Python Error Codes are upshifted

Question

Consider a python script error.py

import sys
sys.exit(3)

Invoking

python error.py; echo $?

yields the expected "3". However, consider runner.py

import os
result = os.system("python error.py")
print result

yields 768. It seems that somehow the result of python code has been leftshifted by 8, but how these two situations are different is not clear. What's going on?

This is occurring in python 2.5 and 2.6.

Answer 1

From the docs:

On Unix, the return value is the exit status of the process encoded in the format specified for wait(). Note that POSIX does not specify the meaning of the return value of the C system() function, so the return value of the Python function is system-dependent.

os.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.

In your case, the return value 768 in binary is 00000011 00000000. The high byte is 3.

Answer 2

Docs

os.system()

On Unix, the return value is the exit status of the process encoded in the format specified for wait()

os.wait()

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)

---

High/low byte for 768

00000011 00000000
 status   signal

---

Specific code

It should be safe to >> 8 the result to get your actual exit status (the result of system() isn't very portable across platforms, however).

import os
result = os.system("python error.py")
print result >> 8