[commands] Add the concept of global checks.

Global checks are checks that are executed before regular per-command
checks except done to every command that the bot has registered. This
allows you to have checks that apply to every command without having to
override `on_message` or appending the check to every single command.

discord/ext/commands/bot.py

@@ -208,6 +208,7 @@ class Bot(GroupMixin, discord.Client):
         self.extra_events = {}
         self.cogs = {}
         self.extensions = {}
+        self._checks = []
         self.description = inspect.cleandoc(description) if description else ''
         self.pm_help = pm_help
         self.command_not_found = options.pop('command_not_found', 'No command called "{}" found.')
@@ -443,6 +444,70 @@ class Bot(GroupMixin, discord.Client):
         destination = _get_variable('_internal_channel')
         return self.send_typing(destination)
 
+    # global check registration
+
+    def check(self):
+        """A decorator that adds a global check to the bot.
+
+        A global check is similar to a :func:`check` that is applied
+        on a per command basis except it is run before any command checks
+        have been verified and applies to every command the bot has.
+
+        .. warning::
+
+            This function must be a *regular* function and not a coroutine.
+
+        Similar to a command :func:`check`\, this takes a single parameter
+        of type :class:`Context` and can only raise exceptions derived from
+        :exc:`CommandError`.
+
+        Example
+        ---------
+
+        .. code-block:: python
+
+            @bot.check
+            def whitelist(ctx):
+                return ctx.message.author.id in my_whitelist
+
+        """
+        def decorator(func):
+            self.add_check(func)
+            return func
+        return decorator
+
+    def add_check(self, func):
+        """Adds a global check to the bot.
+
+        This is the non-decorator interface to :meth:`check`.
+
+        Parameters
+        -----------
+        func
+            The function that was used as a global check.
+        """
+        self._checks.append(func)
+
+    def remove_check(self, func):
+        """Removes a global check from the bot.
+
+        This function is idempotent and will not raise an exception
+        if the function is not in the global checks.
+
+        Parameters
+        -----------
+        func
+            The function to remove from the global checks.
+        """
+
+        try:
+            self._checks.remove(func)
+        except ValueError:
+            pass
+
+    def can_run(self, ctx):
+        return all(f(ctx) for f in self._checks)
+
     # listener registration
 
     def add_listener(self, func, name=None):
@@ -543,6 +608,9 @@ class Bot(GroupMixin, discord.Client):
         They are meant as a way to organize multiple relevant commands
         into a singular class that shares some state or no state at all.
 
+        The cog can also have a ``__check`` member function that allows
+        you to define a global check. See :meth:`check` for more info.
+
         More information will be documented soon.
 
         Parameters
@@ -552,6 +620,14 @@ class Bot(GroupMixin, discord.Client):
         """
 
         self.cogs[type(cog).__name__] = cog
+
+        try:
+            check = getattr(cog, '_{.__class__.__name__}__check'.format(cog))
+        except AttributeError:
+            pass
+        else:
+            self.add_check(check)
+
         members = inspect.getmembers(cog)
         for name, member in members:
             # register commands the cog has
@@ -613,11 +689,20 @@ class Bot(GroupMixin, discord.Client):
             if name.startswith('on_'):
                 self.remove_listener(member)
 
+        try:
+            check = getattr(cog, '_{0.__class__.__name__}__check'.format(cog))
+        except AttributeError:
+            pass
+        else:
+            self.remove_check(check)
+
         unloader_name = '_{0.__class__.__name__}__unload'.format(cog)
         try:
-            getattr(cog, unloader_name)()
+            unloader = getattr(cog, unloader_name)
         except AttributeError:
             pass
+        else:
+            unloader()
 
         del cog
 

discord/ext/commands/core.py

@@ -395,8 +395,11 @@ class Command:
         if self.no_pm and ctx.message.channel.is_private:
             raise NoPrivateMessage('This command cannot be used in private messages.')
 
+        if not ctx.bot.can_run(ctx):
+            raise CheckFailure('The global check functions for command {0.qualified_name} failed.'.format(self))
+
         if not self.can_run(ctx):
-            raise CheckFailure('The check functions for command {0.name} failed.'.format(self))
+            raise CheckFailure('The check functions for command {0.qualified_name} failed.'.format(self))
 
     @asyncio.coroutine
     def invoke(self, ctx):