Split generic subcommand code off its own module.

subcommand.py

Index: subcommand.py
diff --git a/subcommand.py b/subcommand.py
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+# Copyright (c) 2013 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Manages subcommands in a script.
+
+Each subcommand should look like this:
+  @usage('[pet name]')
+  def CMDpet(parser, args):
+    '''Prints a pet.
+
+    Many people likes pet. This command prints a pet for your pleasure.
+    '''
+    parser.add_option('--color', help='color of your pet')
+    options, args = parser.parse_args(args)
+    if len(args) != 1:
+      parser.error('A pet name is required')
+    pet = args[0]
+    if options.color:
+      print('Nice %s %d' % (options.color, pet))
+    else:
+      print('Nice %s' % pet)
+    return 0
+
+Explanation:
+  - usage decorator alters the 'usage: %prog' line in the command's help.
+  - docstring is used to both short help line and long help line.
+  - parser can be augmented with arguments.
+  - return the exit code.
+  - Every function in the specified module with a name starting with 'CMD' will
+    be a subcommand.
+  - The module's docstring will be used in the default 'help' page.
+  - If a command has no docstring, it will not be listed in the 'help' page.
+    Useful to keep compatibility commands around or aliases.
+  - If a command is an alias to another one, it won't be documented. E.g.:
+      CMDoldname = CMDnewcmd
+    will result in oldname not being documented but supported and redirecting to
+    newcmd. Make it a real function that calls the old function if you want it
+    to be documented.
+"""
+
+import difflib
+import sys
+
+
+def usage(more):
+  """Adds a 'usage_more' property to a CMD function."""
+  def hook(fn):
+    fn.usage_more = more
+    return fn
+  return hook
+
+
+def CMDhelp(parser, args):
+  """Prints list of commands or help for a specific command."""
+  # This is the default help implementation. It can be disabled or overriden if
+  # wanted.
+  if not any(i in ('-h', '--help') for i in args):
+    args = args + ['--help']
+  _, args = parser.parse_args(args)
+  # Never gets there.
+  assert False
+
+
+class CommandDispatcher(object):
+  def __init__(self, module):
+    """module is the name of the main python module where to look for commands.
+
+    The python builtin variable __name__ MUST be used for |module|. If the
+    script is executed in the form 'python script.py', __name__ == '__main__'
+    and sys.modules['script'] doesn't exist. On the other hand if it is unit
+    tested, __main__ will be the unit test's module so it has to reference to
+    itself with 'script'. __name__ always match the right value.

[iannucci, 2013/08/16 19:53:06]

Ah, good call. This works :)

+    """
+    self.module = sys.modules[module]
+
+  def enumerate_commands(self):
+    """Returns a dict of command and their handling function.
+
+    The commands must be in the '__main__' modules. To import a command from a
+    submodule, use:
+      from mysubcommand import CMDfoo
+
+    Automatically adds 'help' if not already defined.
+
+    A command can be effectively disabled by defining a global variable to None,
+    e.g.:
+      CMDhelp = None
+    """
+    cmds = dict(
+        (fn[3:], getattr(self.module, fn))
+        for fn in dir(self.module) if fn.startswith('CMD'))
+    cmds.setdefault('help', CMDhelp)
+    return cmds
+
+  def find_nearest_command(self, name):
+    """Retrieves the function to handle a command.
+
+    It automatically tries to guess the intended command by handling typos or
+    incomplete names.
+    """
+    commands = self.enumerate_commands()
+    if name in commands:
+      return commands[name]
+
+    # An exact match was not found. Try to be smart and look if there's
+    # something similar.
+    commands_with_prefix = [c for c in commands if c.startswith(name)]
+    if len(commands_with_prefix) == 1:
+      return commands[commands_with_prefix[0]]
+
+    # A #closeenough approximation of levenshtein distance.
+    def close_enough(a, b):
+      return difflib.SequenceMatcher(a=a, b=b).ratio()
+
+    hamming_commands = sorted(
+        ((close_enough(c, name), c) for c in commands),
+        reverse=True)
+    if (hamming_commands[0][0] - hamming_commands[1][0]) < 0.3:
+      # Too ambiguous.
+      return
+
+    if hamming_commands[0][0] < 0.8:
+      # Not similar enough. Don't be a fool and run a random command.
+      return
+
+    return commands[hamming_commands[0][1]]
+
+  def _add_command_usage(self, parser, command):
+    """Modifies an OptionParser object with the function's documentation."""
+    name = command.__name__[3:]
+    more = getattr(command, 'usage_more', '')
+    if name == 'help':
+      name = '<command>'
+      # Use the module's docstring as the description for the 'help' command if
+      # available.
+      parser.description = self.module.__doc__
+    else:
+      # Use the command's docstring if available.
+      parser.description = command.__doc__
+    parser.description = (parser.description or '').strip()
+    if parser.description:
+      parser.description += '\n'
+    parser.set_usage(
+        'usage: %%prog %s [options]%s' % (name, '' if not more else ' ' + more))
+
+  @staticmethod
+  def _create_command_summary(name, command):
+    """Creates a oneline summary from the command's docstring."""
+    if name != command.__name__[3:]:
+      # Skip aliases.
+      return ''
+    doc = command.__doc__ or ''
+    line = doc.split('\n', 1)[0].rstrip('.')
+    if not line:
+      return line
+    return (line[0].lower() + line[1:]).strip()
+
+  def execute(self, parser, args):
+    """Dispatches execution to the right command.
+
+    Fallbacks to 'help' if not disabled.
+    """
+    commands = self.enumerate_commands()
+    length = max(len(c) for c in commands)
+
+    # Lists all the commands in 'help'.
+    if commands['help']:
+      docs = sorted(
+          (name, self._create_command_summary(name, handler))
+          for name, handler in commands.iteritems())
+      # Skip commands without a docstring.
+      commands['help'].usage_more = (
+          '\n\nCommands are:\n' + '\n'.join(
+              '  %-*s %s' % (length, name, doc) for name, doc in docs if doc))
+
+    if args:
+      if args[0] in ('-h', '--help') and len(args) > 1:
+        # Inverse the argument order so 'tool --help cmd' is rewritten to
+        # 'tool cmd --help'.
+        args = [args[1], args[0]] + args[2:]
+      command = self.find_nearest_command(args[0])
+      if command:
+        if command.__name__ == 'CMDhelp' and len(args) > 1:
+          # Inverse the arguments order so 'tool help cmd' is rewritten to
+          # 'tool cmd --help'. Do it here since we want 'tool hel cmd' to work
+          # too.
+          args = [args[1], '--help'] + args[2:]
+          command = self.find_nearest_command(args[0]) or command
+
+        # "fix" the usage and the description now that we know the subcommand.
+        self._add_command_usage(parser, command)
+        return command(parser, args[1:])
+
+    if commands['help']:
+      # Not a known command. Default to help.
+      self._add_command_usage(parser, commands['help'])
+      return commands['help'](parser, args)
+
+    # Nothing can be done.
+    return 2