Zero or one argument [duplicate]

Question

Which action I have to use, when parameter can be with zero or one argument? I have to run code like this and will be set default value 4 (this variant exit with error)

--pretty-xml
error: argument --pretty-xml: expected one argument

but alse I can run code like this

--pretty-xml = 2 

my code looks like this

parameter.add_argument('--pretty-xml',  dest="pretty", action="append", default=[4])

What needs to be changed to work for both options parameters ? Thanks

Answer 1

You need to use nargs argument with ? and the default and const parameters:

'?'. One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from default will be produced. ...

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parser.add_argument('--pretty-xml', dest='pretty', default=4, const=5, nargs='?', type=int)

This will have the value n if called like --pretty-xml=n, or 5 if called like --pretty-xml or 4 if --pretty-xml isn't present on the command line at all.

NOTE: used type=int to check type and convert to int type. Otherwise returns string if the argument is specified, returns int 4 if no argument is specified.