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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+++ b/subcommand.py
@@ -0,0 +1,178 @@
+# 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 alter the usage: 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 main module with a name starting with 'CMD' will be a
+    subcommand.
+  - The module's docstring will be used in the default help page.
+"""
+
+import difflib
+import sys
+
+# This is the main module where to search for commands. It should never be
+# modified except for unit tests, where the main module is the unit test so this
+# value needs to be updated manually.
+MAIN_MODULE = sys.modules['__main__']

[iannucci, 2013/08/15 20:46:04]

Hm.. I don't like this (esp. modifying it in the test).

Why not put the functionality into a class, and then you can just pass the class
the module to handle. Then you don't need to rely on magic globals.

You could even have the default module value for the class be
sys.modules['__main__']

[M-A Ruel, 2013/08/15 20:56:55]

I'll prototype this and will reply.

+
+
+def usage(more):
+  """Adds a 'usage_more' property to a CMD function."""
+  def hook(fn):
+    fn.usage_more = more
+    return fn
+  return hook

[iannucci, 2013/08/15 20:46:04]

I've actually always wondered why this doesn't just the docstring of the
function (or maybe just the first line). Seems redundant.

In any event you should probably also use functools.wraps

[M-A Ruel, 2013/08/15 20:56:55]

wraps() is not applicable because it's not a hook, it's modifying the function
itself.

It's not in the docstring because it's a separate OptionParser item,
parser.usage != parser.description. The usage is printed first.

[iannucci, 2013/08/15 21:23:22]

Ah right.
Right I see that, I'm just meaning that maybe there could just be a convention
of

def CMDthingy(...):
  """"[lots of things]
  
  This runs the thingy command on |lots of things|
  """"
  ...

I guess I'm +-0 on this though :)


Also, if you're going to require the @usage decorator and modify the function,
why not just look for [x for x in module.__dict__.values() if hasattr(x,
'usage_more')] instead of the weird CMD name hacking?

+
+
+def get_commands():
+  """Returns a dict of command and their handling function.
+
+  The commands must be in the '__main__' modules. To import a command from a

[iannucci, 2013/08/15 20:46:04]

I think some of these docstrings need to be updated a bit too.

+  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(MAIN_MODULE, fn))
+      for fn in dir(MAIN_MODULE) if fn.startswith('CMD'))
+  cmds.setdefault('help', CMDhelp)
+  return cmds
+
+
+def get_command(name):

[iannucci, 2013/08/15 20:46:04]

Hm... I would expect this to be equivalent to get_commands()[name], but it has
some additional magic. Maybe the function name needs to be clarified a bit?

[M-A Ruel, 2013/08/15 20:56:55]

That was ripped directly from git_cl.py The docstring explains what it does. And
the function name says it gets a command. What's your suggestion?

[iannucci, 2013/08/15 21:23:22]

maybe find_closest_command?

[M-A Ruel, 2013/08/16 13:47:20]

Done.

+  """Retrieves the function to handle a command.
+
+  It automatically tries to guess the intended command by handling typos or
+  incomplete names.
+  """
+  commands = get_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 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
+
+
+def add_command_usage(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 __main__ module docstring as the help.
+    parser.description = sys.modules['__main__'].__doc__.strip() + '\n'

[iannucci, 2013/08/15 20:46:04]

This shouldn't be sys.modules['__main__'], I don't think?

[M-A Ruel, 2013/08/15 20:56:55]

Exact, fixing.

+  else:
+    parser.description = command.__doc__.strip() or ''
+    if parser.description:
+      parser.description += '\n'
+  parser.set_usage('usage: %%prog %s [options] %s' % (name, more))
+
+
+def dispatch_command(parser, args):
+  """Dispatches execution to the right command.
+
+  Fallbacks to 'help'.
+  """
+  commands = get_commands()
+  length = max(len(c) for c in commands)
+
+  def gen_summary(x):
+    """Creates a oneline summary from the docstring."""
+    line = x.__doc__.split('\n', 1)[0].rstrip('.')
+    return (line[0].lower() + line[1:]).strip()
+
+  docs = sorted(
+      (name, gen_summary(handler)) for name, handler in commands.iteritems())
+
+  # Lists all the commands in 'help'.
+  if commands['help']:
+    commands['help'].usage_more = (
+        '\n\nCommands are:\n' + '\n'.join(
+            '  %-*s %s' % (length, name, doc) for name, doc in docs))
+
+  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 = get_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 = get_command(args[0])
+
+      # "fix" the usage and the description now that we know the subcommand.
+      add_command_usage(parser, command)
+      return command(parser, args[1:])
+
+  if commands['help']:
+    # Not a known command. Default to help.
+    add_command_usage(parser, commands['help'])
+    return commands['help'](parser, args)
+
+  # Nothing can be done.
+  return 2