POP3 stuff, particularly a streaming downloader and a simple command line which runs it.

Latest release 20260531: POP3Command.cmd_dl: add -1 (once) option for testing, notice SIGINT (though the RETRs get queued so fast it hardly helps), new --limit=n option to limit the number of messages fetched.

I spend some time on a geostationary satellite connection, where round trip ping times are over 600ms when things are good.

My mail setup involves fetching messages from my inbox for local storage in my laptop, usually using POP3. The common standalone tools for this are fetchmail and getmail. However, both are very subject to the link latency, in that they request a message, collect it, issue a delete, then repeat. On a satellite link that incurs a cost of over a second per message, making catch up after a period offline a many minutes long exercise in tedium.

This module does something I've been meaning to do for literally years: a bulk fetch. It issues RETRieves for every message up front as fast as possible. A separate thread collects the messages as they are delivered and issues DELEtes for the saved messages as soon as each is saved.

This results in a fetch process which is orders of magnitude faster. Even on a low latency link the throughput is much faster; on the satellite it is gobsmackingly faster.

Short summary:

• ConnectionSpec: A specification for a POP3 connection.
• main: The pop3 command line mode.
• NetrcEntry: A namedtuple representation of a netrc entry.
• POP3: Simple POP3 class with support for streaming use.
• POP3Command: Command line implementation for POP3 operations.

Module contents:

• class ConnectionSpec(ConnectionSpec): A specification for a POP3 connection.

ConnectionSpec.connect(self): Connect according to this ConnectionSpec, return the socket.

ConnectionSpec.from_spec(spec): Construct an instance from a connection spec string of the form [tcp:|ssl:][user@][tcp_host!]server_hostname[:port].

The optional prefixes tcp: and ssl: indicate that the connection should be cleartext or SSL/TLS respectively. The default is SSL/TLS.

ConnectionSpec.netrc_entry: The default NetrcEntry for this ConnectionSpec.

ConnectionSpec.password: The password for this connection, obtained from the .netrc file via the key user@host:port.

• main(argv=None): The pop3 command line mode.
• class NetrcEntry(NetrcEntry): A namedtuple representation of a netrc entry.

NetrcEntry.by_account(account_name, netrc_hosts=None): Look up an entry by the account field value.

NetrcEntry.get(machine, netrc_hosts=None): Look up an entry by the machine field value.

• class POP3(cs.resources.MultiOpenMixin): Simple POP3 class with support for streaming use.

POP3.client_auth(self, user, password): Perform a client authentication.

POP3.client_begin(self): Read the opening server response.

POP3.client_bg(self, rq_line, is_multiline=False, notify=None): Dispatch a request rq_line in the background. Return a Result to collect the request result.

Parameters:

• rq_line: POP3 request text, without any terminating CRLF
• is_multiline: true if a multiline response is expected, default False
• notify: a optional handler for Result.notify, applied if not None

Note: DOES NOT flush the send stream. Call self.flush() when a batch of requests has been submitted, before trying to collect the Results.

The Result will receive [etc,lines] on success where:

• etc is the trailing portion of an ok response line
• lines is a list of unstuffed text lines from the response if is_multiline is true, None otherwise The Result gets a list instead of a tuple so that a handler may clear it in order to release memory.

Example:

R = self.client_bg(f'RETR {msg_n}', is_multiline=True, notify=notify)

POP3.client_dele_bg(self, msg_n): Queue a delete request for message msg_n, return Result for collection.

POP3.client_quit_bg(self): Queue a QUIT request. return Result for collection.

POP3.client_retr_bg(self, msg_n, notify=None): Queue a retrieve request for message msg_n, return Result for collection.

If notify is not None, apply it to the Result.

POP3.client_uidl(self): Return a mapping of message number to message UID string.

POP3.dl_bg(self, msg_n, maildir, deleRs): Download message msg_n to Maildir maildir. Return the Result for the RETR request.

After a successful save, queue a DELE for the message and add its Result to deleRs.

POP3.flush(self): Flush the send stream.

POP3.get_multiline(self): Generator yielding unstuffed lines from a multiline response.

POP3.get_ok(self): Read server response, require it to be 'OK+'. Returns the etc part.

POP3.get_response(self): Read a server response. Return (ok,status,etc) where ok is true if status is '+OK', false otherwise; status is the status word and etc is the following text. Return (None,None,None) on EOF from the receive stream.

POP3.readline(self): Read a CRLF terminated line from self.recvf. Return the text preceeding the CRLF. Return None at EOF.

POP3.readlines(self): Generator yielding lines from self.recf.

POP3.sendline(self, line, do_flush=False): Send a line (excluding its terminating CRLF). If do_flush is true (default False) also flush the sending stream.

POP3.startup_shutdown(self): Connect to the server and log in.

• class POP3Command(cs.cmdutils.BaseCommand): Command line implementation for POP3 operations.

  Credentials are obtained via the .netrc file presently.

  Connection specifications consist of an optional leading mode prefix followed by a netrc(5) account name or an explicit connection specification from which to derive:

  • user: the user name to log in as
  • tcp_host: the hostname to which to establish a TCP connection
  • port: the TCP port to connect on, default 995 for TLS/SSL or 110 for cleartext
  • sni_host: the TLS/SSL SNI server name, which may be different from the tcp_host

  The optional mode prefix is one of:

  • ssl:: use TLS/SSL - this is the default
  • tcp:: use cleartext - this is useful for ssh port forwards to some not-publicly-exposed clear text POP service; in particular streaming performs better this way, I think because the Python SSL layer does not buffer writes

  Example connection specifications:

  • username@mail.example.com: use TLS/SSL to connect to the POP3S service at mail.example.com, logging in as username
  • mail.example.com: use TLS/SSL to connect to the POP3S service at mail.example.com, logging in with the same login as the local effective user
  • tcp:username@localhost:1110: use cleartext to connect to localhost:1110, typically an ssh port forward to a remote private cleartext POP service, logging in as username
  • username@localhost!mail.example.com:1995: use TLS/SSL to connect to localhost:1995, usually an ssh port forward to a remote private TLS/SSL POP service, logging in as username and passing mail.exampl.com as the TLS/SSL server name indication (which allows certificate verification to proceed correctly)

  Note that the specification may also be a netrc account name. If the specification matches such an account name then values are derived from the netrc entry. The entry's machine name becomes the TCP connection specification, the entry's login provides a default for the username, the entry's account host part provides the sni_host.

  Example netrc entry:

  machine username@localhost:1110
    account username@mail.example.com
    password ************
  

  Such an entry allows you to use the specification tcp:username@mail.example.com and obtain the remaining detail via the netrc entry.Credentials are obtained via the .netrc file presently.

  Connection specifications consist of an optional leading mode prefix followed by a netrc(5) account name or an explicit connection specification from which to derive:

  • user: the user name to log in as
  • tcp_host: the hostname to which to establish a TCP connection
  • port: the TCP port to connect on, default 995 for TLS/SSL or 110 for cleartext
  • sni_host: the TLS/SSL SNI server name, which may be different from the tcp_host

  The optional mode prefix is one of:

  • ssl:: use TLS/SSL - this is the default
  • tcp:: use cleartext - this is useful for ssh port forwards to some not-publicly-exposed clear text POP service; in particular streaming performs better this way, I think because the Python SSL layer does not buffer writes

  Example connection specifications:

  • username@mail.example.com: use TLS/SSL to connect to the POP3S service at mail.example.com, logging in as username
  • mail.example.com: use TLS/SSL to connect to the POP3S service at mail.example.com, logging in with the same login as the local effective user
  • tcp:username@localhost:1110: use cleartext to connect to localhost:1110, typically an ssh port forward to a remote private cleartext POP service, logging in as username
  • username@localhost!mail.example.com:1995: use TLS/SSL to connect to localhost:1995, usually an ssh port forward to a remote private TLS/SSL POP service, logging in as username and passing mail.exampl.com as the TLS/SSL server name indication (which allows certificate verification to proceed correctly)

  Note that the specification may also be a netrc account name. If the specification matches such an account name then values are derived from the netrc entry. The entry's machine name becomes the TCP connection specification, the entry's login provides a default for the username, the entry's account host part provides the sni_host.

  Example netrc entry:

  machine username@localhost:1110
    account username@mail.example.com
    password ************
  

  Such an entry allows you to use the specification tcp:username@mail.example.com and obtain the remaining detail via the netrc entry.

  Usage summary:

  Usage: pop3 [common-options...] subcommand [options...]
    Command line implementation for POP3 operations.
    Subcommands:
      dl [common-options...] [{ssl,tcp}:]{netrc_account|[user@]host[!sni_name][:port]} maildir
        Collect messages from a POP3 server and deliver to a Maildir.
        Options:
          -1             Download only 1 message.
          --limit limit  Limit the number of messages fetched.
      help [common-options...] [-l] [-s] [subcommand-names...]
        Print help for subcommands.
        This outputs the full help for the named subcommands,
        or the short help for all subcommands if no names are specified.
        Options:
          -l  Long listing.
          -r  Recurse into subcommands.
          -s  Short listing.
      info [common-options...] [field-names...]
        Recite general information.
        Explicit field names may be provided to override the default listing.
      repl [common-options...]
        Run a REPL (Read Evaluate Print Loop), an interactive Python prompt.
        Options:
          --banner banner  Banner.
      shell [common-options...]
        Run a command prompt via cmd.Cmd using this command's subcommands.
  

POP3Command.cmd_dl(self, argv): Usage: {cmd} [{{ssl,tcp}}:]{{netrc_account|[user@]host[!sni_name][:port]}} maildir Collect messages from a POP3 server and deliver to a Maildir. Options: -1 Download only 1 message. --limit limit Limit the number of messages fetched.

Release Log

Release 20260531: POP3Command.cmd_dl: add -1 (once) option for testing, notice SIGINT (though the RETRs get queued so fast it hardly helps), new --limit=n option to limit the number of messages fetched.

Release 20240316: Fixed release upload artifacts.

Release 20240201.1: Another test release, nothing new.

Release 20240201: Test release with better DISTINFO.

Release 20221221: Fix stray %s in format string, modernise MultiOpenMixin startup/shutdown, catch ConnectionRefusedError and report succintly.

Release 20220918:

• Emit an error instead of stack trace for messages which cannot be saved (and do not delete).
• POP3Command.cmd_dl: new -n (no action) option.

Release 20220606: Minor updates.

Release 20220312: Make POP3Command.cmd_dl an instance method (static methods broke with the latest cs.cmdutils release).

Release 20211208:

• POP3.startup: do not start the worker queue until authenticated.
• POP3.get_response: return (None,None,None) at EOF.
• POP3.shutdown: catch exceptions from client QUIT.

Release 20210407.2: Provide "pop3" console_script.

Release 20210407.1: Bump for cs.cmdutils minor bugfix, also fix a few docstring typos.

Release 20210407: Initial PyPI release.