assert expressions

Wanderer wanderer at dialup4less.com
Tue Jul 24 22:44:57 CEST 2012


On Jul 24, 4:31 pm, Ian Kelly <ian.g.ke... at gmail.com> wrote:
> On Tue, Jul 24, 2012 at 1:57 PM, Wanderer <wande... at dialup4less.com> wrote:
> > If I use the code
>
> > assert False, "unhandled option"
>
> > I get output like:
>
> > option -q not recognized
> > for help use --help
>
> > What other expressions can I use other than "unhandled option"? Is there a list somewhere?
>
> Are you using argparse or optparse or getopt or something else
> altogether?  And where are you placing this assert?  It would be
> helpful to see some actual code to understand what you are doing.
>
> And by the way, assert is a very bad way to check user input or to
> unconditionally raise an exception.  The reason is that if Python is
> invoked with -O, then all assertions are removed from the compiled
> bytecode, and then your unconditional exception code doesn't raise any
> exception at all.  If you want to raise an exception, just do it:
>
> raise Exception("unhandled option")
>
> Ideally, you would also subclass Exception to create a more specific
> exception class for your custom exception:
>
> class UnhandledOptionException(Exception):
>     pass
>
> # Then, later on...
>
> raise UnhandledOptionException("-q")

I'm using getopt but not at that point. I really don't have a problem.
I'm just curious. I've never seen anything else after
assert False,

Here is some code.

def main(argv=None):

    help_message = \
    ("\nOtFixture.py:\n Set the Optics Test Fixture Light Source Light
Level\n" +
     "Options:\n"
     "  -l, --level= <The light level percent of Max: 0.0 to 100.0>\n"
+
     "  -v, --verbose: Print messages to the terminal.\n"
     "  -h, --help: This message\n")

    level = None
    verbose = False
    helpflag = False

    options = "hl:v"
    long_options = ["help","level=","verbose"]
    if argv is None:
        argv = sys.argv
    try:
        try:
            opts, _args = getopt.getopt(argv[1:],
options,long_options)
        except getopt.error, msg:
            raise Usage(msg)

        for o, a in opts:
            if o in ("-h", "--help"):
                print help_message
                helpflag = True
            elif o in ("-l", "--level"):
                level = a
            elif o in ("-v", "--verbose"):
                verbose = True
            else:
                assert False, "unhandled option"

        if not helpflag:
            if level == None:
                level = raw_input("Enter the light level from 0.0 to
100.0%: ")
            if level.replace(".", "", 1).isdigit():
                level = float(level)
            else:
                msg = "\n" + str(level) + " is not a number.\n"
                raise Usage(msg)

        if verbose and level is not None:
            print "The level is ", level, " percent"

        if level is not None:
            if 0.0 <= level <= 100.0:
                ot = OtFixture(verbose)
                ot.setLightLevel(level)
                print "Light Level set to ", level,"%."
            else:
                msg = "\n" + str(level) + " is not in the range 0.0 to
100.0%\n"
                raise Usage(msg)


    except Usage, err:
        print >>sys.stderr, err.msg
        print >>sys.stderr, "for help use --help"
        return 2

if __name__ == "__main__":
    sys.exit(main())






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