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   gt   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   gt   2    And how the 1 is used here    name   sys argv 1

User · Answer

So if I wanted to return a first name and last name like: Hello Fred Gerbig I would use the code below, this code works but is it actually the most correct way to do it?

import sys
def main():
  if len(sys.argv) >= 2:
    fname = sys.argv[1]
    lname = sys.argv[2]
  else:
    name = 'World'
  print 'Hello', fname, lname
if __name__ == '__main__':
  main()

Edit: Found that the above code works with 2 arguments but crashes with 1. Tried to set len to 3 but that did nothing, still crashes (re-read the other answers and now understand why the 3 did nothing). How do I bypass the arguments if only one is entered? Or how would error checking look that returned "You must enter 2 arguments"?

Edit 2: Got it figured out:

import sys
def main():
  if len(sys.argv) >= 2:
    name = sys.argv[1] + " " + sys.argv[2]
  else:
    name = 'World'
  print 'Hello', name
if __name__ == '__main__':
  main()

User · Answer

In a nutshell  sys argv is a list of the words that appear in the command used to run the program  The first word  first element of the list  is the name of the program  and the rest of the elements of the list are any arguments provided  In most computer languages  including Python   lists are indexed from zero  meaning that the first element in the list  in this case  the program name  is sys argv 0   and the second element  first argument  if there is one  is sys argv 1   etc   The test len sys argv   gt   2 simply checks wither the list has a length greater than or equal to 2  which will be the case if there was at least one argument provided to the program

User · Answer

As explained in the different asnwers already  sys argv contains the command line arguments that called your Python script   However  Python comes with libraries that help you parse command line arguments very easily  Namely  the new standard argparse  Using argparse would spare you the need to write a lot of boilerplate code

User · Answer

let s say on the command-line you have   C   gt  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   gt  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   gt   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

User · Answer

sys argv is the list of arguments passed to the Python program  The first argument  sys argv 0   is actually the name of the program as it was invoked  That s not a Python thing  but how most operating systems work  The reason sys argv 0  exists is so you can change your program s behaviour depending on how it was invoked  sys argv 1  is thus the first argument you actually pass to the program   Because lists  like most sequences  in Python start indexing at 0  and because indexing past the end of the list is an error  you need to check if the list has length 2 or longer before you can access sys argv 1

[python] Python sys.argv lists and indexes

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