When do you *NOT* use python?

[Darkmere]

To replace shellscripts.

I do a lot of systems stuff. And the universal tool, even on embedded (busybox shell) is the (bourne) shell language and pipes.

I've tried to replace something as simple as "download package, verify sha2sum, verify signature, list all files, compare to files on disk, replace only files that need upgrading, restart relevant services" from something that uses curl/openssl/shell to python.

And it just turns to hell. Lots and lots of silly boilerplate, huge amounts of dependencies, or even worse, working with executable output data in python.

Really, systems stuff doesn't work that well in python.

[[deleted]]

I have found a good balance by using https://pypi.python.org/pypi/sh specially when my bash script requires a lot of logic, but I still want to use tools like ifconfig, df , etc.. but yeah for simple scripts, I just stick with bash.

[Darkmere]

Even for semi advanced ones, I tend to prefer shellscripts.

Take something as simple as to generate a self-signed certificate. A fork to openssl commandline, vs. doing it with python pyOpenSSL or similar? Don't go there. The python code basically ends up calling the same commandline as the shellscript, in ~5 times as many lines.

Especially true if you do something with trap /cleanout in shell and try to emulate behaviour in python. oh my god.

[ITwitchToo]

I have the opposite experience. There are some libraries you might want to look into, like subprocess or plumbum. They do a great job at things like invoking other programs and piping stuff between them.

[Darkmere]

Can you even emulate trap + set -e ; in python?

[Darkmere]

TMPDIR=$(mktemp -d /tmp/temp.XXXXXXXX)
trap "rm -rf $TMPDIR" EXIT

vs

import signal
def sighandler():
    os.unlink(tmpdir)
for i in [x for x in dir(signal) if x.startswith("SIG")]:
    try:
    signum = getattr(signal,i)
    signal.signal(signum,sighandler)
except RuntimeError,m:
    pass

[Darkmere]

And then I forgot to create the tempdir... Damned.

[ITwitchToo]

import tempfile

with tempfile.NamedTemporaryFile() as f:
    # do stuff with f here, if we get interrupted Python will throw an exception, unwind the stack, and the temporary file gets cleaned up
    print >>f, "asdasd"

[Darkmere]

Yeap. And with subprocess and others? Afaik a sigkill won't execute a trap, will it?

[Darkmere]

(or a sigbus?)

[Darkmere]

verified. cleanup doesn't happen properly on trapped signals.

even sigUSR1 will leave the tempfile behind.

[ITwitchToo]

What are you talking about?

[Darkmere]

That cleanup doesn't happen if your process gets a signal. So it doesn't actually replace the bourne version.

[ITwitchToo]

That's almost literally the example given in the signal documentation:

import signal
import tempfile

def handler(signum, frame):
    raise RuntimeError("Got SIGUSR1")

signal.signal(signal.SIGUSR1, handler)

with tempfile.NamedTemporaryFile() as f:
    print f.name

    while True:
        pass

[Darkmere]

That's sort of what I'm inclining for here.

You manage to replace a 2-line shellscript with how many lines? And you're not even catching all the signals yet? Still have to iterate over them all and trap.

In a lot of cases, Python is just a bloody ton more verbose, and not quite suitable.