Implement foldersort in a python3 compatible way

By default we sort folders alphabetically (for IMAP) according to their
transposed names. For python3, we need to bend a bit backwards to still
allow the use of a cmp() function for foldersort. While going through, I
discovered that we never sort folders for Maildir.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap.conf

@@ -496,13 +496,14 @@ remoteuser = username
 # one.  For example:
 # folderincludes = ['debian.user', 'debian.personal']
 
-# You can specify foldersort to determine how folders are sorted.
+# You can specify 'foldersort' to determine how folders are sorted.
 # This affects order of synchronization and mbnames. The expression
-# should return -1, 0, or 1, as the default Python cmp() does.  The
+# should return -1, 0, or 1, as the default Python cmp() does.  The two
-# two arguments, x and y, are strings representing the names of the folders
+# arguments, x and y, are strings representing the names of the folders
-# to be sorted.  The sorting is applied *AFTER* nametrans, if any.
+# to be sorted.  The sorting is applied *AFTER* nametrans, if any.  The
-#
+# default is to sort IMAP folders alphabetically
-# To reverse the sort:
+# (case-insensitive). Usually, you should never have to modify this.  To
+# eg. reverse the sort:
 #
 # foldersort = lambda x, y: -cmp(x, y)
 

offlineimap/repository/Base.py

@@ -45,7 +45,7 @@ class BaseRepository(CustomConfig.ConfigHelperMixin, object):
         self.nametrans = lambda foldername: foldername
         self.folderfilter = lambda foldername: 1
         self.folderincludes = []
-        self.foldersort = cmp
+        self.foldersort = None
         if self.config.has_option(self.getsection(), 'nametrans'):
             self.nametrans = self.localeval.eval(
                 self.getconf('nametrans'), {'re': re})

offlineimap/repository/IMAP.py

@@ -309,7 +309,23 @@ class IMAPRepository(BaseRepository):
             finally:
                 self.imapserver.releaseconnection(imapobj)
 
-        retval.sort(lambda x, y: self.foldersort(x.getvisiblename(), y.getvisiblename()))
+        if self.foldersort is None:
+            # default sorting by case insensitive transposed name
+            retval.sort(key=lambda x: str.lower(x.getvisiblename()))
+        else:
+            # do foldersort in a python3-compatible way
+            # http://bytes.com/topic/python/answers/844614-python-3-sorting-comparison-function
+            def cmp2key(mycmp):
+                """Converts a cmp= function into a key= function
+                We need to keep cmp functions for backward compatibility"""
+                class K:
+                    def __init__(self, obj, *args):
+                        self.obj = obj
+                    def __cmp__(self, other):
+                        return mycmp(self.obj, other.obj)
+                return K
+            retval.sort(key=cmp2key(self.foldersort))
+
         self.folders = retval
         return self.folders