Bitwise operations in Python

Question

I'm looking for recommendations on how to do bitwise math in python.

The main problem I have is that python's bitwise operators have infinite precision, which means that -1 is really "111.......111". That's not what I want. I want to emulate real hardware which will have some fixed precision, say 32 bits.

Here are some gotchas:

1) -n should return a 32 bit 2's complement number ( this is easily achieved by taking the lower 32 bits of the infinite precision -n )

2) n >> 3, should be an arithmetic shift of a 32 bit number, which means if bit 31 is '1', then bits 31:28 should be '1' after the shift by 3.

You could use numpy, it has built in int32 types. docs.scipy.org/doc/numpy-1.5.x/reference/arrays.scalars.html — GWW
– GWW, Commented May 13, 2011 at 15:41
GWW: I was just about to post this as an answer. Maybe you'd like to make it an answer yourself? — Sven Marnach
– Sven Marnach, Commented May 13, 2011 at 15:43
@Sven Marnach: Sure I can post it thanks, I wasn't sure if he wanted to use external libraries or not. — GWW
– GWW, Commented May 13, 2011 at 15:45
Thanks for the suggestion. Ideally I would want to have arbitrary sized integers though, not necessarily limited to 32, 64 etc. The hardware might very well have < 32 bit integers if 32 bit precision is not necessary. — Himadri Choudhury
– Himadri Choudhury, Commented May 13, 2011 at 15:45
@DasBoot: When you say you want a 2's complement number that means you want to use negative integers as well correct? — GWW
– GWW, Commented May 13, 2011 at 15:49

GWW · Accepted Answer · 2011-05-13 15:44:47Z

6

You could use numpy, it has built in int32 types and much more.

answered May 13, 2011 at 15:44

GWW

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Comments

kennytm · Accepted Answer · 2011-05-13 15:58:55Z

3

You could always add a & ((1<<32) - 1) mask to limit the number to 32-bits before performing any operation, e.g.

class Int32(int):
    def __neg__(self):
        return Int32(int.__neg__(self) & ((1 << 32) - 1))
    def __rshift__(self, other):
        if self & (-1 << 31):
             retval = int.__rshift__(int.__sub__(self, 1<<32), other)
             return Int32(retval & ((1 << 32) - 1))
        else:
             return Int32(int.__rshift__(self, other))
    ...

>>> -Int32(5)
4294967291
>>> (-Int32(5)) >> 1
4294967293

edited May 13, 2011 at 15:58

answered May 13, 2011 at 15:46

kennytm

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3 Comments

Himadri Choudhury Over a year ago

This doesn't quite work for the >> operator though right? It will shift 0s instead of 1s in some cases.

Himadri Choudhury Over a year ago

Ok. Nice. Anyway I can template this class? Have something like Int<N>?

kindall Over a year ago

You can't really template classes in Python, but you can write the class so that it takes the desired word size at instantiation and behaves accordingly.

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