Hex string to signed int in Python 3.2?

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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
...
>>> twos_complement('FFFE',16)
-2
>>> twos_complement('7FFF',16)
32767
>>> twos_complement('7F',8)
127
>>> twos_complement('FF',8)
-1

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