Revive the UserError/UsageError distinction

UsageError was removed back in

  commit bf3d434b244c57556bec979acbc658c30eb58221
  Author: W. Trevor King <wking@drexel.edu>
  Date:   Sat Dec 12 00:31:55 2009 -0500

      Added libbe.command.base (with Command class)...

because the distinction between UsageError and UserError was unclear.
I've brought it back to satisfy a request by Christian Heinrich:

On Sun, May 01, 2011 at 02:52:13AM +0200, Christian Heinrich wrote:
> 3.) Using wrong syntax should receive better help messages.
>
> Current:
>
> "be new" -> ERROR:
> Missing required argument SUMMARY
>
> Should be:
>
> "be new" -> usage: be new [options] SUMMARY
> ...

He suggested we print the full option list as well, but I've decided
to just print the usage summary and remind the user how to get the
full help message if they want it.

2 files changed, 30 insertions, 6 deletions

diff --git a/libbe/command/__init__.py b/libbe/command/__init__.py
index 1cad096..832218c 100644
--- a/libbe/command/__init__.py
+++ b/libbe/command/__init__.py
@@ -19,6 +19,7 @@
 import base
 
 UserError = base.UserError
+UsageError = base.UsageError
 UnknownCommand = base.UnknownCommand
 get_command = base.get_command
 get_command_class = base.get_command_class
@@ -33,7 +34,7 @@ UnconnectedStorageGetter = base.UnconnectedStorageGetter
 StorageCallbacks = base.StorageCallbacks
 UserInterface = base.UserInterface
 
-__all__ = [UserError, UnknownCommand,
+__all__ = [UserError, UsageError, UnknownCommand,
            get_command, get_command_class, commands,
            Option, Argument, Command,
            InputOutput, StdInputOutput, StringInputOutput,
diff --git a/libbe/command/base.py b/libbe/command/base.py
index b5f5a22..48bb7ab 100644
--- a/libbe/command/base.py
+++ b/libbe/command/base.py
@@ -27,13 +27,36 @@ import libbe.ui.util.user
 import libbe.util.encoding
 import libbe.util.plugin
 
-class UserError(Exception):
+
+class UserError (Exception):
+    "An error due to improper BE usage."
     pass
 
-class UnknownCommand(UserError):
-    def __init__(self, cmd):
-        Exception.__init__(self, "Unknown command '%s'" % cmd)
-        self.cmd = cmd
+
+class UsageError (UserError):
+    """A serious parsing error due to invalid BE command construction.
+
+    The distinction between `UserError`\s and the more specific
+    `UsageError`\s is that when displaying a `UsageError` to the user,
+    the user is pointed towards the command usage information.  Use
+    the more general `UserError` if you feel that usage information
+    would not be particularly enlightening.
+    """
+    def __init__(self, command=None, command_name=None, message=None):
+        super(UsageError, self).__init__(message)
+        self.command = command
+        if command_name is None and command is not None:
+            command_name = command.name
+        self.command_name = command_name
+        self.message = message
+
+
+class UnknownCommand (UsageError):
+    def __init__(self, command_name):
+        super(UnknownCommand, self).__init__(
+            command_name=command_name,
+            message="Unknown command '%s'" % command_name)
+
 
 def get_command(command_name):
     """Retrieves the module for a user command