[python] Checking if sys.argv[x] is defined

What would be the best way to check if a variable was passed along for the script:

try:
    sys.argv[1]
except NameError:
    startingpoint = 'blah'
else:
    startingpoint = sys.argv[1]

Another way I haven't seen listed yet is to set your sentinel value ahead of time. This method takes advantage of Python's lazy evaluation, in which you don't always have to provide an else statement. Example:

startingpoint = 'blah'
if len(sys.argv) >= 2:
  startingpoint = sys.argv[1]

Or if you're going syntax CRAZY you could use Python's ternary operator:

startingpoint = sys.argv[1] if len(sys.argv) >= 2 else 'blah'

Pretty close to what the originator was trying to do. Here is a function I use:

def get_arg(index):
    try:
        sys.argv[index]
    except IndexError:
        return ''
    else:
        return sys.argv[index]

So a usage would be something like:

if __name__ == "__main__":
    banner(get_arg(1),get_arg(2))

It's an ordinary Python list. The exception that you would catch for this is IndexError, but you're better off just checking the length instead.

if len(sys.argv) >= 2:
  startingpoint = sys.argv[1]
else:
  startingpoint = 'blah'

Check the length of sys.argv:

if len(sys.argv) > 1:
    blah = sys.argv[1]
else:
    blah = 'blah'

Some people prefer the exception-based approach you've suggested (eg, try: blah = sys.argv[1]; except IndexError: blah = 'blah'), but I don't like it as much because it doesn't “scale” nearly as nicely (eg, when you want to accept two or three arguments) and it can potentially hide errors (eg, if you used blah = foo(sys.argv[1]), but foo(...) raised an IndexError, that IndexError would be ignored).

A solution working with map built-in fonction !

arg_names = ['command' ,'operation', 'parameter']
args = map(None, arg_names, sys.argv)
args = {k:v for (k,v) in args}

Then you just have to call your parameters like this:

if args['operation'] == "division":
    if not args['parameter']:
        ...
    if args['parameter'] == "euclidian":
        ...

I use this - it never fails:

startingpoint = 'blah'
if sys.argv[1:]:
   startingpoint = sys.argv[1]

You can simply append the value of argv[1] to argv and then check if argv[1] doesn't equal the string you inputted Example:

from sys import argv
argv.append('SomeString')
if argv[1]!="SomeString":
            print(argv[1])