Rework sub-command dispatching

The dispatching of sub-commands has always been a bit messy, with string comparisons that are prone to become outdated once changes are made. To that end, this change reworks this dispatching and gets rid of this problem. We use the subparser's set_defaults method to unconditionally set the 'method' attribute which can be invoked uniformly by callers.

Author: d-e-s-o

btrfs-backup/src/deso/btrfs/main.py

@@ -304,6 +304,7 @@ def addBackupParser(parser):
  "backup", add_help=False, formatter_class=SubLevelHelpFormatter,
  help="Backup one or more subvolumes.",
  )
+ backup.set_defaults(method=lambda x: x.backup)
 
  required = backup.add_argument_group("Required arguments")
  addRequiredArgs(required)
@@ -329,6 +330,7 @@ def addRestoreParser(parser):
  "restore", add_help=False, formatter_class=SubLevelHelpFormatter,
  help="Restore subvolumes or snapshots from a repository.",
  )
+ restore.set_defaults(method=lambda x: x.restore)
 
  required = restore.add_argument_group("Required arguments")
  addRequiredArgs(required)
@@ -386,7 +388,7 @@ def prepare(filters):
  else:
  return split(filters)
 
- command = ns.command
+ method = ns.method
  src_repo = ns.src
  dst_repo = ns.dst
  remote_cmd = ns.remote_cmd
@@ -422,8 +424,9 @@ def prepare(filters):
  del ns.src
  del ns.dst
  del ns.command
+ del ns.method
 
- return command, subvolumes, src_repo, dst_repo
+ return method, subvolumes, src_repo, dst_repo
 
 
 def main(argv):
@@ -455,15 +458,7 @@ def main(argv):
  namespace = parser.parse_args(args)
 
  with alias(namespace) as ns:
- command, subvolumes, src_repo, dst_repo = prepareNamespace(ns)
-
- if command == "backup":
- method = lambda x: x.backup
- elif command == "restore":
- method = lambda x: x.restore
- else:
- assert False
-
+ method, subvolumes, src_repo, dst_repo = prepareNamespace(ns)
  return run(method, subvolumes, src_repo, dst_repo, **vars(ns))