How to make sys.argv arguments optional?

Question

sys.argv takes arguments at the shell command line when running a program. How do I make these arguments optional?

I know I can use try - except. But this forces you to insert either no extra arguments or all extra arguments, unless you nest more try - except which makes the code look much less readable.

Edit

Suppose I would want the following functionality, how do I implement this?

$ python program.py add Peter 
'Peter' was added to the list of names.

This add argument (and not --add) is optional such that

$ python program.py

just runs the program normally.

Ratite

Answer 1

plac is an alternative to the standard library modules given in the other answers. It allows to define command line arguments through annotations. From the documentation, exemple 8 demonstrate optional arguments syntax :

example8.py
def main(command: ("SQL query", 'option', 'q'), dsn):
    if command:
        print('executing %s on %s' % (command, dsn))
        # ...

if __name__ == '__main__':
    import plac; plac.call(main)

Argparse exemple :

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("--add", help="Add prefix to string")
args = parser.parse_args()

Note that the convention is for optional argument to be provided as "--add" while subcommands are provided as "add". There is a subcommand implementation in argparse.

Echidna

This is a very incomplete accepted answer to the original stated question. Although I agree that usually another module like argparse is advisable, that doesn't answer the question. It is in fact possible to do optional args with sys.argv and for a simple script, that may be enough! – Brier

Answer 2

EDIT to address your edit,

import sys

sys.argv = sys.argv[1:]
names = []
while sys.argv and sys.argv[0] == 'add':
    #while the list is not empty and there is a name to add
    names.append(sys.argv[1])
    print sys.argv[1], 'was added to the list of names.'
    sys.argv = sys.argv[2:]

all of the following work with this

$ python program.py add Peter
Peter was added to the list of names.

$ python program.py add Peter add Jane
Peter was added to the list of names.
Jane was added to the list of names.

$ python program.py

if the advantage to requiring 'add' before each name is that if there are any other arguments you want to look for after adding names, you can. If you want to pass multiple names by saying python program.py add Peter Jane this can be done with a fairly simple change

import sys

names = []
if len(sys.argv) > 2 and sys.argv[1] == 'add':
    names = sys.argv[2:]

for n in names:
    print n, 'was added to the list of names.'

---

ORIGINAL

it seems like you would be better off with something like optparse. However since sys.argv is a list you can check the length of it.

arg1 = sys.argv[1] if len(sys.argv) > 1 else 0 # replace 0 with whatever default you want
arg2 = sys.argv[2] if len(sys.argv) > 2 else 0

and then use arg1 and arg2 as your "optional" command line arguments. this will allow you to pass 1, 2, or 0 command line arguments (actually you can pass more than 2 and they will be ignored). this also assumes that the arguments have a known order, if you want to use flags like -a followed by a value, look into optparse http://docs.python.org/library/optparse.html?highlight=optparse#optparse

Filings

Answer 3

You should be using a command-line parser such as getopt or argparse. These allow you define options that are optional and have default values.

Froemming

argparse forces you to use the -- or - before an optional argument. – Ratite

Answer 4

As a simple answer to your question, you can have an optional argument with sys.argv, but if you foresee your module expanding much you may want to consider another module like argparse as suggested elsewhere.

This will ignore no args, but allow an arbitrary number of args after that.

#!/usr/bin/env python

import sys

added = sys.argv[1:]
if added:
    for add in added:
        print("{0} was added to the list of names.".format(add))

$ ./test.py
$ ./test.py Peter
Peter was added to the list of names.

Of course you are not passing add Peter just Peter. But if that is acceptable, then this works.

My point is you can certainly do optional args with sys, and in fact for very simple scripts it is often not necessary to use argparse if that suffices!

Brier