Python sys.argv lists and indexes

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, let's just shorten this 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?