CODEWITHSUNDEEP
python
bofimpact
bofimpact

Reputation: 13

Question about sys.argv (python)

sorry to ask a basic question, but it is hard to find on google. anyway, I have a program that does math from numbers found in various .txt files. one thing I would like to add is a -show argument to show the math step by step (finding bugs in math, finding numbers from certian steps, etc.). I have it set up in the code like so:

import sys
sys.argv[0]
filename = sys.argv[1]
prop = sys.argv[2]
show = sys.argv[3]
    if show == "-show":
        show = 1

(show = 1 does something later on). my problem is when i don't put anything for sys.argv[3] like if i put:

python program.py examplefile.txt exampleline

then the program doesn't run, I know it is becauce it is expecting an argument and thats why its messing up, but is there a way to tell it that sys.argv[3] is not always used and can be blank?

Upvotes: 1

Views: 206

Answers (3)

Anand S Kumar
Anand S Kumar

Reputation: 91007

On a side note, it might be good to checkout argparse module in python, from the documentation -

The argparse module makes it easy to write user-friendly command-line interfaces. The program defines what arguments it requires, and argparse will figure out how to parse those out of sys.argv. The argparse module also automatically generates help and usage messages and issues errors when users give the program invalid arguments.

It also has support for optional arguments. Maybe you can take a look at this to get started on that.

Upvotes: 1

user208139
user208139

Reputation:

You can check the length of sys.argv, but why bother? Use try/catch:

try: flag = sys.argv[1]
except: flag = False

That way variable flag always has a value, and you can write code the knows that it always has a value. The code has fewer lines than a if/else testing. It's a win all around.

Upvotes: 1

John
John

Reputation: 727

You should be getting an IndexError on that line when you run it without the third argument. If you're not getting an IndexError, something else is wrong and you should fix it. If you are, all is well - and you simply need to check for the length of sys.argv, taking into account in your code what should happen when that value is 3 or 4:

if len(sys.argv) == 3:
  # Stuff without sys.argv[3]
if len(sys.argv) == 4:
  # Stuff with sys.argv[3]

Upvotes: 1

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