[Python-Dev] PEP 389: argparse - new command line parsing module (original) (raw)

Robert Kern robert.kern at gmail.com
Wed Sep 30 23:01:32 CEST 2009

On 2009-09-30 15:17 PM, Andrew McNabb wrote:

On Wed, Sep 30, 2009 at 02:22:53PM -0400, Barry Warsaw wrote:

Right. I've made it kind of work in Mailman 3, but it would be nice for argparse to support this out of the box. Note that I think you want two forms: foo help subcommand foo subcommand --help to basically print the same help text. This is the way bzr does it for example and it works great. In some commands, options as well as subcommands can change subsequent parsing. The iptables command is a good example of a command-line program that follows this practice. From the man page: after [a module name is specified], various extra command line options become available, depending on the specific module. You can specify multiple extended match modules in one line, and you can use the -h or --help options after the module has been specified to receive help specific to that module. In the case of iptables, module names are specified as options, not as subcommands. From my cursory reading of the documentation, it looks like argparse can only add subparsers for subcommands. Is there any way to add subparsers based on options instead (as iptables does)?

I have not done so, but I suspect so. The implementation of .add_subparsers() adds to the positional argument list, but one could be written to append to the option list.

Also, is it possible to add these subparsers dynamically? For example, you would want to be able to load a module immediately after parsing the name instead of having to keep a predetermined list of all module names. I'm pretty sure that bzr dynamically loads modules this way. Can argparse help with this?

Not out-of-box, but it looks fairly straightforward to plug in. The subparser logic is mostly encapsulated in the _SubparsersAction class. You can register a new class for it:

 parser.register('action', 'parsers', MySubParsersAction)

-- Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco

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