matrix.o -o matrix.so
$ cat t.py from numpy import array from matrix import mysum a = array([1., 2., 3., -8]) print a print mysum(a) print a $ python t.py [ 1. 2. 3. -8.] -2.0 [ 13. 2. 3. -8.]
One can also cimport the numpy types from the c_numpy.pxd file (for example in Debian it is located at /usr/share/doc/python-numpy/pyrex/c_numpy.pxd).
This would be very nice to grow up into an example of manipulating an array of extension types.
Other examples are in the numpy upstream tarball in the directory:
and the soon to be deprecated:
You can also browse the sources of Sage, which is a production usage of Cython.