Hex string to signed int in Python 3.2?

import struct

For Python 3 (with comments' help):

h = '9DA92DAB'
struct.unpack('>i', bytes.fromhex(h))

For Python 2:

h = '9DA92DAB'
struct.unpack('>i', h.decode('hex'))

or if it is little endian:

h = '9DA92DAB'
struct.unpack('<i', h.decode('hex'))

This works for 16 bit signed ints, you can extend for 32 bit ints. It uses the basic definition of 2's complement signed numbers. Also note xor with 1 is the same as a binary negate.

# convert to unsigned
x = int('ffbf', 16) # example (-65)
# check sign bit
if (x & 0x8000) == 0x8000:
    # if set, invert and add one to get the negative value, then add the negative sign
    x = -( (x ^ 0xffff) + 1)

Here's a general function you can use for hex of any size:

import math

# hex string to signed integer
def htosi(val):
    uintval = int(val,16)
    bits = 4 * (len(val) - 2)
    if uintval >= math.pow(2,bits-1):
        uintval = int(0 - (math.pow(2,bits) - uintval))
    return uintval

And to use it:

h = str(hex(-5))
h2 = str(hex(-13589))
x = htosi(h)
x2 = htosi(h2)

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