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How do I convert a hex string to a signed int in Python 3.2?
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
824
If you are using the python interpreter, you can just type 0x(your hex value) and the interpreter will convert it automatically for you.
The int data type in python simply the same as the signed integer. A signed integer is a 32-bit integer in the range of -(2^31) = -2147483648 to (2^31) – 1=2147483647 which contains positive or negative numbers. It is represented in two's complement notation.
Python hex() function is used to convert an integer to a lowercase hexadecimal string prefixed with “0x”. We can also pass an object to hex() function, in that case the object must have __index__() function defined that returns integer. The input integer argument can be in any base such as binary, octal etc.
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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