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Date: Sun, 26 Jan 2014 21:04:57 -0800
From: Gary Herron <gary.herron@islandtraining.com>
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To: python-list@python.org
Subject: Re: buggy python interpretter or am I missing something here?
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Xref: csiph.com comp.lang.python:64825

On 01/26/2014 07:42 PM, me wrote:
> I'm writing a linux daemon in python 2.x to process batches of GPS/GIS
> data and I'm running into something that seems to break the expected
> program flow in a REALLY BAD WAY.
>
> Consider the attached template script and execute it with the -h option.
> It is falling through to the except: clause even though try to manually
> exit with sys.exit(0).  However, if I insert a "raise" into the except
> clause then the sys.exit(0) executes properly.
>
> See the attached code and output from when I run it.
>
> Not interested in work-arounds.  I want to understand why it doesn't work
> as expected.
>
> Thanks!
>
> TCdaemon.py ----
>
> #! /usr/bin/python
>
> import sys
>
>
> #
> ------------------------------------------------------------------------------
>
> defaultparams={ \
>      "basedir": "/process", \
>      "inpipe": "/tmp/TCdaemonIN", \
>      "maxjobs":8, \
>      "outpipe": "/tmp/TCdaemonOUT", \
>      "ZZZ": 0
>      }
>
> #
> ------------------------------------------------------------------------------
>
> def parse_args(a,d):
>      l=len(a)
>      idx=1
>      try:
>          while (idx<l):
>              if (a[idx]=="-#"):
>                  idx=idx+1
>                  d["maxjobs"]=int(a[idx])
>              elif (a[idx]=="-d"):
>                  idx=idx+1
>                  d["basedir"]=a[idx]
>              elif (a[idx]=="-h"):
>                  print "help goes here"
>                  sys.exit(0)
>              elif (a[idx]=="-i"):
>                  idx=idx+1
>                  d["inpipe"]=a[idx]
>              elif (a[idx]=="-o"):
>                  idx=idx+1
>                  d["outpipe"]=a[idx]
>              idx=idx+1
>      except:
>          # raise # -- WTF!!!??? required for -h to not fall through to
> here?
>          print "%s: error in command line - %s"%(a[0],a[idx:])
>          sys.exit(1)

Never *ever* have a bare except like that.  If it gets invoked, you have 
no idea why.   A simple typo like ixd instead of idx or a(idx) instead 
of a[idx] would raise an exception but give you no idea why.

Do
   try:
       ...
   except Exception,e:
       print e
at the absolute minimum.
(Python 3 syntax would differ slightly, but the advice is the same.)

Perhaps printing a traceback along with the exception would help. Add
     traceback.print_exc()


Gary Herron


>
>      
> #
> ------------------------------------------------------------------------------
> #
> ------------------------------------------------------------------------------
> #
> ------------------------------------------------------------------------------
>
> if (__name__=="__main__"):
>      print defaultparams
>      parse_args(sys.argv,defaultparams)
>      print defaultparams
>      
>
>
>
>
> output.txt ---
> [@blackbox new]$ ./TCdaemon.py -h
> {'ZZZ': 0, 'basedir': '/process', 'outpipe': '/tmp/TCdaemonOUT',
> 'maxjobs': 8, 'inpipe': '/tmp/TCdaemonIN'}
> help goes here
> ./TCdaemon.py: error in command line - ['-h']
> [@blackbox new]$ echo $?
> 1
> [@blackbox new]$
> [@blackbox new]$
> [@blackbox new]$ # editing to add "raise" at line 40
> [@blackbox new]$
> [@blackbox new]$
> [@blackbox new]$
> [@blackbox new]$ ./TCdaemon.py -h
> {'ZZZ': 0, 'basedir': '/process', 'outpipe': '/tmp/TCdaemonOUT',
> 'maxjobs': 8, 'inpipe': '/tmp/TCdaemonIN'}
> help goes here
> [@blackbox new]$ echo $?
> 0
> [@blackbox new]$
>
>