list matches what is on the server, using the TOP command in
POP to fetch headers text; use if inbox can change due to
deletes in other client, or automatic action by email server;
raises except if out of synch, or error while talking to server;

for speed, only checks last in last: this catches inbox deletes,
but assumes server won't insert before last (true for incoming
mails); check inbox size first: smaller if just deletes; else
top will differ if deletes and newly arrived messages added at
end; result valid only when run: inbox may change after return;
"""
self.trace('synch check')
errormsg = 'Message index out of synch with mail server.\n'
errormsg += 'Mail client may require restart or reload.'
server = self.connect()
try:
lastmsgnum = len(synchHeaders) # 1..N
(msgCount, msgBytes) = server.stat() # inbox size
if lastmsgnum > msgCount: # fewer now?
raise MessageSynchError(errormsg) # none to cmp
if self.srvrHasTop:
resp, hdrlines, respsz = server.top(lastmsgnum, 0) # hdrs only
hdrlines = self.decodeFullText(hdrlines)
lastmsghdrs = '\n'.join(hdrlines)
if not self.headersMatch(lastmsghdrs, synchHeaders[-1]):
raise MessageSynchError(errormsg)
finally:
server.quit()

def headersMatch(self, hdrtext1, hdrtext2):
""""
may not be as simple as a string compare: some servers add
a "Status:" header that changes over time; on one ISP, it
begins as "Status: U" (unread), and changes to "Status: RO"
(read, old) after fetched once - throws off synch tests if
new when index fetched, but have been fetched once before
delete or last-message check; "Message-id:" line is unique
per message in theory, but optional, and can be anything if
forged; match more common: try first; parsing costly: try last
"""

try match by simple string compare

if hdrtext1 == hdrtext2:
self.trace('Same headers text')
return True

try match without status lines

split1 = hdrtext1.splitlines() # s.split('\n'), but no final ''
split2 = hdrtext2.splitlines()
strip1 = [line for line in split1 if not line.startswith('Status:')]
strip2 = [line for line in split2 if not line.startswith('Status:')]
if strip1 == strip2:
self.trace('Same without Status')
return True

try mismatch by message-id headers if either has one

974 | Chapter 13: Client-Side Scripting