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> optparse, which argparse attempts to replace, permitted positional 
> arguments to be intermixed with optional arguments
Sure, but optparse didn't actually parse positional arguments - it just threw them into a bag, and then you had to group them and convert them however you wanted afterwards. Argparse, instead, was designed to let you specify the groups of positional arguments. Your situation is a little different because you just want to throw all the positional arguments into a bag again. Not that there's anything wrong with that - it's just not the primary use case argparse had in mind.
> The only definition of positional parameters I could find...
Yeah, it looks like there's no good documentation on positional vs. optional parameters. Somewhere obvious, perhaps right at the beginning of the add_argument() documentation, there should probably be something like:
Argparse groups the command line argument strings into two types of groups: optional arguments, which are a sequence of command line strings that begin with a flag like "-v" or "--verbose", and positional arguments, which are a sequence of command line strings that do not begin with a flag. The add_argument() method allows you to specify how many command line strings each of your optional or positional arguments should consume, how those strings should be converted into Python objects, etc.