assert expressions

[Wanderer]

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?

Thanks

 

[Ian Kelly]

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")

 

[Wanderer]

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())

 

[Wanderer]

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 left out the Usage class

class Usage(Exception):
def __init__(self, msg):
self.msg = msg

 

[Wanderer]

I left out the Usage class

class Usage(Exception):
    def __init__(self, msg):
        self.msg = msg

I seem to be missing a post.

Here is the code.

class Usage(Exception):
def __init__(self, msg):
self.msg = msg

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())

I don't really have a problem. I'm was just curious.

How do you invoke python -O? When I run python.exe -O OtFixture.py -q,
I get the same response. It's a capital letter O, right?

 

[Ian Kelly]

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.

It doesn't matter what you put after the assert False, because that
line is not actually reached. When getopt sees the -q, it immediately
raises a getopt.error, which you catch and then immediately reraise as
a Usage exception, which is then caught and printed at the end. The
chained ifs with the assert statement never even execute in this
scenario.

Seeing the assert in context, it makes more sense. It's not
intercepting some unimplemented option and preventing the program from
proceeding; it's there as a development tool to catch programming
errors where an option is added to the getopt configuration but is not
implemented in the if chain.

 

[Wanderer]

It doesn't matter what you put after the assert False, because that
line is not actually reached.  When getopt sees the -q, it immediately
raises a getopt.error, which you catch and then immediately reraise as
a Usage exception, which is then caught and printed at the end.  The
chained ifs with the assert statement never even execute in this
scenario.

Seeing the assert in context, it makes more sense.  It's not
intercepting some unimplemented option and preventing the program from
proceeding; it's there as a development tool to catch programming
errors where an option is added to the getopt configuration but is not
implemented in the if chain.

Thanks. Now it makes more sense.