python, argparse: enable input parameter when another one has been specified

问题:

In my python script, I want to be able to use an optional input parameter only when another optional parameter has been specified. Example:

$ python myScript.py --parameter1 value1 $ python myScript.py --parameter1 value1 --parameter2 value2

But NOT:

$ python myScript.py --parameter2 value2

How do I do this with argparse?

Thanks!

回答1:

Use a custom action:

import argparse  foo_default=None      class BarAction(argparse.Action):     def __call__(self,parser,namespace,values,option_string=None):         didfoo=getattr(namespace,'foo',foo_default)         if(didfoo == foo_default):             parser.error( "foo before bar!")         else:             setattr(namespace,self.dest,values)  parser=argparse.ArgumentParser() parser.add_argument('--foo',default=foo_default) parser.add_argument('--bar',action=BarAction,help="Only use this if --foo is set")  #testing. print parser.parse_args('--foo baz'.split()) print parser.parse_args('--foo baz --bar cat'.split()) print parser.parse_args('--bar dog'.split())

This can even be done in a little easier to maintain way if you're OK with relying on some undocumented behavior of argparse:

import argparse  parser=argparse.ArgumentParser() first_action=parser.add_argument('--foo',dest='cat',default=None)  class BarAction(argparse.Action):     def __call__(self,parser,namespace,values,option_string=None):         didfoo=getattr(namespace,first_action.dest,first_action.default)         if(didfoo == first_action.default):             parser.error( "foo before bar!")         else:             setattr(namespace,self.dest,values)  parser.add_argument('--bar',action=BarAction,                     help="Only use this if --foo is set")  #testing. print parser.parse_args('--foo baz'.split()) print parser.parse_args('--foo baz --bar cat'.split()) print parser.parse_args('--bar dog'.split())

In this example, we get the default for foo and it's destination from the action object returned by add_argument (add_argument's return value isn't documented anywhere that I can find). This is still a little fragile (If you want to specify a type= keyword to the --foo argument for example).

Finally, you can check sys.argv before parsing.

import sys if ("--parameter2" in sys.argv) and ("--parameter1" not in sys.argv):     parser.error("parameter1 must be given if parameter2 is given")

This gets a little more tricky if --parameter1 could also be triggered by --p1, but you get the idea. Then you could use

if (set(sys.argv).intersection(('--p2',...)) and      not set(sys.argv).intersection(('--p1',...)))

The advantage here is that it doesn't require any particular order. (--p2 doesn't need to follow --p1 on the commandline). And, as before, you can get the list of command strings that will trigger your particular action via the option_strings attribute returned by parser.add_argument(...). e.g.

import argparse import sys    parser=argparse.ArgumentParser() action1=parser.add_argument('--foo') action2=parser.add_argument('--bar',                             help="Only use this if --foo is set")  argv=set(sys.argv) if (( argv &