bit manipulation - Two's Complement in Python

Question

Is there a built in function in python which will convert a binary string, for example '111111111111', to the two's complement integer -1?

Answer 1

Two's complement subtracts off (1<<bits) if the highest bit is 1. Taking 8 bits for example, this gives a range of 127 to -128.

A function for two's complement of an int...

def twos_comp(val, bits):
    """compute the 2's complement of int value val"""
    if (val & (1 << (bits - 1))) != 0: # if sign bit is set e.g., 8bit: 128-255
        val = val - (1 << bits)        # compute negative value
    return val                         # return positive value as is

Going from a binary string is particularly easy...

binary_string = '1111' # or whatever... no '0b' prefix
out = twos_comp(int(binary_string,2), len(binary_string))

A bit more useful to me is going from hex values (32 bits in this example)...

hex_string = '0xFFFFFFFF' # or whatever... '0x' prefix doesn't matter
out = twos_comp(int(hex_string,16), 32)