Convert highest byte of 16-bits to signed int in python

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?

What system are you on that uses signed return statuses? (GNU make should never return a value other than 0, 1, or 2). — Wooble
– Wooble, Commented Jun 15, 2012 at 19:19

Ignacio Vazquez-Abrams · Accepted Answer · 2012-06-15 19:15:28Z

8

if number > 127:
  number -= 256

answered Jun 15, 2012 at 19:15

Ignacio Vazquez-Abrams

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1 Comment

Ajoy Over a year ago

Any explanation of why the bitwise operation gives an unsigned number, when the input was a signed int, and the OP is only right shifting?

senderle · Accepted Answer · 2012-06-15 20:19:58Z

4

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)

edited Jun 15, 2012 at 20:19

answered Jun 15, 2012 at 19:15

senderle

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1 Comment

Ignacio Vazquez-Abrams Over a year ago

It's not quite as bad as using numpy.int8, but it's still more work than necessary.

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