How to get the signed integer value of a long in python?

Question

If lv stores a long value, and the machine is 32 bits, the following code:

iv = int(lv & 0xffffffff)

results an iv of type long, inst

Answer 1

Essentially, the problem is to sign extend from 32 bits to... an infinite number of bits, because Python has arbitrarily large integers. Normally, sign extension is done automatically by CPU instructions when casting, so it's interesting that this is harder in Python than it would be in, say, C.

By playing around, I found something similar to BreizhGatch's function, but that doesn't require a conditional statement. n & 0x80000000 extracts the 32-bit sign bit; then, the - keeps the same 32-bit representation but sign-extends it; finally, the extended sign bits are set on n.

def toSigned32(n):
    n = n & 0xffffffff
    return n | (-(n & 0x80000000))

Bit Twiddling Hacks suggests another solution that perhaps works more generally. n ^ 0x80000000 flips the 32-bit sign bit; then - 0x80000000 will sign-extend the opposite bit. Another way to think about it is that initially, negative numbers are above positive numbers (separated by 0x80000000); the ^ swaps their positions; then the - shifts negative numbers to below 0.

def toSigned32(n):
    n = n & 0xffffffff
    return (n ^ 0x80000000) - 0x80000000