Introduction to Python

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The Command Prompt: Program Arguments

sys.argv to capture command line arguments

sys.argv is a list that holds string arguments entered at the command line

a python script get_args.py

import sys                           # import the sys library

print('first arg: ' + sys.argv[1])   # print first command line arg
print('second arg: ' + sys.argv[2])  # print second command line arg

running the script from the command line, with two arguments

$ python myscript.py hello there
first arg: hello
second arg: there

• sys.argv is a list that is automatically provided by the sys module.
• This list contains any string arguments to the program that were entered at the command line by the user.
• If the user does not type arguments at the command line, then they will not be added to the sys.argv list.

The default item in sys.argv: the program name

sys.argv[0] will always contain the name of our program.

• sys.argv[0] always contains the name of the program itself
• or, it may contain the pathname that was used to execute the program (i.e., directory path to the program filename)
• even if no arguments are passed at the command line, sys.argv always holds this one value

a python script print_args.py

import sys
print(sys.argv)

(passing 3 arguments)

$ python print_args.py hello there budgie
['myscript2.py', 'hello', 'there', 'budgie']

running the script from the command line (passing no arguments)

$ python print_args.py
['myscript2.py']

IndexError with sys.argv (when user passes no argument)

Since we read arguments from a list, we can trigger an IndexError if we try to read an argument that wasn't passed.

a python script addtwo.py

import sys

firstint = int(sys.argv[1])
secondint = int(sys.argv[2])

mysum = firstint + secondint

print(f'the sum of the two values is {mysum}')

passing 2 arguments

$ python addtwo.py 5 10
the sum of the two values is 15

passing no arguments

$ python addtwo.py
Traceback (most recent call last):
  File "addtwo.py", line 3, in <module>
firstint = int(sys.argv[1])
IndexError: list index out of range

How to handle this exception? Test the len() of sys.argv, or trap the exception.