Is this mailbox manipulation working by luck, or can't I understand my own code?

[MRAB]

On 2018-10-13 16:15, Chris Green wrote:
> I use a Python script (called directly by '| <name of script>' in
> .forward) which routes incoming mail to various mailboxes according to
> the mailing list it's from (plus a few other criteria).  The first
> lines of the program are:-
> 
>      #!/usr/bin/python
>      #
>      #
>      # Mail filtering script
>      #
>      import email
>      import mailbox
>      import os
>      import sys
>      import time
>      import mailLib
>      import shlex
>      #
>      #
>      # Redirect any exceptions to a file
>      #
>      sys.stderr = open("/home/chris/tmp/mail.err", 'a')
>      #
>      #
>      # Some constants (i.e. configuration)
>      #
>      home = "/home/chris"
>      logfile = home + "/tmp/mail.log"
>      filtfile = home + "/.mutt/filter"
>      mldir = home + "/Mail/"
>      indir = mldir + "In/"
>      judir = mldir + "Ju/"
>      #
>      #
>      # Set to log to mail.log in ~/tmp with name 'filter' and the
>      envelope/from
>      #
>      log = mailLib.initLog("filter")
>      #
>      #
>      # Initialise destination mailbox name to empty
>      #
>      dest = ""
>      #
>      #
>      # Read the message from standard input and make a message object from
>      it
>      #
>      # msg = email.message_from_string(sys.stdin.read())
>      msg = mailbox.mboxMessage(sys.stdin.read())
>      #
>      #
>      # Extract the To:, Cc: and Subject: headers and the envelope/from
>      #
>      msgcc = msg.get("Cc", "unknown").lower()
>      msgto = msg.get("To", "unknown").lower()
>      msgsb = msg.get("Subject", "unknown")
>      msgfm = msg.get("From", "unknown").lower()
> 
> I then do various things according to the values in msgcc, msgto,
> msgsb and msgfm.  The program works well and I've been using it for
> quite a few years.
> 
> Now I want to do some minor changes and I can't for the life of me see
> how those lines with msg.get(... in them work.  The mailbox.mboxMessage()
> class doesn't have a get() method and the get() method for the parent
> class mailbox() doesn't seem to do what it appears to be doing here.
> 
> Can anyone suggest how this might be working?  What will those
> msg.get() calls return?  Will it simply be the whole message (there's
> always only one message fed into the program from .froward) or is
> there something going on that means it does work as I intended by
> getting the content of the individual header lines.
> 
> Looking at the rest of the code it *might* work OK even if all of
> msgcc, msgto, msgsb and msgfm contain the whole message.  It just
> wouldn't be working quite as I intended! :-)
> 
'mailbox.mboxMessage' is class 'mboxMessage' in file 'mailbox.py' in the 
stdlib.

In file 'mailbox.py' in the stdlib, class 'mboxMessage' inherits from 
'_mboxMMDFMessage', which inherits from '_mboxMMDFMessage', which 
inherits from 'Message', which inherits from 'email.message.Message' 
(class 'Message' from file 'email/message.py' in the stdlib.

Those msg.get() calls return is the contents (a string) of the relevant 
field in the message's header.

This:

     msg.get("Cc", "unknown").lower()

returns the contents of the "Cc" field if it exists, or the default 
value "unknown" if it doesn't exist.

That string is then converted to lowercase.