[commands] Dispatch command_error on command exec error.

Provide fallback on_command_error - will only fire if no cog handlers and
no local handler.
Propagate exceptions in checks and argument parsing to bot.

discord/ext/commands/bot.py

@@ -29,11 +29,12 @@ import discord
 import inspect
 import importlib
 import sys
+import traceback
 
 from .core import GroupMixin, Command, command
 from .view import StringView
 from .context import Context
-from .errors import CommandNotFound
+from .errors import CommandNotFound, CommandError
 from .formatter import HelpFormatter
 
 def _get_variable(name):
@@ -247,6 +248,26 @@ class Bot(GroupMixin, discord.Client):
                 coro = self._run_extra(event, event_name, *args, **kwargs)
                 discord.compat.create_task(coro, loop=self.loop)
 
+    @asyncio.coroutine
+    def on_command_error(self, exception, context):
+        """|coro|
+
+        The default command error handler provided by the bot.
+
+        By default this prints to ``sys.stderr`` however it could be
+        overridden to have a different implementation.
+
+        This only fires if you do not specify any listeners for command error.
+        """
+        if self.extra_events.get('on_command_error', None):
+            return
+
+        if hasattr(context.command, "on_error"):
+            return
+
+        print('Ignoring exception in command {}'.format(context.command), file=sys.stderr)
+        traceback.print_exception(type(exception), exception, exception.__traceback__, file=sys.stderr)
+
     # utility "send_*" functions
 
     def say(self, *args, **kwargs):
@@ -618,7 +639,12 @@ class Bot(GroupMixin, discord.Client):
             command = self.commands[invoker]
             self.dispatch('command', command, ctx)
             ctx.command = command
+            try:
                 yield from command.invoke(ctx)
+            except CommandError as e:
+                command.handle_local_error(e, ctx)
+                self.dispatch('command_error', e, ctx)
+            else:
                 self.dispatch('command_completion', command, ctx)
         else:
             exc = CommandNotFound('Command "{}" is not found'.format(invoker))

discord/ext/commands/core.py

@@ -44,7 +44,10 @@ def inject_context(ctx, coro):
         _internal_channel = ctx.message.channel
         _internal_author = ctx.message.author
 
+        try:
             ret = yield from coro(*args, **kwargs)
+        except Exception as e:
+            raise CommandError("Exception raised while executing command") from e
         return ret
     return wrapped
 
@@ -306,7 +309,6 @@ class Command:
 
     @asyncio.coroutine
     def _parse_arguments(self, ctx):
-        try:
         ctx.args = [] if self.instance is None else [self.instance]
         ctx.kwargs = {}
         args = ctx.args
@@ -350,15 +352,9 @@ class Command:
                         args.append(transformed)
                     except RuntimeError:
                         break
-
-        except CommandError as e:
-            self.handle_local_error(e, ctx)
-            ctx.bot.dispatch('command_error', e, ctx)
-            return False
         return True
 
     def _verify_checks(self, ctx):
-        try:
         if not self.enabled:
             raise DisabledCommand('{0.name} command is disabled'.format(self))
 
@@ -367,11 +363,6 @@ class Command:
 
         if not self.can_run(ctx):
             raise CheckFailure('The check functions for command {0.name} failed.'.format(self))
-        except CommandError as exc:
-            self.handle_local_error(exc, ctx)
-            ctx.bot.dispatch('command_error', exc, ctx)
-            return False
-
         return True
 
     @asyncio.coroutine