I am not sure what optparse's metavar parameter is used for. I see it is used all around, but I can't see its use.
Can someone make it clear to me? Thanks.
As @Guillaume says, it's used for generating help. If you want to have an option that takes an argument, such as a filename, you can add the metavar parameter to the add_option call so your preferred argument name/descriptor is output in the help message. From the current module documentation:
usage = "usage: %prog [options] arg1 arg2"
parser = OptionParser(usage=usage)
parser.add_option("-f", "--filename",
metavar="FILE", help="write output to FILE"),
would produce help like this:
usage: <yourscript> [options] arg1 arg2
options:
-f FILE, --filename=FILE
The "FILE" after the "-f" and the "--filename" comes from the metavar.