如何将字符串中的原始ascii值转换为整数？

Question

I have a 128 bit value I'm storing as a string in python. I would like to retrieve the last 4 bytes of it, increment it, and then put it back into the 128 bit value. Example:

mybigvalue = "69dda8455c7dd4254bf353b773304eec".decode('hex')
lastInt = mybigvalue [12:]
lastInt =lastInt +1
mybigvalue [12:] = lastInt

This doesn't work though. I'm a python noob and not sure what to try next, or if my entire idea of doing this is wrong. I come from a C background and do not totally understand how python treats data.

Answer 1

Python 2: use struct.unpack() to interpret those last 4 bytes as an integer:

import struct

lastInt = struct.unpack('<I', mybigvalue[-4:])[0]
lastInt += 1
mybigvalue = mybigvalue[:-4] + struct.pack('<I', lastInt & ((1 << 32) - 1))

'<I' means the bytes are interpreted as an unsigned integer, little-endian.

I also masked the value to fit within 32 bits; ffffffff will overflow to 00000000 that way.

Demo:

>>> import struct
>>> mybigvalue = "69dda8455c7dd4254bf353b773304eec".decode('hex')
>>> lastInt = struct.unpack('<I', mybigvalue[-4:])[0]
>>> lastInt += 1
>>> mybigvalue = mybigvalue[:-4] + struct.pack('<I', lastInt & ((1 << 32) - 1))
>>> print mybigvalue.encode('hex')
69dda8455c7dd4254bf353b774304eec

The 73304eec incremented to 74304eec ; if you wanted 73304eed instead, use big-endian; '>I' .