Pythonic way to return a boolean value and a message [duplicate]

6

This question already has an answer here:

I have a simple script that checks various Linux processes and, finding one of them, logs a particular message ("particular" in terms of referencing the service's name).

My question: what is the proper, Pythonic way to have a multi-conditional function return a boolean and a string (for use in a printed message)?

Here's a stripped-down version of my current solution (using tuples):

import subprocess
import time

def _isProcessRunning(cmd):
    return int(
            subprocess.check_output(
                '{} | grep -v grep | wc -l'.format(cmd),
                shell=True
                )
            ) > 0

def processGroupFound():
    if _isProcessRunning('ps auwxx | grep duplicity'):
        return (True, 'Duplicity')
    elif _isProcessRunning('who | grep pts'):
        return (True, 'SSH')
    elif _isProcessRunning('netstat -na | grep ESTA | grep 5901'):
        return (True, 'VNC')
    else:
        return (False, '')

def worker():
    while True:
        process_found, service_string = processGroupFound()
        if process_found:
            print('{} found; skipping'.format(service_string))
        else:
            print('Continuing on')
        time.sleep(10)


if __name__ == "__main__":
    worker()

This works, but I care about doing it correctly (stylistically, in particular, but if you glean incorrect logic in this brief sample, please feel free to comment there, too. Appreciate your assistance!

ABach

출처 python linux shell
  • This is fine. Perfectly idiomatic. I might omit the parens (return True, 'Duplicity'), but then again I might not in some cases. - Mad Physicist
  • Lots of other options here too stackoverflow.com/questions/354883/… - Alex G Rice
  • I wouldn't bother with the parentheses around the return statements, but otherwise, seems okay. I'm always suspicious of shell=True, and using external utilities for things Python can do (the grep and wc work), but that's separate from what you're asking about. - ShadowRanger
  • Thanks, all, for your comments (especially yours, @ShadowRanger – I'm going to use shell processes for the bare minimum, then Python for the bulk of the work). - ABach

2 답변

5

An empty string in Python is "falsey", so it's somewhat redundant to return (False, ''). I might do this instead:

def processGroupFound():
    if _isProcessRunning('ps auwxx | grep duplicity'):
        return 'Duplicity'
    elif _isProcessRunning('who | grep pts'):
        return 'SSH'
    elif _isProcessRunning('netstat -na | grep ESTA | grep 5901'):
        return 'VNC'
    else:
        return ''

def worker():
    while True:
        service_string = processGroupFound()
        if service_string:
            print('{} found; skipping'.format(service_string))
        else:
            print('Continuing on')
        time.sleep(10)

(See 4.1 Truth Value Testing)

  • Testing the returned value is C-ish instead of pythonic, that's why I prefer the exception solution. - Gribouillis
  • Thanks for your answer! I had forgotten about the "falsey-ness" of empty strings. - ABach
  • The answer @Gribouillis gave wasn't there when I started writing mine. I agree that his is more pythonic - it gets points under "ask forgiveness, not permission" at the least. - Joe Carey
3

I think this would be pythonic too (but may be it's just me)

class NoRunningService(RuntimeError): pass

def findService():
    if _isProcessRunning('ps auwxx | grep duplicity'):
        return 'Duplicity'
    elif _isProcessRunning('who | grep pts'):
        return 'SSH'
    elif _isProcessRunning('netstat -na | grep ESTA | grep 5901'):
        return 'VNC'
    else:
        raise NoRunningService

def worker():
    while True:
        try:
            service_string = findService()
        except NoRunningService:
            print('Continuing on')
        else:
            print('{} found; skipping'.format(service_string))
        time.sleep(10)

  • Thanks so much. Psychologically, I grapple with the idea of an "exception" as a "fatal error that needs correcting." I'll work on that. ;) - ABach
  • It's much more than that. When a function does not handle a case, raise an exception! - Gribouillis

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