Checking if sys.argv[x] is defined

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).

In the end, the difference between try, except and testing len(sys.argv) isn't all that significant. They're both a bit hackish compared to argparse.

This occurs to me, though -- as a sort of low-budget argparse:

arg_names = ['command', 'x', 'y', 'operation', 'option']
args = dict(zip(arg_names, sys.argv))

You could even use it to generate a namedtuple with values that default to None -- all in four lines!

Arg_list = collections.namedtuple('Arg_list', arg_names)
args = Arg_list(*(args.get(arg, None) for arg in arg_names))

In case you're not familiar with namedtuple, it's just a tuple that acts like an object, allowing you to access its values using tup.attribute syntax instead of tup[0] syntax.

So the first line creates a new namedtuple type with values for each of the values in arg_names. The second line passes the values from the args dictionary, using get to return a default value when the given argument name doesn't have an associated value in the dictionary.

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'