Allowing for optional sys.argv for a Python script

Question

I have a Python script that requires at least one command line argument to run with default values. I want to allow the user to optionally change these default values from the command line.

Right now, I run it with python script.py arg1 and it works great but I want to be able to run it with python script.py arg1 [optional arg2 optional arg3].

Also to clarify, I'm using the following block of code to check for number of agreements and to print some information:

if len(sys.argv) < 2:
    print "Error. Required: python "+sys.argv[0]+" arg1 + [optional] arg2 (default 2) + [optional] arg3 (default 100)"
    sys.exit(0)

After that, I tried the following:

if sys.argv[2] == None:
    default_value1 == 2
else:
    default_value1 == sys.argv[2]

if sys.argv[3] == None:
    default_value2 == 100
else:
    default_value2 == sys.argv[3]

I also tried replacing the None above for an empty string ("") or a space (" ") but I always get the error:

if sys.argv[2] == None:
IndexError: list index out of range

How do you allow for additional sys.argv to be passed or ignored on the command line? I think to understand that I would have to either give it both additional arguments (i.e.,sys.argv[2] and sys.argv[3]) or none at all for the above code to work. Is this correct?

Thanks!

use `len(sys.argv)` to check whether it has at least 3 elements... — Antti Haapala -- Слава Україні, Sep 27 '17 at 17:09
I'm already are. I have this block of code at the beginning of the script: `if len(sys.argv) < 2:` `print "Error, required: python "+sys.argv[0]+" arg1 + [optional] arg2 (default 2) + [optional] arg3 (default 100)"` `sys.exit(0)` — Uribe-Convers, Sep 27 '17 at 17:26
OK, I figured it out. I used `if len(sys.argv) == 2:` and gave default values to the 2 optional variables. Then I used `if len(sys.argv) == 4:` to give the user specified values to the variables, i.e., `arg2 = int(sys.argv[2])` and `arg3 = int(sys.argv[3])` — Uribe-Convers, Sep 27 '17 at 18:10
Thanks @AnttiHaapala, I'll check that out too. It will probably give me a more elegant way of doing it. — Uribe-Convers, Sep 27 '17 at 18:21
Oh, to be a beginner! I wrote this question almost two years ago when I was first starting to code! I now use `click` and `argparse` on a daily basis to write CLIs with required and optional arguments, flags, defaults, and more! But we all have to start somewhere and with help from the stack overflow community we can all learn and grow. — Uribe-Convers, Jun 22 '19 at 02:27
should we find better duplicates too, maybe something using argparse or detailing click — Antti Haapala -- Слава Україні, Jun 22 '19 at 06:35

Allowing for optional sys.argv for a Python script

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