Python argparse and controlling/overriding the exit status code

Solution 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.

Solution 2:

All the answers nicely explain the details of argparse implementation.

Indeed, as proposed in PEP (and pointed by Rob Cowie) one should inherit ArgumentParser and override the behavior of error or exit methods.

In my case I just wanted to replace usage print with full help print in case of the error:

class ArgumentParser(argparse.ArgumentParser):

    def error(self, message):
        self.print_help(sys.stderr)
        self.exit(2, '%s: error: %s\n' % (self.prog, message))

In case of override main code will continue to contain the minimalistic..

# Parse arguments.
args = parser.parse_args()
# On error this will print help and cause exit with explanation message.

Solution 3:

Perhaps catching the SystemExit exception would be a simple workaround:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('foo')
try:
    args = parser.parse_args()
except SystemExit:
    print("do something else")

Works for me, even in an interactive session.

Edit: Looks like @Rob Cowie beat me to the switch. Like he said, this doesn't have very much diagnostic potential, unless you want get silly and try to glean info from the traceback.