math - Python: unsigned 32 bit bitwise arithmetic

33
votes

Trying to answer to another post whose solution deals with IP addresses and netmasks, I got stuck with plain bitwise arithmetic.

Is there a standard way, in Python, to carry on bitwise AND, OR, XOR, NOT operations assuming that the inputs are "32 bit" (maybe negative) integers or longs, and that the result must be a long in the range [0, 2**32]?

In other words, I need a working Python counterpart to the C bitwise operations between unsigned longs.

EDIT: the specific issue is this:

>>> m = 0xFFFFFF00   # netmask 255.255.255.0
>>> ~m
-4294967041L         # wtf?! I want 255
5
pythonmath
I imagine Pythons loose typing would play hell with any algorithm like this... I want to see one too, just out of curiousity. - Matthew Scharley
As of Python 3.3, some users encountering this might be interested in the ipaddress module. - gerrit

5 Answers

45
votes

You can mask everything by 0xFFFFFFFF:

>>> m = 0xFFFFFF00
>>> allf = 0xFFFFFFFF
>>> ~m & allf
255L
56
votes

You can use ctypes and its c_uint32:

>>> import ctypes
>>> m = 0xFFFFFF00
>>> ctypes.c_uint32(~m).value
255L

So what I did here was casting ~m to a C 32-bit unsigned integer and retrieving its value back in Python format.

11
votes
from numpy import uint32
2
votes

You could also xor with 0xFFFFFFFF, which is equivalent to the "unsigned complement".

>>> 0xFFFFFF00 ^ 0xFFFFFFFF
255
1
votes

This is a module that I created a long time ago, and it might be of help to you:

IPv4Utils

It provides at least a CIDR class with subnet arithmetic. Check the test cases at the end of the module for examples.