This worked for me:

import sys
firstarg=sys.argv[1]
secondarg=sys.argv[2]
thirdarg=sys.argv[3]

You can use the Optional Arguments like so.

With this program:

#!/usr/bin/env python3

import argparse, sys

parser=argparse.ArgumentParser()

parser.add_argument("--bar", help="Do the bar option")
parser.add_argument("--foo", help="Foo the program")

args=parser.parse_args()

print(f"Args: {args}\nCommand Line: {sys.argv}\nfoo: {args.foo}")
print(f"Dict format: {vars(args)}")

Make it executable:

$ chmod +x prog.py

Then if you call it with:

$ ./prog.py --bar=bar-val --foo foo-val

It prints:

Args: Namespace(bar='bar-val', foo='foo-val')
Command Line: ['./prog.py', '--bar=bar-val', '--foo', 'foo-val']
foo: foo-val
Dict format: {'bar': 'bar-val', 'foo': 'foo-val'}

Or, if the user wants help argparse builds that too:

 $ ./prog.py -h
usage: prog.py [-h] [--bar BAR] [--foo FOO]

options:
  -h, --help  show this help message and exit
  --bar BAR   Do the bar option
  --foo FOO   Foo the program

2022-08-30: Updated to Python3 this answer...

There are a few modules specialized in parsing command line arguments: getopt, optparse and argparse. optparse is deprecated, and getopt is less powerful than argparse, so I advise you to use the latter, it'll be more helpful in the long run.

Here's a short example:

import argparse

# Define the parser
parser = argparse.ArgumentParser(description='Short sample app')

# Declare an argument (`--algo`), saying that the 
# corresponding value should be stored in the `algo` 
# field, and using a default value if the argument 
# isn't given
parser.add_argument('--algo', action="store", dest='algo', default=0)

# Now, parse the command line arguments and store the 
# values in the `args` variable
args = parser.parse_args()

# Individual arguments can be accessed as attributes...
print args.algo

That should get you started. At worst, there's plenty of documentation available on line (say, this one for example)...

You can also use the sys module. Here is an example :

import sys

first_arg = sys.argv[1]
second_arg = sys.argv[2]

def greetings(word1=first_arg, word2=second_arg):
    print("{} {}".format(word1, word2))

if __name__ == "__main__":
    greetings()
    greetings("Bonjour", "monde")

It has the behavior your are looking for :

$ python parse_args.py Hello world
Hello world
Bonjour monde

I think what you're looking for is the argparse module https://docs.python.org/dev/library/argparse.html.

It will allow you to use command line option and argument parsing.

e.g. Assume the following for script.py

import argparse

if __name__ == '__main__':
   parser = argparse.ArgumentParser()
   parser.add_argument('--arg1')
   parser.add_argument('--arg2')
   args = parser.parse_args()

   print args.arg1
   print args.arg2

   my_dict = {'arg1': args.arg1, 'arg2': args.arg2}
   print my_dict

Now, if you try:

  $ python script.py --arg1 3 --arg2 4

you will see:

3
4
{'arg1': '3', 'arg2': '4'}

as output. I think this is what you were after.

But read the documentation, since this is a very watered down example of how to use argparse. For instance the '3' and '4' I passed in are viewed as str's not as integers