diff --git a/ldapcherry/backend/__init__.py b/ldapcherry/backend/__init__.py
index 1957589..5634ede 100644
--- a/ldapcherry/backend/__init__.py
+++ b/ldapcherry/backend/__init__.py
@@ -11,36 +11,107 @@ from ldapcherry.exceptions import MissingParameter
 class Backend:
 
     def __init__(self, config, logger, name, attrslist, key):
+        """ Initialize the backend
+        :param config: the configuration of the backend
+        :type config: dict {'config key': 'value'}
+        :param logger: the cherrypy error logger object
+        :type logger: python logger
+        :param name: id of the backend
+        :type name: string
+        :param attrslist: list of the backend attributes
+        :type attrslist: list of strings
+        :param key: the key attribute
+        :type key: string
+        """
         raise Exception()
 
     def auth(self, username, password):
+        """ Check authentication against the backend
+        :param username: 'key' attribute of the user
+        :type username: string
+        :param password: password of the user
+        :type password: string
+        :rtype: boolean (True is authentication success, False otherwise)
+        """
         return False
 
     def add_user(self, attrs):
+        """ Add a user to the backend
+        :param attrs: attributes of the user
+        :type attrs: dict ({<attr>: <value>})
+        """
         pass
 
     def del_user(self, username):
+        """ Delete a user from the backend
+        :param username: 'key' attribute of the user
+        :type username: string
+        """
         pass
 
     def set_attrs(self, username, attrs):
+        """ Set a list of attributes for a given user
+        :param username: 'key' attribute of the user
+        :type username: string
+        :param attrs: attributes of the user
+        :type attrs: dict ({<attr>: <value>})
+        """
         pass
 
     def add_to_groups(self, username, groups):
+        """ Add a user to a list of groups
+        :param username: 'key' attribute of the user
+        :type username: string
+        :param groups: list of groups
+        :type groups: list of strings
+        """
         pass
 
     def del_from_groups(self, username, groups):
+        """ Delete a user from a list of groups
+        :param username: 'key' attribute of the user
+        :type username: string
+        :param groups: list of groups
+        :type groups: list of strings
+        """
         pass
 
     def search(self, searchstring):
+        """ Search backend for users
+        :param searchstring: the search string
+        :type searchstring: string
+        :rtype: dict of dict ( {<user attr key>: {<attr>: <value>}} )
+        """
         return {}
 
     def get_user(self, username):
+        """ Get a user's attributes
+        :param username: 'key' attribute of the user
+        :type username: string
+        :rtype: dict ( {<attr>: <value>} )
+        """
         return {}
 
     def get_groups(self, username):
+        """ Get a user's groups
+        :param username: 'key' attribute of the user
+        :type username: string
+        :rtype: list of groups
+        """
         return []
 
     def get_param(self, param, default=None):
+        """ Get a parameter in config (handle default value)
+
+        :param param: name of the parameter to recover
+        :type param: string
+        :param default: the default value, raises an exception
+            if param is not in configuration and default
+            is None (which is the default value).
+        :type default: string or None
+        :rtype: the value of the parameter or the default value if
+            not set in configuration
+        """
         if param in self.config:
             return self.config[param]
         elif default is not None: