Casting in Jython 2.5.3 - casting

Question

There is a Python function, that runs in CPython 2.5.3 but crashes in Jython 2.5.3 .
It is part of a user defined function in Apache Pig, which uses Jython 2.5.3 so i cannot change it.
The input is a array of singed bytes, but in fact that are unsigned bytes, so i need to cast it.
from StringIO import StringIO
import array
import ctypes
assert isinstance(input, array.array), 'unexpected input parameter'
assert input.typecode == 'b', 'unexpected input type'
buffer = StringIO()
for byte in input:
s_byte = ctypes.c_byte(byte)
s_byte_p = ctypes.pointer(s_byte)
u_byte = ctypes.cast(s_byte_p, ctypes.POINTER(ctypes.c_ubyte)).contents.value
buffer.write(chr(u_byte))
buffer.seek(0)
output = buffer.getvalue()
assert isinstance(output, str)
The error is:
s_byte = ctypes.cast(u_byte_p, ctypes.POINTER(ctypes.c_byte)).contents.value
AttributeError: 'module' object has no attribute 'cast'
I guess the ctypes.cast functions is not implemeted in Jython 2.5.3 . Is there a workaround for that issue?
Thanks,
Steffen

Answer 1

Here is my solution, that is quite ugly but works without additional dependecies.
It uses the bit representation of usinged und signed bytes (https://de.wikipedia.org/wiki/Zweierkomplement).
import array
assert isinstance(input, array.array), 'unexpected input parameter'
assert input.typecode == 'b', 'unexpected input type'
output = array.array('b', [])
for byte in input:
if byte > 127:
byte = byte & 127
byte = -128 + byte
output.append(byte)