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diff --git a/cliapp/hook.py b/cliapp/hook.py
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+# Copyright (C) 2009-2012  Lars Wirzenius
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+
+'''Hooks with callbacks.
+
+In order to de-couple parts of the application, especially when plugins
+are used, hooks can be used. A hook is a location in the application
+code where plugins may want to do something. Each hook has a name and
+a list of callbacks. The application defines the name and the location
+where the hook will be invoked, and the plugins (or other parts of the
+application) will register callbacks.
+
+'''
+
+
+class Hook(object):
+
+    '''A hook.'''
+
+    def __init__(self):
+        self.callbacks = []
+
+    def add_callback(self, callback):
+        '''Add a callback to this hook.
+
+        Return an identifier that can be used to remove this callback.
+
+        '''
+
+        if callback not in self.callbacks:
+            self.callbacks.append(callback)
+        return callback
+
+    def call_callbacks(self, *args, **kwargs):
+        '''Call all callbacks with the given arguments.'''
+        for callback in self.callbacks:
+            callback(*args, **kwargs)
+
+    def remove_callback(self, callback_id):
+        '''Remove a specific callback.'''
+        if callback_id in self.callbacks:
+            self.callbacks.remove(callback_id)
+
+
+class FilterHook(Hook):
+
+    '''A hook which filters data through callbacks.
+
+    Every hook of this type accepts a piece of data as its first argument
+    Each callback gets the return value of the previous one as its
+    argument. The caller gets the value of the final callback.
+
+    Other arguments (with or without keywords) are passed as-is to
+    each callback.
+
+    '''
+
+    def call_callbacks(self, data, *args, **kwargs):
+        for callback in self.callbacks:
+            data = callback(data, *args, **kwargs)
+        return data
+