Getting IndexError when trying to use default function arguments, if no commandline arguments are passed to script

Question

I'm having some issues with some python logic. Not sure if that's possible but if it's possible with bash I guess that's possible in every other language.

I want to use default function argument if no commandline argument is passed to script:

#!/usr/bin/env python3

import sys

def get_file_name(filename: str = "watched_movies"):
    if sys.argv[1]:
        filename = sys.argv[1]
        return filename
    return filename
    
print(get_file_name())

My code might be wrong because I was just doing some poking around.

I'm getting error:

Traceback (most recent call last):
  File "/home/os/scripts/python/./movies.py", line 11, in <module>
    print(get_file_name())
  File "/home/os/scripts/python/./movies.py", line 6, in get_file_name
    if sys.argv[1]:
IndexError: list index out of range

yes, but how can i bypass that, and if there are no sys.argv passed to script i would like to use default function argument — wilk85, Aug 01 '23 at 13:17
well i have read it, but my goal is to get default name from function argument if no sys.argv provided, and i haven't found any clue in link you have gave — wilk85, Aug 01 '23 at 13:45
To be clear: the code worked when you *did* pass command line arguments? And you *already understood* that `sys.argv` will contain the command line arguments? If you do not pass any command line arguments, what do you think will be in `sys.argv`? How long would it be? Do you understand how to check the length of a list? Does that give you some idea about how to detect the condition and work around it? — Karl Knechtel, Aug 01 '23 at 14:00
In a comment to the accepted answer you said "looking for len of sys.argv is the key" which is what I have been trying to tell you, and you already seem to know from your original code how you can use the default filename, just your `if` condition was wrong. — mkrieger1, Aug 01 '23 at 14:22

score 0 · Accepted Answer · answered Aug 01 '23 at 13:12

0

Try it:

#!/usr/bin/env python3
    
import sys

def get_file_name(filename: str = "watched_movies"):
    if len(sys.argv) > 1 and sys.argv[1]:
        filename = sys.argv[1]
        return filename
    return filename 
    
print(get_file_name())

In a single line:

def get_file_name(filename: str = "watched_movies"):
    return filename if len(sys.argv) == 1 else sys.argv[1]

answered Aug 01 '23 at 13:12

André de Mattos Ferraz

612
5
13

I was going to say that `sys.argv[1]` was always going to have something in it if it exists, but users can certainly pass empty arguments if they try hard enough. So good job on idiot-proofing your condition! – kindall Aug 01 '23 at 13:14
thank you, that was the functionality i was looking for so len of sys.argv is the key – wilk85 Aug 01 '23 at 13:48

score 0 · Answer 2 · answered Aug 01 '23 at 13:35

0

#!/usr/bin/env python3

import sys

def get_file_name(filename: str = "watched_movies"):
    if len(sys.argv) < 2:
        print("no file name!")
        return
    if sys.argv[1]:
        filename = sys.argv[1]
        return filename
    return filename
    
print(get_file_name())

output

python main.py
no file name!
None

python main.py test
test

sys.argv is an array.If there is no parameter then its length is 1(just one element)，index 1 is out of the range.

answered Aug 01 '23 at 13:35

Wildptr

15
3

That's not the correct thing i would like to achieve, So i'm just wondering if i do not provide sys.argv, could filename: str = "watched_movies" from function can be passed as file name – wilk85 Aug 01 '23 at 13:43

Getting IndexError when trying to use default function arguments, if no commandline arguments are passed to script

2 Answers2