# Command Errors

Handling errors in OpenStackClient commands is fairly straightforward. An exception is thrown and handled by the application-level caller.

Note: There are many cases that need to be filled out here. The initial version of this document considers the general command error handling as well as the specific case of commands that make multiple REST API calls and how to handle when one or more of those calls fails.

## General Command Errors

The general pattern for handling OpenStackClient command-level errors is to raise a CommandError exception with an appropriate message. This should include conditions arising from arguments that are not valid/allowed (that are not otherwise enforced by `argparse`) as well as errors arising from external conditions.

### External Errors

External errors are a result of things outside OpenStackClient not being as expected.

#### Example

This example is taken from `keypair create` where the `--public-key` option specifies a file containing the public key to upload. If the file is not found, the `IOError` exception is trapped and a more specific `CommandError` exception is raised that includes the name of the file that was attempted to be opened.

    class CreateKeypair(command.ShowOne):
        """Create new public key"""

        ## ...

        def take_action(self, parsed_args):
            compute_client = self.app.client_manager.compute

            public_key = parsed_args.public_key
            if public_key:
                try:
                    with open(
                        os.path.expanduser(parsed_args.public_key),
                        "rb"
                    ) as p:
                        public_key = p.read()
                except IOError as e:
                    msg = _("Key file %s not found: %s")
                    raise exceptions.CommandError(
                        msg % (parsed_args.public_key, e),
                    )

            keypair = compute_client.create_keypair(
                name=parsed_args.name,
                public_key=public_key,
            )

            ## ...

## REST API Errors

Most commands make a single REST API call via the supporting client library or SDK. Errors based on HTML return codes are usually handled well by default, but in some cases more specific or user-friendly messages need to be logged. Trapping the exception and raising a CommandError exception with a useful message is the correct approach.

### Multiple REST API Calls

Some CLI commands make multiple calls to library APIs and thus REST APIs. Most of the time these are `create` or `set` commands that expect to add or change a resource on the server. When one of these calls fails, the behaviour of the remainder of the command handler is defined as such:

- Whenever possible, all API calls will be made. This may not be possible for specific commands where the subsequent calls are dependent on the results of an earlier call.

- Any failure of an API call will be logged for the user

- A failure of any API call results in a non-zero exit code

- In the cases of failures in a `create` command a follow-up mode needs to be present that allows the user to attempt to complete the call, or cleanly remove the partially-created resource and re-try.

The desired behaviour is for commands to appear to the user as idempotent whenever possible, i.e. a partial failure in a `set` command can be safely retried without harm. `create` commands are a harder problem and may need to be handled by having the proper options in a set command available to allow recovery in the case where the primary resource has been created but the subsequent calls did not complete.

#### Example 1

This example is taken from the `volume snapshot set` command where `--property` arguments are set using the volume manager’s `set_metadata()` method, `--state` arguments are set using the `reset_state()` method, and the remaining arguments are set using the `update()` method.

    class SetSnapshot(command.Command):
    """Set snapshot properties"""

    ## ...

    def take_action(self, parsed_args):
        volume_client = self.app.client_manager.volume
        snapshot = utils.find_resource(
            volume_client.volume_snapshots,
            parsed_args.snapshot,
        )

        kwargs = {}
        if parsed_args.name:
            kwargs['name'] = parsed_args.name
        if parsed_args.description:
            kwargs['description'] = parsed_args.description

        result = 0
        if parsed_args.property:
            try:
                volume_client.volume_snapshots.set_metadata(
                    snapshot.id,
                    parsed_args.property,
                )
            except SomeException:      # Need to define the exceptions to catch here
                LOG.error(_("Property set failed"))
                result += 1

        if parsed_args.state:
            try:
                volume_client.volume_snapshots.reset_state(
                    snapshot.id,
                    parsed_args.state,
                )
            except SomeException:      # Need to define the exceptions to catch here
                LOG.error(_("State set failed"))
                result += 1

        try:
            volume_client.volume_snapshots.update(
                snapshot.id,
                **kwargs
            )
        except SomeException:      # Need to define the exceptions to catch here
            LOG.error(_("Update failed"))
            result += 1

        # NOTE(dtroyer): We need to signal the error, and a non-zero return code,
        #                without aborting prematurely
        if result > 0:
            raise SomeNonFatalException

#### Example 2

This example is taken from the `network delete` command which takes multiple networks to delete. All networks will be deleted in a loop, which makes multiple `delete_network()` calls.

    class DeleteNetwork(common.NetworkDelete):
        """Delete network(s)"""

        def get_parser(self, prog_name):
            parser = super().get_parser(prog_name)
            parser.add_argument(
                'network',
                metavar="<network>",
                nargs="+",
                help=_("Network(s) to delete (name or ID)")
            )
            return parser

        def take_action_delete(self, parsed_args):
            client = self.app.client_manager.network
            obj = client.find_network(network, ignore_missing=False)
            client.delete_network(obj)

this page last updated: 2026-07-15 15:33:41

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