Accessing argument values for argparse in Python

Question

I am trying to set up some simple flag arguments for my program but cannot figure out how to access them. I have the argparser:

 parser = argparse.ArgumentParser(description='Simple PostScript Interpreter')
 parser.add_argument('-s', action="store_true")
 parser.add_argument('-d', action="store_true")
 parser.parse_args(sys.argv[1:])

The program should take either sps.py -s, sps.py -d, or sps.py on the command line. Then I just want to check whether or not the -s flag was set or the -d flag was set. If neither were set, then just default to -d.

What do I need to do to access the boolean values that are set by the parser?

Answer 1

You don't need to give parse_args() any parameters. You call it like this:

>>> args = parser.parse_args()

which will return a NameSpace object. You can access your arguments using the dot notation:

>>> args.s
False
>>> args.d
False

Working example:

import argparse
parser = argparse.ArgumentParser(description='Simple PostScript Interpreter')
parser.add_argument('-s', action="store_true")
parser.add_argument('-d', action="store_true")
args = parser.parse_args()
print args

Running it like so:

msvalkon@Lunkwill:/tmp$ python sps.py
Namespace(d=False, s=False)
msvalkon@Lunkwill:/tmp$ python sps.py -d
Namespace(d=True, s=False)
msvalkon@Lunkwill:/tmp$ python sps.py -s
Namespace(d=False, s=True)

• Matt Hintzke: when I do that and enter sps.py -d it says error: unrecognized arguments: -d
• msvalkon: Running python sps.py -d or -s works fine for me with your code, when I remove the sys.argv[1:] from parse_args(). How exactly are you running this?
• Matt Hintzke: i took out the sys.argv[1:] as well. I got it working, but I had to keep the add_argument() calls for both -d and -s. After doing that, the Namespace obj was created correctly
• msvalkon: Sorry if I was unclear, the only part where you had a problem with the code was the sys.argv[1:] which would've resulted in weird option values. You do need to have the add_argument() for any command line argument you wish to implement. Perhaps I should've talked about function arguments, or parameters instead of just arguments to avoid confusion.
• elijahfhopp: Thanks so much @msvalkon for this answer, I spent forever looking through the docs, and missed the section on the namespace class (docs.python.org/3/library/argparse.html#the-namespace-object). This is a super helpful answer!

Answer 2

Try adding this:

args = parser.parse_args()
print args.s
print args.d

Answer 3

Your existing code is mostly correct:

parser = argparse.ArgumentParser(description='Simple PostScript Interpreter')
parser.add_argument('-s', action="store_true")
parser.add_argument('-d', action="store_true")
args = parser.parse_args()

although the default argument to parse_args makes passing sys.argv[1:] unnecessary. Since each argument is parsed independently, you'll need a post-processing step after the arguments are parsed:

if not args.s and not args.d:
 args.s = True