core (memory/persistent): added clear method + initialise with a dict:

A "clear" method has been added to delete all data for a specific namespace (and profile).
Internal cache (self._cache) is not initialised with an empty dict instead of None to allow using methods like __setitem__ or clear without having to do a potentially expansive `load()` before. The only counterpart is that we won't have an exception anymore if developer forget to load() data before using them, which can make debugging a bit more difficult.

sat/memory/persistent.py

@@ -44,7 +44,7 @@ class PersistentDict(object):
         if not self.storage:
             log.error(_("PersistentDict can't be used before memory initialisation"))
             raise MemoryNotInitializedError
-        self._cache = None
+        self._cache = {}
         self.namespace = namespace
         self.profile = profile
 
@@ -111,13 +111,19 @@ class PersistentDict(object):
         return self._cache.__getitem__(key)
 
     def __setitem__(self, key, value):
-        self.storage.setPrivateValue(self.namespace, key, value, self.binary, self.profile)
+        self.storage.setPrivateValue(self.namespace, key, value, self.binary,
+                                     self.profile)
         return self._cache.__setitem__(key, value)
 
     def __delitem__(self, key):
         self.storage.delPrivateValue(self.namespace, key, self.binary, self.profile)
         return self._cache.__delitem__(key)
 
+    def clear(self):
+        """Delete all values from this namespace"""
+        self._cache.clear()
+        return self.storage.delPrivateNamespace(self.namespace, self.binary, self.profile)
+
     def get(self, key, default=None):
         return self._cache.get(key, default)
 
@@ -133,7 +139,8 @@ class PersistentDict(object):
 
         @return: deferred fired when data is actually saved
         """
-        return self.storage.setPrivateValue(self.namespace, name, self._cache[name], self.binary, self.profile)
+        return self.storage.setPrivateValue(self.namespace, name, self._cache[name],
+                                            self.binary, self.profile)
 
 
 class PersistentBinaryDict(PersistentDict):