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I now set the help string for an
argparse option with two parameters: 
parser.add_argument('-s', '--substitute', nargs=2,
 help='Replace first string with second',
 metavar='string')
which generates a help message
 -s string string, --substitute string string
 Replace first string with second
Instead I would *like* to generate the help message
 -s fromString toString, --substitute fromString toString
 Replace fromString with toString
 
At present metavar replaces each of multiple parameters with the same thing,
but with a fixed number of parameters it makes sense that each may have a
different meaning.
It seems to me that when nargs=k is set for an integer k,
we could have metavar be a list of k strings to go in the k places,
as in
parser.add_argument('-s', '--substitute', nargs=2,
 help='Replace fromString with toString',
 metavar=['fromString', 'toString'])
and have this generate my desired help message.
There is no need to break the current behavior:
The parser can distinguish a list from a single string. 
The same text is in the attached file.

Withdrawn. My error. I missed the part of the documentation that says a *tuple*, not the list that I tried, does just what I wanted.