"poplib" --- POP3 protocol client
*********************************

**Source code:** Lib/poplib.py

======================================================================

This module defines a class, "POP3", which encapsulates a connection
to a POP3 server and implements the protocol as defined in **RFC
1939**. The "POP3" class supports both the minimal and optional
command sets from **RFC 1939**. The "POP3" class also supports the
"STLS" command introduced in **RFC 2595** to enable encrypted
communication on an already established connection.

Additionally, this module provides a class "POP3_SSL", which provides
support for connecting to POP3 servers that use SSL as an underlying
protocol layer.

Note that POP3, though widely supported, is obsolescent.  The
implementation quality of POP3 servers varies widely, and too many are
quite poor. If your mailserver supports IMAP, you would be better off
using the "imaplib.IMAP4" class, as IMAP servers tend to be better
implemented.

The "poplib" module provides two classes:

class poplib.POP3(host, port=POP3_PORT[, timeout])

   This class implements the actual POP3 protocol.  The connection is
   created when the instance is initialized. If *port* is omitted, the
   standard POP3 port (110) is used. The optional *timeout* parameter
   specifies a timeout in seconds for the connection attempt (if not
   specified, the global default timeout setting will be used).

   Raises an auditing event "poplib.connect" with arguments "self",
   "host", "port".

   All commands will raise an auditing event "poplib.putline" with
   arguments "self" and "line", where "line" is the bytes about to be
   sent to the remote host.

   在 3.9 版更改: If the *timeout* parameter is set to be zero, it
   will raise a "ValueError" to prevent the creation of a non-blocking
   socket.

class poplib.POP3_SSL(host, port=POP3_SSL_PORT, keyfile=None, certfile=None, timeout=None, context=None)

   This is a subclass of "POP3" that connects to the server over an
   SSL encrypted socket.  If *port* is not specified, 995, the
   standard POP3-over-SSL port is used.  *timeout* works as in the
   "POP3" constructor. *context* is an optional "ssl.SSLContext"
   object which allows bundling SSL configuration options,
   certificates and private keys into a single (potentially long-
   lived) structure.  Please read Security considerations for best
   practices.

   *keyfile* and *certfile* are a legacy alternative to *context* -
   they can point to PEM-formatted private key and certificate chain
   files, respectively, for the SSL connection.

   Raises an auditing event "poplib.connect" with arguments "self",
   "host", "port".

   All commands will raise an auditing event "poplib.putline" with
   arguments "self" and "line", where "line" is the bytes about to be
   sent to the remote host.

   在 3.2 版更改: *context* parameter added.

   在 3.4 版更改: The class now supports hostname check with
   "ssl.SSLContext.check_hostname" and *Server Name Indication* (see
   "ssl.HAS_SNI").

   3.6 版后已移除: *keyfile* and *certfile* are deprecated in favor of
   *context*. Please use "ssl.SSLContext.load_cert_chain()" instead,
   or let "ssl.create_default_context()" select the system's trusted
   CA certificates for you.

   在 3.9 版更改: If the *timeout* parameter is set to be zero, it
   will raise a "ValueError" to prevent the creation of a non-blocking
   socket.

One exception is defined as an attribute of the "poplib" module:

exception poplib.error_proto

   Exception raised on any errors from this module (errors from
   "socket" module are not caught). The reason for the exception is
   passed to the constructor as a string.

参见:

  Module "imaplib"
     The standard Python IMAP module.

  Frequently Asked Questions About Fetchmail
     The FAQ for the **fetchmail** POP/IMAP client collects
     information on POP3 server variations and RFC noncompliance that
     may be useful if you need to write an application based on the
     POP protocol.


POP3 Objects
============

All POP3 commands are represented by methods of the same name, in
lower-case; most return the response text sent by the server.

An "POP3" instance has the following methods:

POP3.set_debuglevel(level)

   Set the instance's debugging level.  This controls the amount of
   debugging output printed.  The default, "0", produces no debugging
   output.  A value of "1" produces a moderate amount of debugging
   output, generally a single line per request.  A value of "2" or
   higher produces the maximum amount of debugging output, logging
   each line sent and received on the control connection.

POP3.getwelcome()

   Returns the greeting string sent by the POP3 server.

POP3.capa()

   Query the server's capabilities as specified in **RFC 2449**.
   Returns a dictionary in the form "{'name': ['param'...]}".

   3.4 新版功能.

POP3.user(username)

   Send user command, response should indicate that a password is
   required.

POP3.pass_(password)

   Send password, response includes message count and mailbox size.
   Note: the mailbox on the server is locked until "quit()" is called.

POP3.apop(user, secret)

   Use the more secure APOP authentication to log into the POP3
   server.

POP3.rpop(user)

   Use RPOP authentication (similar to UNIX r-commands) to log into
   POP3 server.

POP3.stat()

   Get mailbox status.  The result is a tuple of 2 integers: "(message
   count, mailbox size)".

POP3.list([which])

   Request message list, result is in the form "(response, ['mesg_num
   octets', ...], octets)". If *which* is set, it is the message to
   list.

POP3.retr(which)

   Retrieve whole message number *which*, and set its seen flag.
   Result is in form "(response, ['line', ...], octets)".

POP3.dele(which)

   Flag message number *which* for deletion.  On most servers
   deletions are not actually performed until QUIT (the major
   exception is Eudora QPOP, which deliberately violates the RFCs by
   doing pending deletes on any disconnect).

POP3.rset()

   Remove any deletion marks for the mailbox.

POP3.noop()

   Do nothing.  Might be used as a keep-alive.

POP3.quit()

   Signoff:  commit changes, unlock mailbox, drop connection.

POP3.top(which, howmuch)

   Retrieves the message header plus *howmuch* lines of the message
   after the header of message number *which*. Result is in form
   "(response, ['line', ...], octets)".

   The POP3 TOP command this method uses, unlike the RETR command,
   doesn't set the message's seen flag; unfortunately, TOP is poorly
   specified in the RFCs and is frequently broken in off-brand
   servers. Test this method by hand against the POP3 servers you will
   use before trusting it.

POP3.uidl(which=None)

   Return message digest (unique id) list. If *which* is specified,
   result contains the unique id for that message in the form
   "'response mesgnum uid", otherwise result is list "(response,
   ['mesgnum uid', ...], octets)".

POP3.utf8()

   Try to switch to UTF-8 mode. Returns the server response if
   successful, raises "error_proto" if not. Specified in **RFC 6856**.

   3.5 新版功能.

POP3.stls(context=None)

   Start a TLS session on the active connection as specified in **RFC
   2595**. This is only allowed before user authentication

   *context* parameter is a "ssl.SSLContext" object which allows
   bundling SSL configuration options, certificates and private keys
   into a single (potentially long-lived) structure.  Please read
   Security considerations for best practices.

   This method supports hostname checking via
   "ssl.SSLContext.check_hostname" and *Server Name Indication* (see
   "ssl.HAS_SNI").

   3.4 新版功能.

Instances of "POP3_SSL" have no additional methods. The interface of
this subclass is identical to its parent.


POP3 Example
============

Here is a minimal example (without error checking) that opens a
mailbox and retrieves and prints all messages:

   import getpass, poplib

   M = poplib.POP3('localhost')
   M.user(getpass.getuser())
   M.pass_(getpass.getpass())
   numMessages = len(M.list()[1])
   for i in range(numMessages):
       for j in M.retr(i+1)[1]:
           print(j)

At the end of the module, there is a test section that contains a more
extensive example of usage.
