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Compatibility Between POP Servers
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POP servers are often notoriously bad at correctly following standards. Standards also simply do not exist for some POP behaviors, so these details are left up to the authors of server software. So basic operations will generally work fine, but certain behaviors can vary from server to server. For instance, some servers will mark all of your messages as read whenever you connect to the server whether you download any of them or not! while other servers will mark a given message as read only when it is downloaded. Some servers, on the other hand, never mark any messages as read at all. The standard itself seems to assume the latter behavior, but is not clear either way. Keep these differences in mind as you read this chapter.
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Connecting and Authenticating
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POP supports several authentication methods. The two most common are basic username-password authentication, and APOP, which is an optional extension to POP that helps protect passwords from being sent in plain-text if you are using an ancient POP server that does not support SSL.
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CHAPTER 14 POP
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The process of connecting and authenticating to a remote server looks like this in Python: 1. 2. Create a POP3_SSL or just a plain POP3 object, and pass the remote hostname and port to it. Call user() and pass_() to send the username and password. Note the underscore in pass_()! It is present because pass is a keyword in Python and cannot be used for a method name. If the exception poplib.error_proto is raised, it means that the login has failed and the string value of the exception contains the error explanation sent by the server.
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The choice between POP3 and POP3_SSL is governed by whether your e-mail provider offers or, in this day and age, even requires that you connect over an encrypted connection. Consult 6 for more information about SSL, but the general guideline should be to use it whenever it is at all feasible for you to do so. Listing 14 1 uses the foregoing steps to log in to a remote POP server. Once connected, it calls stat(), which returns a simple tuple giving the number of messages in the mailbox and the messages total size. Finally, the program calls quit(), which closes the POP connection. Listing 14 1. A Very Simple POP Session #!/usr/bin/env python # POP connection and authentication - 14 - popconn.py import getpass, poplib, sys if len(sys.argv) != 3: print 'usage: %s hostname user' % sys.argv[0] exit(2) hostname, user = sys.argv[1:] passwd = getpass.getpass() p = poplib.POP3_SSL(hostname) # or "POP3" if SSL is not supported try: p.user(user) p.pass_(passwd) except poplib.error_proto, e: print "Login failed:", e else: status = p.stat() print "You have %d messages totaling %d bytes" % status finally: p.quit() You can test this program if you have a POP account somewhere. Most people do even large webmail services like GMail provide POP as an alternate means of checking your mailbox. Run the preceding program, giving it two command-line arguments: the hostname of your POP server, and your username. If you do not know this information, contact your Internet provider or network administrator; note that on some services your username will be a plain string (like guido), whereas on others it will be your full e-mail address (guido@example.com). The program will then prompt you for your password. Finally, it will display the mailbox status, without touching or altering any of your mail.
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CHAPTER 14 POP
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Caution! While this program does not alter any messages, some POP servers will nonetheless alter mailbox flags simply because you connected. Running the examples in this chapter against a live mailbox could cause you to lose information about which messages are read, unread, new, or old. Unfortunately, that behavior is serverdependent, and beyond the control of POP clients. I strongly recommend running these examples against a test mailbox rather than your live mailbox!
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Here is how you might run the program: $ ./popconn.py pop.example.com guido Password: (type your password) You have 3 messages totaling 5675 bytes If you see output like this, then your first POP conversation has taken place successfully! When POP servers do not support SSL to protect your connection from snooping, they sometimes at least support an alternate authentication protocol called APOP, which uses a challenge-response scheme to assure that your password is not sent in the clear. (But all of your e-mail will still be visible to any third party watching the packets go by!) The Python Standard Library makes this very easy to attempt: just call the apop() method, then fall back to basic authentication if the POP server you are talking to does not understand. To use APOP but fall back to plain authentication, you could use a stanza like the one shown in Listing 14 2 inside your POP program (like Listing 14 1). Listing 14 2. Attempting APOP and Falling Back print "Attempting APOP authentication..." try: p.apop(user, passwd) except poplib.error_proto: print "Attempting standard authentication..." try: p.user(user) p.pass_(passwd) except poplib.error_proto, e: print "Login failed:", e sys.exit(1)
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