python - Argv - String into Integer

Question

I'm pretty new at python and I've been playing with argv. I wrote this simple program here and getting an error that says :

TypeError: %d format: a number is required, not str

from sys import argv

file_name, num1, num2 = argv
int(argv[1])
int(argv[2])
def addfunc(num1, num2):
    print "This function adds %d and %d" % (num1, num2)
    return num1 + num2

addsum = addfunc(num1, num2)
print "The final sum of addfunc is: " + str(addsum)

When I run filename.py 2 2, does argv put 2 2 into strings? If so, how do I convert these into integers?

Thanks for your help.

Answer 1

sys.argv is indeed a list of strings. Use the int() function to turn a string to a number, provided the string can be converted.

You need to assign the result, however:

num1 = int(argv[1])
num2 = int(argv[2])

or simply use:

num1, num2 = int(num1), int(num2)

You did call int() but ignored the return value.

Answer 2

Assign the converted integers to those variables:

num1 = int(argv[1])  #assign the return int to num1
num2 = int(argv[2])

Doing just:

int(argv[1])
int(argv[2])

won't affect the original items as int returns a new int object, the items inside sys.argv are not affected by that.

Yo modify the original list you can do this:

argv[1:] = [int(x) for x in argv[1:]]
file_name, num1, num2 = argv  #now num1 and num2 are going to be integers

Answer 3

Running int(argv[1]) doesn't actually change the value of argv[1] (or of num1, to which it is assigned).

Replace this:

int(argv[1])
int(argv[2])

With this:

num1 = int(num1)
num2 = int(num2)

and it should work.

The int(..), str(...) etc functions do not modify the values passed to them. Instead, they return a reinterpretation of the data as a different type.