--- a/wokkel/mam.py	Thu Apr 23 10:57:40 2015 +0200
+++ b/wokkel/mam.py	Thu Apr 23 13:35:21 2015 +0200
@@ -85,15 +85,11 @@
     @itype form: C{unicode}
     """
 
-    form = None
-    rsm = None
-    node = None
-
-    def __init__(self, form=None, rsm=None, node=None):
+    def __init__(self, form=None, rsm_=None, node=None):
         if form is not None:
-            assert(form.formType == 'submit')
+            assert form.formType == 'submit'
         self.form = form
-        self.rsm = rsm
+        self.rsm = rsm_
         self.node = node
 
     @classmethod
@@ -141,7 +137,7 @@
         @return: MAM request element.
         @rtype: L{Element<twisted.words.xish.domish.Element>}
         """
-        assert(parent.name == 'iq')
+        assert parent.name == 'iq'
         mam_elt = self.toElement()
         parent.addChild(mam_elt)
         return mam_elt
@@ -152,7 +148,7 @@
     A Message Archive Management <prefs/> request.
 
     @param default: A value in ('always', 'never', 'roster').
-    @type : C{unicode}
+    @type : C{unicode} or C{None}
 
     @param always (list): A list of JID instances.
     @type always: C{list}
@@ -161,24 +157,19 @@
     @type never: C{list}
     """
 
-    default = None
-    always = None
-    never = None
-
-    def __init__(self, default=None, always=None, never=None):
-        if default:
-            assert(default in ('always', 'never', 'roster'))
-            self.default = default
-        if always:
-            assert(isinstance(always, list))
-            self.always = always
+    def __init__(self, default, always=None, never=None):
+        assert default in ('always', 'never', 'roster')
+        self.default = default
+        if always is not None:
+            assert isinstance(always, list)
         else:
-            self.always = []
-        if never:
-            assert(isinstance(never, list))
-            self.never = never
+            always = []
+        self.always = always
+        if never is not None:
+            assert isinstance(never, list)
         else:
-            self.never = []
+            never = []
+        self.never = never
 
     @classmethod
     def parse(cls, element):
@@ -229,7 +220,7 @@
         @return: MAM request element.
         @rtype: L{Element<twisted.words.xish.domish.Element>}
         """
-        assert(parent.name == 'iq')
+        assert parent.name == 'iq'
         mam_elt = self.toElement()
         parent.addChild(mam_elt)
         return mam_elt
@@ -329,7 +320,7 @@
         @rtype: L{Deferred<twisted.internet.defer.Deferred>}
         """
         # http://xmpp.org/extensions/xep-0313.html#prefs
-        assert(default is not None)
+        assert default is not None
         iq = IQ(self.xmlstream, 'set')
         MAMPrefs(default, always, never).render(iq)
         if sender is not None:
@@ -408,14 +399,13 @@
                                },
                       }
 
-    extra_filters = {}
-
     def __init__(self, resource):
         """
         @param resource: instance implementing IMAMResource
         @type resource: L{object}
         """
         self.resource = resource
+        self.extra_filters = {}
 
     def connectionInitialized(self):
         """