ArgumentParser: optional argument with optional value
If I have an optional argument with an optional argument value, is there a way to check if an argument is set when no value is set?
For example:
parser = argparse.ArgumentParser() parser.add_argument('--abc', nargs='?') args = parser.parse_args()
I would correctly give me:
optional arguments:
--abc [ABC]
How can I distinguish between 1 and 2 below?
- '' => args.abc is None
- '- abc' => args.abc is still missing
- '- abc something' => args.abc is something.
...
Update:
Found a trick to solve this problem: you can use "nargs = '*'" instead of "nargs = '?". So # 1 will return None and # 2 will return an empty list. The disadvantage is that this will also allow multiple values for the arguments; so you need to add a check for it if needed.
Alternatively, you can also set a default value for the argument; see answer from Chapner and Anand S. Kumar.
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Use a different default for the parameter. Compare
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--abc', nargs='?', default="default")
>>> parser.parse_args()
Namespace(abc='default')
>>> parser.parse_args(['--abc'])
Namespace(abc=None)
>>> parser.parse_args(['--abc', 'value'])
Namespace(abc='value')
I'm not sure how you could provide a different value if --abc
used without an argument, instead of using a custom action instead of an argument nargs
.
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Not sure if this is the standard way, but you can set an argument default
to something, and then that value will be used in case it is --abc
not in the argument list.
Sample code -
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--abc', nargs='?', default="-1")
args = parser.parse_args()
print(args)
Result -
>python a.py
Namespace(abc='-1')
>python a.py --abc
Namespace(abc=None)
>python a.py --abc something
Namespace(abc='something')
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With, nargs='?'
you can specify both default
and const
.
In [791]: parser=argparse.ArgumentParser()
In [792]: parser.add_argument('--abc', nargs='?', default='default', const='const')
If no argument is specified, it uses the default:
In [793]: parser.parse_args([])
Out[793]: Namespace(abc='default')
If given but without an argument string, a constant is used:
In [794]: parser.parse_args(['--abc'])
Out[794]: Namespace(abc='const')
Otherwise, it uses the argument string:
In [795]: parser.parse_args(['--abc','test'])
Out[795]: Namespace(abc='test')
In [796]: parser.print_help()
usage: ipython3 [-h] [--abc [ABC]]
optional arguments:
-h, --help show this help message and exit
--abc [ABC]
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