Question

In the below code I understand that sys.argv uses lists, however I am not clear on how the index’s are used here.

def main():

  if len(sys.argv) >= 2:
    name = sys.argv[1]
  else:
    name = 'World'
  print 'Hello', name

if __name__ == '__main__':
  main()

If I change

name = sys.argv[1]

to

name = sys.argv[0] 

and type something for an argument it returns:

Hello C:\Documents and Settings\fred\My Documents\Downloads\google-python-exercises
\google-python-exercises\hello.py

Which kind of make sense.

Can someone explain how the 2 is used here:

if len(sys.argv) >= 2:

And how the 1 is used here:

name = sys.argv[1] 

Answer 1

Let’s say on the command-line you have:

C:\> C:\Documents and Settings\fred\My Documents\Downloads\google-python-exercises
\google-python-exercises\hello.py John

To make it easier to read, shorten it to:

C:\> hello.py John

argv represents all the items that come along via the command-line input, but counting starts at zero (0) not one (1): in this case, "hello.py" is element 0, "John" is element 1

In other words, sys.argv[0] == 'hello.py' and sys.argv[1] == 'John' … but look, how many elements is this? 2, right! so even though the numbers are 0 and 1, there are 2 elements here.

len(sys.argv) >= 2 just checks whether you entered at least two elements. in this case, we entered exactly 2.

Now let’s translate your code into English:

define main() function:
    if there are at least 2 elements on the cmd-line:
        set 'name' to the second element located at index 1, e.g., John
    otherwise there is only 1 element... the program name, e.g., hello.py:
        set 'name' to "World" (since we did not get any useful user input)
    display 'Hello' followed by whatever i assigned to 'name'

So what does this mean? It means that if you enter:

• "hello.py", the code outputs "Hello World" because you didn’t give a name
• "hello.py John", the code outputs "Hello John" because you did
• "hello.py John Paul", the code still outputs "Hello John" because it does not save nor use sys.argv[2], which was "Paul" — can you see in this case that len(sys.argv) == 3 because there are 3 elements in the sys.argv list?