Python: unsigned 32 bit bitwise arithmetic

Question

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

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
– Matthew Scharley, Commented Oct 16, 2008 at 23:40
As of Python 3.3, some users encountering this might be interested in the ipaddress module. — gerrit
– gerrit, Commented Feb 5, 2014 at 9:41

Dzinx · Accepted Answer · 2008-10-17 00:27:26Z

64

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.

answered Oct 17, 2008 at 0:27

Dzinx

58.2k10 gold badges63 silver badges78 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

John Millikin · Accepted Answer · 2008-10-17 00:28:43Z

54

You can mask everything by 0xFFFFFFFF:

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

answered Oct 17, 2008 at 0:28

John Millikin

202k41 gold badges216 silver badges228 bronze badges

5 Comments

bstpierre Over a year ago

This strategy also helps with bash arithmetic.

HardlyKnowEm Over a year ago

You're doing one more operation than necessary. ANDing a NOT is the same as an XOR. So, m ^ allf is what you need.

Ned Batchelder Over a year ago

@mlefavor I don't know where you got the idea that ~a & b is the same as a ^ b. It's not in general.

HardlyKnowEm Over a year ago

I guess that was overexuberant of me. But I think it's true in the case of 0xFFFFFFFF.

OozeMeister Over a year ago

See this wiki for the reason for needing the mask: wiki.python.org/moin/BitwiseOperators In a nutshell, negative numbers are treated as if they have an infinite number of leading 1s. The bitmask shaves off all but the number of digits that you want.

pixelbeat · Accepted Answer · 2008-10-17 00:04:03Z

13

from numpy import uint32

answered Oct 17, 2008 at 0:04

pixelbeat

32k9 gold badges55 silver badges62 bronze badges

1 Comment

penguin359 Over a year ago

@gerrit Is he referring to the fact that it requires the system to have the numpy module installed whereas ctypes is a part of core python?

Maarten · Accepted Answer · 2016-01-22 16:10:58Z

6

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

>>> 0xFFFFFF00 ^ 0xFFFFFFFF
255

answered Jan 22, 2016 at 16:10

Maarten

4,7895 gold badges33 silver badges37 bronze badges

Comments

tzot · Accepted Answer · 2008-10-17 07:45:07Z

1

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.

answered Oct 17, 2008 at 7:45

tzot

96.6k30 gold badges151 silver badges210 bronze badges

Collectives™ on Stack Overflow

Python: unsigned 32 bit bitwise arithmetic

5 Answers 5

Comments

5 Comments

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

5 Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related