Hex string to signed int in Python

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

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

But bit n-1 has value 2n-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

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