diff --git a/offlineimap/folder/Base.py b/offlineimap/folder/Base.py
index c89b191..7900099 100644
--- a/offlineimap/folder/Base.py
+++ b/offlineimap/folder/Base.py
@@ -223,7 +223,7 @@ class BaseFolder():
         :returns: Boolean indicating the match. Returns True in case it
             implicitely saved the UIDVALIDITY."""
 
-        if self.get_saveduidvalidity() != None:
+        if self.get_saveduidvalidity() is not None:
             return self.get_saveduidvalidity() == self.get_uidvalidity()
         else:
             self.save_uidvalidity()
@@ -939,7 +939,7 @@ class BaseFolder():
 
         # Execute new mail hook if we have new mail.
         if self.have_newmail:
-            if self.newmail_hook != None:
+            if self.newmail_hook is not None:
                 self.newmail_hook()
 
     def __syncmessagesto_delete(self, dstfolder, statusfolder):
diff --git a/offlineimap/folder/Gmail.py b/offlineimap/folder/Gmail.py
index 8f1b8ef..f3d2cf3 100644
--- a/offlineimap/folder/Gmail.py
+++ b/offlineimap/folder/Gmail.py
@@ -148,7 +148,7 @@ class GmailFolder(IMAPFolder):
         for messagestr in response:
             # looks like: '1 (FLAGS (\\Seen Old) X-GM-LABELS (\\Inbox \\Favorites) UID 4807)' or None if no msg
             # Discard initial message number.
-            if messagestr == None:
+            if messagestr is None:
                 continue
             messagestr = messagestr.split(' ', 1)[1]
             # e.g.: {'X-GM-LABELS': '("Webserver (RW.net)" "\\Inbox" GInbox)', 'FLAGS': '(\\Seen)', 'UID': '275440'}
diff --git a/offlineimap/folder/IMAP.py b/offlineimap/folder/IMAP.py
index bbbd32a..e574a38 100644
--- a/offlineimap/folder/IMAP.py
+++ b/offlineimap/folder/IMAP.py
@@ -140,7 +140,7 @@ class IMAPFolder(BaseFolder):
             # SELECT (if not already done) and get current UIDVALIDITY.
             self.__selectro(imapobj)
             typ, uidval = imapobj.response('UIDVALIDITY')
-            assert uidval != [None] and uidval != None, \
+            assert uidval != [None] and uidval is not None, \
                 "response('UIDVALIDITY') returned [None]!"
             self._uidvalidity = int(uidval[-1])
             return self._uidvalidity
@@ -237,16 +237,16 @@ class IMAPFolder(BaseFolder):
 
         conditions = []
         # 1. min_uid condition.
-        if min_uid != None:
+        if min_uid is not None:
             conditions.append("UID %d:*" % min_uid)
         # 2. date condition.
-        elif min_date != None:
+        elif min_date is not None:
             # Find out what the oldest message is that we should look at.
             conditions.append("SINCE %02d-%s-%d" % (
                 min_date[2], MonthNames[min_date[1]], min_date[0]))
         # 3. maxsize condition.
         maxsize = self.getmaxsize()
-        if maxsize != None:
+        if maxsize is not None:
             conditions.append("SMALLER %d" % maxsize)
 
         if len(conditions) >= 1:
@@ -535,7 +535,7 @@ class IMAPFolder(BaseFolder):
 
         if rtime is None:
             rtime = emailutil.get_message_date(content)
-            if rtime == None:
+            if rtime is None:
                 return None
         datetuple = time.localtime(rtime)
 
diff --git a/offlineimap/folder/Maildir.py b/offlineimap/folder/Maildir.py
index 590a93c..86f4f6e 100644
--- a/offlineimap/folder/Maildir.py
+++ b/offlineimap/folder/Maildir.py
@@ -199,9 +199,9 @@ class MaildirFolder(BaseFolder):
                     nouidcounter -= 1
                 else:
                     uid = int(uidmatch.group(1))
-            if min_uid != None and uid > 0 and uid < min_uid:
+            if min_uid is not None and uid > 0 and uid < min_uid:
                 continue
-            if min_date != None and not self._iswithintime(filename, min_date):
+            if min_date is not None and not self._iswithintime(filename, min_date):
                 # Keep track of messages outside of the time limit, because they
                 # still might have UID > min(UIDs of within-min_date). We hit
                 # this case for maxage if any message had a known/valid datetime
@@ -218,7 +218,7 @@ class MaildirFolder(BaseFolder):
                 retval[uid] = self.msglist_item_initializer(uid)
                 retval[uid]['flags'] = flags
                 retval[uid]['filename'] = filepath
-        if min_date != None:
+        if min_date is not None:
             # Re-include messages with high enough uid's.
             positive_uids = [uid for uid in retval if uid > 0]
             if positive_uids: