fix: folder/*: never set self.messagelist to None

Empty the list by setting an empty dict.
Introduce BaseFolder().ismessagelistempty().

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Base.py

@@ -248,8 +248,17 @@ class BaseFolder(object):
 
         raise NotImplementedError
 
+    def ismessagelistempty(self):
+        """Empty everythings we know about messages."""
+
+        if len(self.messagelist.keys()) < 1:
+            return True
+        return False
+
     def dropmessagelistcache(self):
-        raise NotImplementedException
+        """Empty everythings we know about messages."""
+
+        self.messagelist = {}
 
     def getmessagelist(self):
         """Gets the current message list.

offlineimap/folder/GmailMaildir.py

@@ -65,7 +65,7 @@ class GmailMaildirFolder(MaildirFolder):
 
 
     def cachemessagelist(self):
-        if self.messagelist is None:
+        if self.ismessagelistempty():
             self.messagelist = self._scanfolder()
 
         # Get mtimes

offlineimap/folder/IMAP.py

@@ -48,7 +48,7 @@ class IMAPFolder(BaseFolder):
         self.expunge = repository.getexpunge()
         self.root = None # imapserver.root
         self.imapserver = imapserver
-        self.messagelist = None
+        self.messagelist = {}
         self.randomgenerator = random.Random()
         #self.ui is set in BaseFolder
         self.imap_query = ['BODY.PEEK[]']
@@ -249,7 +249,7 @@ class IMAPFolder(BaseFolder):
                 self.messagelist[uid] = {'uid': uid, 'flags': flags, 'time': rtime}
 
     def dropmessagelistcache(self):
-        self.messagelist = None
+        self.messagelist = {}
 
     # Interface from BaseFolder
     def getmessagelist(self):

offlineimap/folder/LocalStatus.py

@@ -155,9 +155,6 @@ class LocalStatusFolder(BaseFolder):
         self.readstatus(cachefd)
         cachefd.close()
 
-    def dropmessagelistcache(self):
-        self.messagelist = None
-
     def save(self):
         """Save changed data to disk. For this backend it is the same as saveall."""
 

offlineimap/folder/LocalStatusSQLite.py

@@ -202,7 +202,7 @@ class LocalStatusSQLiteFolder(BaseFolder):
             self.messagelist[uid]['mtime'] = row[2]
 
     def dropmessagelistcache(self):
-        self.messagelist = None
+        self.messagelist = {}
 
     # Interface from LocalStatusFolder
     def save(self):

offlineimap/folder/Maildir.py

@@ -63,7 +63,7 @@ class MaildirFolder(BaseFolder):
         super(MaildirFolder, self).__init__(name, repository)
         self.dofsync = self.config.getdefaultboolean("general", "fsync", True)
         self.root = root
-        self.messagelist = None
+        self.messagelist = {}
         # check if we should use a different infosep to support Win file systems
         self.wincompatible = self.config.getdefaultboolean(
             "Account "+self.accountname, "maildir-windows-compatible", False)
@@ -220,12 +220,9 @@ class MaildirFolder(BaseFolder):
 
     # Interface from BaseFolder
     def cachemessagelist(self):
-        if self.messagelist is None:
+        if self.ismessagelistempty():
             self.messagelist = self._scanfolder()
 
-    def dropmessagelistcache(self):
-        self.messagelist = None
-
     # Interface from BaseFolder
     def getmessagelist(self):
         return self.messagelist