A numerical agnostic pyrex class

Date:2006-11-04 (last modified), 2006-03-29 (created)

NOTE: This entry was last updated 2006-11-04 and contains information that is not relevant today (as of 2013).


Here is presented !NumInd (Numerical Independent Extension), an example of a class written in Pyrex. This class can wrap information from any object coming from Numeric, numarray or !NumPy without any dependency of those packages for compiling the extension. It lets you to create a uniform interface in both Python and C spaces. In addition, you can personalize it by adding new methods or properties.

For this extension to work, you need a numerical package that supports the array interface. Any of these versions would be good enough:

 NumPy (all versions) Numeric (>=24.2) * numarray (>=1.5.1)

NumInd: a Numerical Independent Pyrex-based extension

The !NumInd class shown below takes a Numeric/numarray/!NumPy object and creates another object that can be accessed in an uniform way from both Python and Pyrex (and hence, C) space. Moreover, it exposes an array interface so that you can re-wrap this object with any Numeric/numarray/!NumPy. All of these features are achieved without actually copying the data itself. This opens the door to the possibility to develop applications that supports the Numeric/numarray/!NumPy triad without a need to compile against any of them.

Warning: This class supports mainly homogeneous datasets, but it wouldn't be difficult to support recarrays as well. This is a work in-progress anyway.

In [ ]:
# This Pyrex extension class can take a numpy/numarray/Numeric object
# as a parameter and wrap it so that its information can be accessed
# in a standard way, both in Python space and C space.
# Heavily based on an idea of Andrew Straw. See
# http://www.scipy.org/Cookbook/ArrayStruct_and_Pyrex
# Very inspiring! :-)
# First version: 2006-03-25
# Last update: 2006-03-25
# Author: Francesc Altet 

import sys

cdef extern from "Python.h":
    ctypedef int Py_intptr_t
    long PyInt_AsLong(object)
    void Py_INCREF(object)
    void Py_DECREF(object)
    object PyCObject_FromVoidPtrAndDesc(void* cobj, void* desc,
                                        void (*destr)(void *, void *))

cdef extern from "stdlib.h":
    ctypedef long size_t
    ctypedef long intptr_t
    void *malloc(size_t size)
    void free(void* ptr)

# for PyArrayInterface:

# byteorder dictionary
byteorder = {'<':'little', '>':'big'}

ctypedef struct PyArrayInterface:
    int version          # contains the integer 2 as a sanity check
    int nd               # number of dimensions
    char typekind        # kind in array --- character code of typestr
    int itemsize         # size of each element
    int flags            # flags indicating how the data should be interpreted
    Py_intptr_t *shape   # A length-nd array of shape information
    Py_intptr_t *strides # A length-nd array of stride information
    void *data           # A pointer to the first element of the array

cdef void free_array_interface(void *ptr, void *arr):
    arrpy = <object>arr

cdef class NumInd:
    cdef void *data
    cdef int _nd
    cdef Py_intptr_t *_shape, *_strides
    cdef PyArrayInterface *inter
    cdef object _t_shape, _t_strides, _undarray

    def __init__(self, object undarray):
        cdef int i, stride
        cdef object array_shape, array_strides

        # Keep a reference to the underlying object
        self._undarray = undarray
        # Get the shape and strides C arrays
        array_shape = undarray.__array_shape__
        self._t_shape = array_shape
        # The number of dimensions
        self._nd = len(array_shape)
        # The shape
        self._shape = <Py_intptr_t *>malloc(self._nd*sizeof(Py_intptr_t))
        for i from 0 <= i < self._nd:
            self._shape[i] = self._t_shape[i]
        # The strides (compute them if needed)
        array_strides = undarray.__array_strides__
        self._t_strides = array_strides
        self._strides = <Py_intptr_t *>malloc(self._nd*sizeof(Py_intptr_t))
        if array_strides:
            for i from 0 <= i < self._nd:
                self._strides[i] = array_strides[i]
            # strides is None. Compute them explicitely.
            self._t_strides = [0] * self._nd
            stride = int(self.typestr[2:])
            for i from self._nd > i >= 0:
                self._strides[i] = stride
                self._t_strides[i] = stride
                stride = stride * array_shape[i]
            self._t_strides = tuple(self._t_strides)
        # Populate the C array interface
        self.inter = self._get_array_interface()

    # Properties. This are visible from Python space.
    # Add as many as you want.

    property undarray:  # Returns the underlying array
        def __get__(self):
            return self._undarray

    property shape:
        def __get__(self):
            return self._t_shape

    property strides:
        def __get__(self):
            return self._t_strides

    property typestr:
        def __get__(self):
            return self._undarray.__array_typestr__

    property readonly:
        def __get__(self):
            return self._undarray.__array_data__[1]

    property __array_struct__:
        "Allows other numerical packages to obtain a new object."
        def __get__(self):
            if hasattr(self._undarray, "__array_struct__"):
                return self._undarray.__array_struct__
                # No an underlying array with __array_struct__
                # Deliver an equivalent PyCObject.
                return PyCObject_FromVoidPtrAndDesc(<void*>self.inter,

    cdef PyArrayInterface *_get_array_interface(self):
        "Populates the array interface"
        cdef PyArrayInterface *inter
        cdef object undarray, data_address, typestr

        undarray = self._undarray
        typestr = self.typestr
        inter = <PyArrayInterface *>malloc(sizeof(PyArrayInterface))
        if inter is NULL:
            raise MemoryError()

        inter.version = 2
        inter.nd = self._nd
        inter.typekind = ord(typestr[1])
        inter.itemsize = int(typestr[2:])
        inter.flags = 0  # initialize flags
        if typestr[0] == '|':
            inter.flags = inter.flags | NOTSWAPPED
        elif byteorder[typestr[0]] == sys.byteorder:
            inter.flags = inter.flags | NOTSWAPPED
        if not self.readonly:
            inter.flags = inter.flags | WRITEABLE
        # XXX how to determine the ALIGNED flag?
        inter.strides = self._strides
        inter.shape = self._shape
        # Get the data address
        data_address = int(undarray.__array_data__[0], 16)
        inter.data = <void*>PyInt_AsLong(data_address)
        return inter

    # This is just an example on how to modify the data in C space
    # (and at C speed! :-)
    def modify(self):
        "Modify the values of the underlying array"
        cdef int *data, i

        data = <int *>self.inter.data
        # Modify just the first row
        for i from 0 <= i < self.inter.shape[self.inter.nd-1]:
            data[i] = data[i] + 1

    def __dealloc__(self):

An example of use

In order to get an idea of what the above extension offers, try to run this script against the !NumInd extension:

In [ ]:
import Numeric
import numarray
import numpy
import numind

# Create an arbitrary object for each package
nu.shape = (4,3)
na.shape = (4,3)
np.shape = (4,3)

# Wrap the different objects with the NumInd class
# and execute some actions on it
for obj in [nu, na, np]:
    ni = numind.NumInd(obj)
    print "original object type-->", type(ni.undarray)
    # Print some values
    print "typestr -->", ni.typestr
    print "shape -->", ni.shape
    print "strides -->", ni.strides
    npa = numpy.asarray(ni)
    print "object after a numpy re-wrapping -->", npa
    print "object after modification in C space -->", npa

You can check the output here test_output.txt

See also

  • ["Cookbook/Pyrex_and_NumPy"]
  • ["Cookbook/ArrayStruct_and_Pyrex"] (The inspiring recipe)

Section author: FrancescAltet