Python "bug"

[Sean Reifschneider]

Exactly what Stephen said.  The python equivalent of WEXITSTATUS is
os.WEXITSTATUS(exit_code)

On Fri, Feb 9, 2024 at 1:06 PM Stephen Warren <swarren at wwwdotorg.org> wrote:

> This is nothing to do with Python at all; it's the way the POSIX exit()
> and system() functions are specified, which Python's os module is a
> lightweight wrapper around. Python doesn't define these APIs; it simply
> exposes them.
>
> In particular, the value returned by system() (POSIX; called from C or
> Python) is not the exit status, but an encoded value. In C, you must
> (after checking some other conditions...) use WEXITSTATUS() to extract
> the exit status of the invoked process. I don't recall immediately how
> to invoke that macro from Python. WEXITSTATUS happens to be implemented
> as follows on my system, but portable code should not rely on this:
>
> /usr/include/x86_64-linux-gnu/bits/waitstatus.h
> #define __WEXITSTATUS(status) (((status) & 0xff00) >> 8)
>
> /usr/include/stdlib.h
>
> #define WEXITSTATUS(status) __WEXITSTATUS (status)
>
> On 2/9/24 11:43, Brian Sturgill wrote:
> > Python  has some quirks.
> >
> > Anyway, here's the bug I've just wasted an hour on...
> >
> > I have a shell script, which error checked the return of a Python script.
> >
> > The python script autogens an example written in python
> > that automatically creates an output 3d object to be used by my
> > documentation web page.
> >
> > In other words, the shell invokes python, which generates a python
> program,
> > which it then runs.  Error status were being pass as appropriate.
> > However, I had a syntax error.
> > The shell script claims it got 0.
> > Python claimed it was sending 256.
> >
> > It turns out that the sys.exit function recommends sticking to 0-127.
> >
> > The irony is that Python itself was generating the 256 exit status,
> > which it cannot handle.
> >
> > Try these:
> > (piecad) brian at george:~/work/piecad$ python -c "import sys;
> sys.exit(256)"
> > (piecad) brian at george:~/work/piecad$ echo $?
> > 0
> > (piecad) brian at george:~/work/piecad$ python -c "import sys;
> sys.exit(255)"
> > (piecad) brian at george:~/work/piecad$ echo $?
> > 255
> >
> > Bash seems to be limited to 0-255, which is what man 3 exit says should
> > be the range.
> >
> > But the plot thickens... the real culprit was the os.system function.
> > Which does return the exit status, but as the second byte of the
> > returned 16-bit
> > integer.
> > So essentially, python returned a 1 status for the syntax error,
> > which o.system intentionally made into 256. Which the shell then
> > translated to 0.
> >
> > Whose fault is it?  Well, me of course. I should have read the 4 pages
> > of documentation
> > it took to figure out what was going on! :-)
> >
> > --
> >
> > Brian
> >
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.nclug.org/pipermail/nclug/attachments/20240209/689a498a/attachment.htm>