Python argparse and controlling/overriding the exit status code

Question

Apart from tinkering with the argparse source, is there any way to control the exit status code should there be a problem when parse_args() is called, for example, a missing required switch?

Answer 1

I'm not aware of any mechanism to specify an exit code on a per-argument basis. You can catch the SystemExit exception raised on .parse_args() but I'm not sure how you would then ascertain what specifically caused the error.

EDIT: For anyone coming to this looking for a practical solution, the following is the situation:

• ArgumentError() is raised appropriately when arg parsing fails. It is passed the argument instance and a message
• ArgumentError() does not store the argument as an instance attribute, despite being passed (which would be convenient)
• It is possible to re-raise the ArgumentError exception by subclassing ArgumentParser, overriding .error() and getting hold of the exception from sys.exc_info()

All that means the following code - whilst ugly - allows us to catch the ArgumentError exception, get hold of the offending argument and error message, and do as we see fit:

import argparse
import sys

class ArgumentParser(argparse.ArgumentParser):    
    def _get_action_from_name(self, name):
        """Given a name, get the Action instance registered with this parser.
        If only it were made available in the ArgumentError object. It is 
        passed as it's first arg...
        """
        container = self._actions
        if name is None:
            return None
        for action in container:
            if '/'.join(action.option_strings) == name:
                return action
            elif action.metavar == name:
                return action
            elif action.dest == name:
                return action

    def error(self, message):
        exc = sys.exc_info()[1]
        if exc:
            exc.argument = self._get_action_from_name(exc.argument_name)
            raise exc
        super(ArgumentParser, self).error(message)

## usage:
parser = ArgumentParser()
parser.add_argument('--foo', type=int)
try:
    parser.parse_args(['--foo=d'])
except argparse.ArgumentError, exc:
    print exc.message, '\n', exc.argument

Not tested in any useful way. The usual don't-blame-me-if-it-breaks indemnity applies.