Instead of writing execrable shell scripts, I write a lot of Python commandline scripts. I use this template as a starting point, which helps me remember a few things:
- Just how the
__name__ == "__main__"syntax and getting arguments from the commandline work. - The proper incantation for configuring logging.
- The
idb_excepthook()function, which I use to implement a--debugmode where the interactive debugger is started automatically if an unhandled exception is encountered. - How to properly handle broken pipes, as happens if you run
script.py | head.
Template
Available directly at cli.template.py (shortened).
#!/usr/bin/env python3
import argparse
import logging
import os
import pdb
import sys
import traceback
import typing
from collections.abc import Callable
logging.basicConfig(
level=logging.INFO, format="[%(asctime)s] [%(name)s] [%(levelname)s] %(message)s"
)
logger = logging.getLogger(__name__)
def idb_excepthook(type, value, tb):
"""Call an interactive debugger in post-mortem mode
If you do "sys.excepthook = idb_excepthook", then an interactive debugger
will be spawned at an unhandled exception
"""
if hasattr(sys, "ps1") or not sys.stderr.isatty():
sys.__excepthook__(type, value, tb)
else:
traceback.print_exception(type, value, tb)
print
pdb.pm()
def broken_pipe_handler(
func: Callable[[typing.List[str]], int], *arguments: typing.List[str]
) -> int:
"""Handler for broken pipes
Wrap the main() function in this to properly handle broken pipes
without a giant nastsy backtrace.
The EPIPE signal is sent if you run e.g. `script.py | head`.
Wrapping the main function with this one exits cleanly if that happens.
See <https://docs.python.org/3/library/signal.html#note-on-sigpipe>
"""
try:
returncode = func(*arguments)
sys.stdout.flush()
except BrokenPipeError:
devnull = os.open(os.devnull, os.O_WRONLY)
os.dup2(devnull, sys.stdout.fileno())
# Convention is 128 + whatever the return code would otherwise be
returncode = 128 + 1
return returncode
def resolvepath(path):
return os.path.realpath(os.path.normpath(os.path.expanduser(path)))
def parseargs(arguments: typing.List[str]):
"""Parse program arguments"""
parser = argparse.ArgumentParser(description="Python command line script template")
parser.add_argument(
"--debug",
"-d",
action="store_true",
help="Launch a debugger on unhandled exception",
)
parser.add_argument("action", help="The thing to do")
parsed = parser.parse_args(arguments)
return parsed
def main(*arguments):
"""Main program"""
parsed = parseargs(arguments[1:])
if parsed.debug:
sys.excepthook = idb_excepthook
print(f"Taking important action: {parsed.action}")
if __name__ == "__main__":
exitcode = broken_pipe_handler(main, *sys.argv)
sys.exit(exitcode)
