Decide if number is Armstrong number
By PEP 8, Python's official style guide, function names should generally be lower_case_with_underscores.  Also, the final semicolon should be omitted.

The function would typically be written more elegantly using the sum() builtin function, with a generator expression.  (It's not any faster than your original code, though.  Actually, a tiny bit slower.)

I don't recommend using arr as a variable name, since it's neither descriptive (what does the array represent?) nor accurate (it's actually a string).

For the printout, I recommend using str.format() or one of the other formatting mechanisms.

import sys

def is_armstrong(number):
    digits = str(number)
    length = len(digits)
    return number == sum(int(digit) ** length for digit in digits)

if len(sys.argv) &gt; 1:
    arg = int(sys.argv[1])
    print('{} is an Armstrong number? {}'.format(arg, is_armstrong(arg)))
else:
    print("No arguments passed! :-(")