Question

I use os.system to run a make command

os.system('make -C mydir/project all')

I’d like to see if make fails or not. The system documentation states that the return code is in the same format as wait()

Wait for completion of a child process, and return a tuple containing its pid 
and exit status indication: a 16-bit number, whose low byte is the signal number 
that killed the process, and whose high byte is the exit status (if the signal 
number is zero); the high bit of the low byte is set if a core file was produced.

so if make (or another application) returns -1, I have to convert 0xFFxx (I don’t really care about the pid of the called) to -1. After a right shift, I get 0xFF, but I cannot get it to convert that to -1, it always print 255.

So, in python, how can I convert 255 to -1, or how can I tell the interpreter that my 255 is in fact a 8 bits signed integer?

Answer 1

Although Ignacio’s answer may be better for this case, a good general-purpose tool for unpacking bytes from specially formatted data is struct:

>>> val = (255 << 8) + 13
>>> struct.unpack('bb', struct.pack('H', val))
(13, -1)