I am creating a python script where I want to have an argument that manipulates how many search results you get as output. I've currently named the argument --head
. This is the functionality I'd like it to have:
When --head
is not passed at the command line I'd like it to default to one value. In this case, a rather big one, like 80
When --head
is passed without any value, I'd like it to default to another value. In this case, something limited, like 10
When --head
is passed with a value, I'd like it to store the value it was passed.
Here is some code describing the problem:
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('-h',
'--head',
dest='size',
const=80,
default=10,
action="I don't know",
help='Only print the head of the output')
>>> # OFC, that last line will fail because the action is uknown,
... # but here is how I'd like it to work
... parser.parse_args(''.split())
Namespace(size=80)
>>> parser.parse_args('--head'.split())
Namespace(size=10)
>>> parser.parse_args('--head 15'.split())
Namespace(size=15)
I know I probably can write a custom action for this, but I first want to see if there is any default behaviour that does this.
After a little more reading in the documentation I found what I needed: nargs='?'
.
This is used with the store
action, and does exactly what I want.
Here is an example:
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--head',
dest='size',
const=10,
default=80,
action='store',
nargs='?',
type=int,
help='Only print the head of the output')
>>> parser.parse_args(''.split())
... Namespace(size=80)
>>> parser.parse_args('--head'.split())
... Namespace(size=10)
>>> parser.parse_args('--head 15'.split())
... Namespace(size=15)
Source: http://docs.python.org/3/library/argparse.html#nargs