--help doesn't return when argparse is called from inside a program acting on a passed-in string

[schaefer01]

Bug report

Bug description:

If I parse a command line and I use the option -h or --help, the program exits through a path not under my control, i.e. if in a try/except block, the program just exits, this is ok for the usual case, but if my program is modal, that is I have different parsings for different modes, I'd like to capture a bad option (or simply handle a return from --help). In the modal case, a bad parse or even --help will cause the program to lock up.
note: parse_known_args() locks up

the parse routine:

def parse_afe_send(theLine):
    #
    err_f = False

    parser = argparse.ArgumentParser()

    parser.add_argument("-i", "--iter", type=int, 
                                      default=1, help="iterations.")
                           
    parser.add_argument("-d", "--delay",type=int, default=0, 
                                       help="seconds delay between iterations.")

    parser.add_argument("-r", "--response",action='store_true',
                        help="response expected.")

    parser.add_argument("-e", "--expect",  
                              default="", help="expected response.")

    parser.add_argument("-a", "--append",  
                              default="", help="append response to file.")

    #
    # adding listen without wait argument
    # default delay determine stochastically
    #
    theWait = 3
    parser.add_argument("-l", "--listen",type = int, 
                                         default = theWait,
                                         const = theWait,
                                         nargs='?',
                                         help = "listen seconds.")
    #
    # Note 1: bad option causes exit()
    #         which bypasses the exception handler and
    #         prevents cleanup, so vector elsewhere
    #
    # Note 2: argument parser doesn't handle no options, so 
    #         treats a no option parameter as an error, 
    #         Need to have a pass-through for this case
    #
    try:
      (args,unknowns) = parser.parse_known_args(theLine)
    except Exception as eobj:
      #
      # One would think that a bad option leads to unknowns,
      # however option -3 does not
      #
      err_str = "options exception in parse_afe_send()"
      err_code = -1
      eobj_str = repr(eobj)
      errDict = {'err_code': err_code, 'err_str': err_str, 'eobj_str': eobj_str}
      raise UserDefinedException(errDict)     
    else:
        theLast = len(unknowns)
        ii = 0
        while (ii < theLast):
          if (unknowns[ii][0] == '-'):
            print("unknown option=",unknowns[ii])
            err_f = True    # keep going, print all
          ii = ii + 1
        # end while

        if (err_f):
          err_str = "other options exception in parse_afe_send()"
          err_code = -2
          eobj_str = None
          errDict = {'err_code': err_code, 'err_str': err_str, 'eobj_str': eobj_str}
          raise UserDefinedException(errDict)
    # end else

    return args
# end parse_afe_send

the invocation, trimmed:

      try:
        shlex_out = shlex.split(theCmdString)
      except Exception as eobj:        # likely no closing quotations
        print("afe_send: shlex exception:",eobj)
        status = -2
        err_f = True
        return status
      #                                  # still here? 
         
      try:
        args = parse_afe_send(shlex_out) # parse send options
      except Exception as eobj:
        err_f = True
        err_str = "afe_send: other options exception in parse_afe_send()"
        err_code = -3
        eobj_str = repr(eobj)
        errDict = {'err_code': err_code, 'err_str': err_str, 'eobj_str': eobj_str}
        raise UserDefinedException(errDict)

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

[gvanrossum]

Maybe this is helpful? https://docs.python.org/3/library/argparse.html#exiting-methods

I expect you could override error() too. Maybe this is juts a docs defect?

[schaefer01]

thank you, I must have missed, this, I'll self-close if I can get it to work

[gvanrossum]

You will probably have to create a custom exception to catch somewhere.

Note that you could also just catch SystemExit exceptions coming out of plain parser.parse_args(). It's a BaseException, it doesn't inherit from Exception, so your try/except didn't catch it.

[schaefer01]

hi, guido,
ErrorCatchingArgumentParser didn't catch -h but adding SystemExit to the try/except block caught -h
thank you
I'll close this maybe monday if I don't find a worrying side effect between now and then
bob s