Tag Archives: arguments

Using Python’s argparse for a “turn on”/”turn off” argument

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What’s argparse ?

Argparse is a Python standard module and “makes it easy to write user-friendly command-line interfaces”. The 2.x doco is here, the 3.x doco is here. Before 2.7 there was the optparse module supplied as part of Python but that’s been deprecated and replaced with argparse.

“turn off” / “turn off” type arguments

I was working on some code yesterday and I wanted an argument of the “turn on” / “turn off” type. So for instance you might want the output to be verbose or not, it’s not uncommon to see this implemented by means of a

--verbose

argument. When ‘–verbose’ is present the programmer provides verbose output, when it’s absent they don’t.

How then ?

A nice neat way to do this is to make use of the `action` (2.x and 3.x) argument of the `add_argument` method and to combine that with use of the `set_defaults` method so that a value is set in the case when the argument is not used by the user.

Here’s an example taken from my django-row-count project :

parser.add_argument('--echotostdout', dest='echotostdout', action='store_true')
parser.set_defaults(echotostdout=False)
args = parser.parse_args()

In this case a command line argument …

--echotostdout

… sets an attribute

args.echotostdout

… to True if it’s present as an argument on the command line and to False if it’s absent.

optparse: simulating ‘–help’

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optparse: simulating ‘–help’

Summary

Getting optparse to show a summary of script options under program control.

What do you use optparse for ?

optparse is a Python standard library for parsing command-line options.

optparse provides a relatively simple way of

* defining a set of command line arguments which the user may enter
* parsing the command line entered and storing the results of the parse

I need help !

By default optparse will respond to a either of :

<yourscript> -h

or

<yourscript> --help

by printing a summary of your scripts options.

What I learned today

In the script I was working on today I wanted to respond to the user not entering any argument at all by printing a summary of all options available (in other words as if they user had entered ‘–help’ or ‘-h’ as an argument).

There is a way you can do this but for some reason it’s not in the list of OptionParser methods shown in the documentation.

print_help()

If you call the method ‘print_help’ as shown in the code below optparse will respond by by printing the same text that would be shown if the user were to enter an argument of ‘–help’.

parser = OptionParser(description=desc, usage=usage)
parser.add_option(  "-i", "--inbox", action="store",  dest="inbox",
metavar="INBOX", help="Location of INBOX")
parser.add_option(  "-o", "--outpath", action="store", dest="outpath",
metavar="PATH", help="PATH to output csv file")
parser.add_option(  "-v", "--verbose", action="store_true",
dest="verbose", help="Show each file processed")

(options, args) = parser.parse_args()

if (options.inbox is None) and (options.outpath is None):
  parser.print_help()
  exit(-1)
elif not os.path.exists(options.inbox):
  parser.error('inbox location does not exist')
elif not os.path.exists(os.path.dirname(options.outpath)):
  parser.error('path to ouput location does not exist')

return options