How to check for command line argument and if none, prompt for input through console?

Question

This question was part of an assignment for a software class in college. I've long since quit but I'd really like to learn python so I'm revisiting some assignments.

I've created a simple code that will convert the number of inches given, into feet and inches. So far I've been able to have it check that the input is an integer, and is positive. The final step is being able to accept the input from either the command line or the console. Like the title says, how can I check for a command line argument and if none exists, prompt for input through the console and finish executing the script?

Here's my original code made with help from here.

try:
    total_inches = int(input('Enter a distance in inches: '))
    if total_inches > 0:
        INCHES_PER_FOOT = 12
        feet = total_inches//INCHES_PER_FOOT
        final_inches = total_inches%INCHES_PER_FOOT
        
        print(str(total_inches) + '"', 'is equivalent to', str(feet) + '\'', str(final_inches) + '"')
    else:
        print('Must be a positive integer. Try again')
except ValueError:
    print('Not a number. Try again.')

As you can see, it checks the input and executes accordingly.

I've done some searching around here and here but haven't come up with anything solid yet. The first one is about input() which I know and parsing which I'm not familiar with yet. The second one led me to this clunky looking bit of code:

from sys import argv

if len(argv) == 1:
    try:
        total_inches = int(argv[1])
        if total_inches > 0:
            INCHES_PER_FOOT = 12
            feet = total_inches//INCHES_PER_FOOT
            final_inches = total_inches%INCHES_PER_FOOT
            
            print(str(total_inches) + '"', 'is equivalent to', str(feet) + '\'', str(final_inches) + '"')
        else:
            print('Must be positive')
    except ValueError:
        print('Not a number. Try again.')
else:
    try:
        total_inches = int(input('Enter a distance in inches: '))
        if total_inches > 0:
            INCHES_PER_FOOT = 12
            feet = total_inches//INCHES_PER_FOOT
            final_inches = total_inches%INCHES_PER_FOOT
            
            print(str(total_inches) + '"', 'is equivalent to', str(feet) + '\'', str(final_inches) + '"')
        else:
            print('Must be positive')
    except ValueError:
        print('Not a number. Try again.')

When I include a command line argument it still prompts for console input but otherwise executes correctly. When I omit the command line argument and try executing the script I get:

D:\Python\2>python p2e3.py
Traceback (most recent call last):
File "D:\Python\2\p2e3.py", line 5, in
total_inches = int(argv[1])
IndexError: list index out of range

I know that that means it's missing an argument but I don't understand why it doesn't skip down to the else statement or why it still asks for console input when there's a command line argument. Not sure if my indentation is wrong or if there's another function I should be using, any help would be appreciated.