Question

How do I convert a hex string to a signed int in Python 3?

The best I can come up with is

h = '9DA92DAB'
b = bytes(h, 'utf-8')
ba = binascii.a2b_hex(b)
print(int.from_bytes(ba, byteorder='big', signed=True))

Is there a simpler way? Unsigned is so much easier: int(h, 16)

BTW, the origin of the question is itunes persistent id – music library xml version and iTunes hex version

Answer 1

In n-bit two’s complement, bits have value:

bit 0 = 2⁰
bit 1 = 2¹
bit n-2 = 2^n-2
bit n-1 = -2^n-1

But bit n-1 has value 2^n-1 when unsigned, so the number is 2ⁿ too high. Subtract 2ⁿ if bit n-1 is set:

def twos_complement(hexstr, bits):
    value = int(hexstr, 16)
    if value & (1 << (bits - 1)):
        value -= 1 << bits
    return value

print(twos_complement('FFFE', 16))
print(twos_complement('7FFF', 16))
print(twos_complement('7F', 8))
print(twos_complement('FF', 8))

Output:

-2
32767
127
-1