Python argparse command line flags without arguments

Question

How do I add an optional flag to my command line args?

eg. so I can write

python myprog.py 

or

python myprog.py -w

I tried

parser.add_argument('-w')

But I just get an error message saying

Usage [-w W]
error: argument -w: expected one argument

which I take it means that it wants an argument value for the -w option. What's the way of just accepting a flag?

I'm finding http://docs.python.org/library/argparse.html rather opaque on this question.

Answer 1

As you have it, the argument w is expecting a value after -w on the command line. If you are just looking to flip a switch by setting a variable True or False, have a look here (specifically store_true and store_false)

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('-w', action='store_true')

where action='store_true' implies default=False.

Conversely, you could haveaction='store_false', which implies default=True.

Answer 2

Adding a quick snippet to have it ready to execute:

Source: myparser.py

import argparse
parser = argparse.ArgumentParser(description="Flip a switch by setting a flag")
parser.add_argument('-w', action='store_true')

args = parser.parse_args()
print args.w

Usage:

python myparser.py -w
>> True

Answer 3

Your script is right. But by default is of None type. So it considers true of any other value other than None is assigned to args.argument_name variable.

I would suggest you to add a action="store_true". This would make the True/False type of flag. If used its True else False.

import argparse
parser = argparse.ArgumentParser('parser-name')
parser.add_argument("-f","--flag",action="store_true",help="just a flag argument")

usage

$ python3 script.py -f

After parsing when checked with args.f it returns true,

args = parser.parse_args()
print(args.f)

>>>true

Answer 4

If you are looking for a binary flag, then the argparse actions store_true or store_false provide exactly this. This approach is well explained in the accepted answer by @Jdog.

The official docs are also fairly clear. I would only complete the example with one line, so to make it very clear how the store_true/store_false act:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='store_true')
>>> parser.add_argument('--fov', action='store_true')  # this is not in the docs!
>>> parser.add_argument('--bar', action='store_false')
>>> parser.add_argument('--baz', action='store_false')
>>> parser.parse_args('--foo --bar'.split())  # --baz and --fov are missing
Out[4]: Namespace(bar=False, baz=True, foo=True, fov=False)  # mind the fov=False

---

A slightly more powerful approach is to use the count action. You typically have used this type of flag already when setting the verbosity level when running a command.

For example ssh's verbose mode flag -v is a counter:

-v Verbose mode. Causes ssh to print debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum is 3.

So if you run ssh it's non verbose, ssh -v is slightly verbose and ssh -vvv is maximally verbose.

With argparse in python such a counter flag can be defined as follows:

parser.add_argument('--verbose', '-v', action='count', default=0)

If you want to use it as a boolena (True/False) flag, then you need to cast args.verbose into a boolean. You can either do this explicitly yourself, or rely a conditional statement like if args.verbose: ....

Here is a full working example to illustrate how you can use the counter flag:

With the script test.py:

#!/usr/bin/env python3
# test.py
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--verbose', '-v', action='count', default=0)
args = parser.parse_args()

if args.verbose:
    print('verbose')
    print(f'verbosity level: {args.verbose}')
else:
    print('non-verbose')

You get the following outputs:

python test.py 
>> non-verbose 

python test.py -v
>> verbose
>> verbosity level: 1

python test.py -vvv
>> verbose
>> verbosity level: 3

Answer 5

Here's a quick way to do it, won't require anything besides sys.. though functionality is limited:

flag = "--flag" in sys.argv[1:]

[1:] is in case if the full file name is --flag