Undocumented feature: MHMailbox sets the msg object's _mh_msgno

attribute to the (stringized) message number (if this attribute is
settable).  This is useful so users of this class can report the
correct message number (e.g. when classifying spam).

Also added a blank line before the first method of each class.

Lib/mailbox.py

@@ -10,6 +10,7 @@
            "PortableUnixMailbox"]
 
 class _Mailbox:
+
     def __init__(self, fp, factory=rfc822.Message):
         self.fp = fp
         self.seekp = 0
@@ -35,6 +36,7 @@ def next(self):
 
 
 class _Subfile:
+
     def __init__(self, fp, start, stop):
         self.fp = fp
         self.start = start
@@ -94,6 +96,7 @@ def close(self):
 
 # Recommended to use PortableUnixMailbox instead!
 class UnixMailbox(_Mailbox):
+
     def _search_start(self):
         while 1:
             pos = self.fp.tell()
@@ -161,6 +164,7 @@ class PortableUnixMailbox(UnixMailbox):
 
 
 class MmdfMailbox(_Mailbox):
+
     def _search_start(self):
         while 1:
             line = self.fp.readline()
@@ -181,6 +185,7 @@ def _search_end(self):
 
 
 class MHMailbox:
+
     def __init__(self, dirname, factory=rfc822.Message):
         import re
         pat = re.compile('^[1-9][0-9]*$')
@@ -204,7 +209,12 @@ def next(self):
             return None
         fn = self.boxes.pop(0)
         fp = open(os.path.join(self.dirname, fn))
-        return self.factory(fp)
+        msg = self.factory(fp)
+        try:
+            msg._mh_msgno = fn
+        except (AttributeError, TypeError):
+            pass
+        return msg
 
 
 class Maildir:
@@ -238,6 +248,7 @@ def next(self):
 
 
 class BabylMailbox(_Mailbox):
+
     def _search_start(self):
         while 1:
             line = self.fp.readline()