This tutorial will walk you through the creation of an extremely simple SMTP client application. By the time the tutorial is complete, you will understand how to create and start a TCP client speaking the SMTP protocol, have it connect to an appropriate mail exchange server, and transmit a message for delivery.
For the majority of this tutorial, twistd will be used
to launch the application. Near the end we will explore other
possibilities for starting a Twisted application. Until then, make
sure that you have twistd installed and conveniently
accessible for use in running each of the example .tac
files.
The first step is to create smtpclient-1.tac possible for use by twistd .
from twisted.application import service
The first line of the .tac file
imports twisted.application.service , a module which
contains many of the basic service classes and helper
functions available in Twisted. In particular, we will be using
the Application function to create a new application service . An application service simply acts as a
central object on which to store certain kinds of deployment
configuration.
application = service.Application("SMTP Client Tutorial")
The second line of the .tac file creates a
new application service and binds it to the local
name application . twistd requires this
local name in each .tac file it runs. It uses various
pieces of configuration on the object to determine its behavior. For
example, "SMTP Client Tutorial" will be used as the name
of the .tap file into which to serialize application
state, should it be necessary to do so.
That does it for the first example. We now have enough of
a .tac file to pass to twistd . If we
run smtpclient-1.tac using
the twistd command line:
twistd -ny smtpclient-1.tac
we are rewarded with the following output:
exarkun@boson:~/mail/tutorial/smtpclient$ twistd -ny smtpclient-1.tac
18:31 EST [-] Log opened.
18:31 EST [-] twistd 2.0.0 (/usr/bin/python2.4 2.4.1) starting up
18:31 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
18:31 EST [-] Loading smtpclient-1.tac...
18:31 EST [-] Loaded.
As we expected, not much is going on. We can shutdown this server
by issuing ^C :
18:34 EST [-] Received SIGINT, shutting down.
18:34 EST [-] Main loop terminated.
18:34 EST [-] Server Shut Down.
exarkun@boson:~/mail/tutorial/smtpclient$
The first version of our SMTP client wasn’t very interesting. It
didn’t even establish any TCP connections! The smtpclient-2.tac will come a little bit
closer to that level of complexity. First, we need to import a few
more things:
from twisted.application import internet
from twisted.internet import protocol
twisted.application.internet is
another application service module. It provides services for
establishing outgoing connections (as well as creating network
servers, though we are not interested in those parts for the
moment). twisted.internet.protocol provides base
implementations of many of the core Twisted concepts, such
as factories and protocols .
The next line of smtpclient-2.tac
instantiates a new client factory .
smtpClientFactory = protocol.ClientFactory()
Client factories are responsible for constructing protocol instances whenever connections are established. They may be required to create just one instance, or many instances if many different connections are established, or they may never be required to create one at all, if no connection ever manages to be established.
Now that we have a client factory, we’ll need to hook it up to the
network somehow. The next line of smtpclient-2.tac does
just that:
smtpClientService = internet.TCPClient(None, None, smtpClientFactory)
We’ll ignore the first two arguments
to internet.TCPClient for the moment and instead focus on
the third. TCPClient is one of those application service classes. It creates TCP connections to a specified
address and then uses its third argument, a client factory ,
to get a protocol instance . It then associates the TCP
connection with the protocol instance and gets out of the way.
We can try to run smtpclient-2.tac the same way we
ran smtpclient-1.tac , but the results might be a little
disappointing:
exarkun@boson:~/mail/tutorial/smtpclient$ twistd -ny smtpclient-2.tac
18:55 EST [-] Log opened.
18:55 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
18:55 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
18:55 EST [-] Loading smtpclient-2.tac...
18:55 EST [-] Loaded.
18:55 EST [-] Starting factory <twisted.internet.protocol.ClientFactory
instance at 0xb791e46c>
18:55 EST [-] Traceback (most recent call last):
File "twisted/scripts/twistd.py", line 187, in runApp
app.runReactorWithLogging(config, oldstdout, oldstderr)
File "twisted/application/app.py", line 128, in runReactorWithLogging
reactor.run()
File "twisted/internet/posixbase.py", line 200, in run
self.mainLoop()
File "twisted/internet/posixbase.py", line 208, in mainLoop
self.runUntilCurrent()
--- <exception caught here> ---
File "twisted/internet/base.py", line 533, in runUntilCurrent
call.func(*call.args, **call.kw)
File "twisted/internet/tcp.py", line 489, in resolveAddress
if abstract.isIPAddress(self.addr[0]):
File "twisted/internet/abstract.py", line 315, in isIPAddress
parts = string.split(addr, '.')
File "/usr/lib/python2.4/string.py", line 292, in split
return s.split(sep, maxsplit)
exceptions.AttributeError: 'NoneType' object has no attribute 'split'
18:55 EST [-] Received SIGINT, shutting down.
18:55 EST [-] Main loop terminated.
18:55 EST [-] Server Shut Down.
exarkun@boson:~/mail/tutorial/smtpclient$
What happened? Those first two arguments to TCPClient
turned out to be important after all. We’ll get to them in the next
example.
Version three of our SMTP client only changes one thing. The line from version two:
smtpClientService = internet.TCPClient(None, None, smtpClientFactory)
has its first two arguments changed from None to
something with a bit more meaning:
smtpClientService = internet.TCPClient('localhost', 25, smtpClientFactory)
This directs the client to connect to localhost on
port 25 . This isn’t the address we want ultimately, but it’s
a good place-holder for the time being. We can
run smtpclient-3.tac and see what this
change gets us:
exarkun@boson:~/mail/tutorial/smtpclient$ twistd -ny smtpclient-3.tac
19:10 EST [-] Log opened.
19:10 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
19:10 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
19:10 EST [-] Loading smtpclient-3.tac...
19:10 EST [-] Loaded.
19:10 EST [-] Starting factory <twisted.internet.protocol.ClientFactory
instance at 0xb791e48c>
19:10 EST [-] Enabling Multithreading.
19:10 EST [Uninitialized] Traceback (most recent call last):
File "twisted/python/log.py", line 56, in callWithLogger
return callWithContext({"system": lp}, func, *args, **kw)
File "twisted/python/log.py", line 41, in callWithContext
return context.call({ILogContext: newCtx}, func, *args, **kw)
File "twisted/python/context.py", line 52, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "twisted/python/context.py", line 31, in callWithContext
return func(*args,**kw)
--- <exception caught here> ---
File "twisted/internet/selectreactor.py", line 139, in _doReadOrWrite
why = getattr(selectable, method)()
File "twisted/internet/tcp.py", line 543, in doConnect
self._connectDone()
File "twisted/internet/tcp.py", line 546, in _connectDone
self.protocol = self.connector.buildProtocol(self.getPeer())
File "twisted/internet/base.py", line 641, in buildProtocol
return self.factory.buildProtocol(addr)
File "twisted/internet/protocol.py", line 99, in buildProtocol
p = self.protocol()
exceptions.TypeError: 'NoneType' object is not callable
19:10 EST [Uninitialized] Stopping factory
<twisted.internet.protocol.ClientFactory instance at
0xb791e48c>
19:10 EST [-] Received SIGINT, shutting down.
19:10 EST [-] Main loop terminated.
19:10 EST [-] Server Shut Down.
exarkun@boson:~/mail/tutorial/smtpclient$
A meager amount of progress, but the service still raises an exception. This time, it’s because we haven’t specified a protocol class for the factory to use. We’ll do that in the next example.
In the previous example, we ran into a problem because we hadn’t
set up our client factory’s protocol attribute
correctly (or at all). ClientFactory.buildProtocol is
the method responsible for creating a protocol instance . The
default implementation calls the factory’s protocol attribute,
adds itself as an attribute named factory to the
resulting instance, and returns it. In smtpclient-4.tac , we’ll correct the
oversight that caused the traceback in smtpclient-3.tac:
smtpClientFactory.protocol = protocol.Protocol
Running this version of the client, we can see the output is once again traceback free:
exarkun@boson:~/doc/mail/tutorial/smtpclient$ twistd -ny smtpclient-4.tac
19:29 EST [-] Log opened.
19:29 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
19:29 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
19:29 EST [-] Loading smtpclient-4.tac...
19:29 EST [-] Loaded.
19:29 EST [-] Starting factory <twisted.internet.protocol.ClientFactory
instance at 0xb791e4ac>
19:29 EST [-] Enabling Multithreading.
19:29 EST [-] Received SIGINT, shutting down.
19:29 EST [Protocol,client] Stopping factory
<twisted.internet.protocol.ClientFactory instance at
0xb791e4ac>
19:29 EST [-] Main loop terminated.
19:29 EST [-] Server Shut Down.
exarkun@boson:~/doc/mail/tutorial/smtpclient$
But what does this
mean? twisted.internet.protocol.Protocol is the
base protocol implementation. For those familiar with the
classic UNIX network services, it is equivalent to
the discard service. It never produces any output and it
discards all its input. Not terribly useful, and certainly nothing
like an SMTP client. Let’s see how we can improve this in the next
example.
In smtpclient-5.tac , we will begin
to use Twisted’s SMTP protocol implementation for the first time.
We’ll make the obvious change, simply swapping
out twisted.internet.protocol.Protocol in favor
of twisted.mail.smtp.ESMTPClient . Don’t worry about
the E in ESMTP . It indicates we’re actually using a
newer version of the SMTP protocol. There is
an SMTPClient in Twisted, but there’s essentially no
reason to ever use it.
smtpclient-5.tac adds a new import:
from twisted.mail import smtp
All of the mail related code in Twisted exists beneath
the twisted.mail package. More specifically, everything
having to do with the SMTP protocol implementation is defined in
the twisted.mail.smtp module.
Next we remove a line we added in smtpclient-4.tac:
smtpClientFactory.protocol = protocol.Protocol
And add a similar one in its place:
smtpClientFactory.protocol = smtp.ESMTPClient
Our client factory is now using a protocol implementation which behaves as an SMTP client. What happens when we try to run this version?
exarkun@boson:~/doc/mail/tutorial/smtpclient$ twistd -ny smtpclient-5.tac
19:42 EST [-] Log opened.
19:42 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
19:42 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
19:42 EST [-] Loading smtpclient-5.tac...
19:42 EST [-] Loaded.
19:42 EST [-] Starting factory <twisted.internet.protocol.ClientFactory
instance at 0xb791e54c>
19:42 EST [-] Enabling Multithreading.
19:42 EST [Uninitialized] Traceback (most recent call last):
File "twisted/python/log.py", line 56, in callWithLogger
return callWithContext({"system": lp}, func, *args, **kw)
File "twisted/python/log.py", line 41, in callWithContext
return context.call({ILogContext: newCtx}, func, *args, **kw)
File "twisted/python/context.py", line 52, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "twisted/python/context.py", line 31, in callWithContext
return func(*args,**kw)
--- <exception caught here> ---
File "twisted/internet/selectreactor.py", line 139, in _doReadOrWrite
why = getattr(selectable, method)()
File "twisted/internet/tcp.py", line 543, in doConnect
self._connectDone()
File "twisted/internet/tcp.py", line 546, in _connectDone
self.protocol = self.connector.buildProtocol(self.getPeer())
File "twisted/internet/base.py", line 641, in buildProtocol
return self.factory.buildProtocol(addr)
File "twisted/internet/protocol.py", line 99, in buildProtocol
p = self.protocol()
exceptions.TypeError: __init__() takes at least 2 arguments (1 given)
19:42 EST [Uninitialized] Stopping factory
<twisted.internet.protocol.ClientFactory instance at
0xb791e54c>
19:43 EST [-] Received SIGINT, shutting down.
19:43 EST [-] Main loop terminated.
19:43 EST [-] Server Shut Down.
exarkun@boson:~/doc/mail/tutorial/smtpclient$
Oops, back to getting a traceback. This time, the default
implementation of buildProtocol seems no longer to be
sufficient. It instantiates the protocol with no arguments,
but ESMTPClient wants at least one argument. In the next
version of the client, we’ll override buildProtocol to
fix this problem.
smtpclient-6.tac introduces
a twisted.internet.protocol.ClientFactory subclass with
an overridden buildProtocol method to overcome the
problem encountered in the previous example.
class SMTPClientFactory(protocol.ClientFactory):
protocol = smtp.ESMTPClient
def buildProtocol(self, addr):
return self.protocol(secret=None, identity='example.com')
The overridden method does almost the same thing as the base
implementation: the only change is that it passes values for two
arguments to twisted.mail.smtp.ESMTPClient ‘s initializer.
The secret argument is used for SMTP authentication
(which we will not attempt yet). The identity argument
is used as a to identify ourselves Another minor change to note is
that the protocol attribute is now defined in the class
definition, rather than tacked onto an instance after one is created.
This means it is a class attribute, rather than an instance attribute,
now, which makes no difference as far as this example is concerned.
There are circumstances in which the difference is important: be sure
you understand the implications of each approach when creating your
own factories.
One other change is required: instead of
instantiating twisted.internet.protocol.ClientFactory , we
will now instantiate SMTPClientFactory :
smtpClientFactory = SMTPClientFactory()
Running this version of the code, we observe that the code still isn’t quite traceback-free.
exarkun@boson:~/doc/mail/tutorial/smtpclient$ twistd -ny smtpclient-6.tac
21:17 EST [-] Log opened.
21:17 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
21:17 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
21:17 EST [-] Loading smtpclient-6.tac...
21:17 EST [-] Loaded.
21:17 EST [-] Starting factory <__builtin__.SMTPClientFactory instance
at 0xb77fd68c>
21:17 EST [-] Enabling Multithreading.
21:17 EST [ESMTPClient,client] Traceback (most recent call last):
File "twisted/python/log.py", line 56, in callWithLogger
return callWithContext({"system": lp}, func, *args, **kw)
File "twisted/python/log.py", line 41, in callWithContext
return context.call({ILogContext: newCtx}, func, *args, **kw)
File "twisted/python/context.py", line 52, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "twisted/python/context.py", line 31, in callWithContext
return func(*args,**kw)
--- <exception caught here> ---
File "twisted/internet/selectreactor.py", line 139, in _doReadOrWrite
why = getattr(selectable, method)()
File "twisted/internet/tcp.py", line 351, in doRead
return self.protocol.dataReceived(data)
File "twisted/protocols/basic.py", line 221, in dataReceived
why = self.lineReceived(line)
File "twisted/mail/smtp.py", line 1039, in lineReceived
why = self._okresponse(self.code,'\n'.join(self.resp))
File "twisted/mail/smtp.py", line 1281, in esmtpState_serverConfig
self.tryTLS(code, resp, items)
File "twisted/mail/smtp.py", line 1294, in tryTLS
self.authenticate(code, resp, items)
File "twisted/mail/smtp.py", line 1343, in authenticate
self.smtpState_from(code, resp)
File "twisted/mail/smtp.py", line 1062, in smtpState_from
self._from = self.getMailFrom()
File "twisted/mail/smtp.py", line 1137, in getMailFrom
raise NotImplementedError
exceptions.NotImplementedError:
21:17 EST [ESMTPClient,client] Stopping factory
<__builtin__.SMTPClientFactory instance at 0xb77fd68c>
21:17 EST [-] Received SIGINT, shutting down.
21:17 EST [-] Main loop terminated.
21:17 EST [-] Server Shut Down.
exarkun@boson:~/doc/mail/tutorial/smtpclient$
What we have accomplished with this iteration of the example is to navigate far enough into an SMTP transaction that Twisted is now interested in calling back to application-level code to determine what its next step should be. In the next example, we’ll see how to provide that information to it.
SMTP Client 7 is the first version of our SMTP client which
actually includes message data to transmit. For simplicity’s sake,
the message is defined as part of a new class. In a useful program
which sent email, message data might be pulled in from the filesystem,
a database, or be generated based on
user-input. smtpclient-7.tac , however,
defines a new class, SMTPTutorialClient , with three class
attributes (mailFrom , mailTo ,
and mailData ):
class SMTPTutorialClient(smtp.ESMTPClient):
mailFrom = "tutorial_sender@example.com"
mailTo = "tutorial_recipient@example.net"
mailData = '''\
Date: Fri, 6 Feb 2004 10:14:39 -0800
From: Tutorial Guy <tutorial_sender@example.com>
To: Tutorial Gal <tutorial_recipient@example.net>
Subject: Tutorate!
Hello, how are you, goodbye.
'''
This statically defined data is accessed later in the class
definition by three of the methods which are part of the
SMTPClient callback API . Twisted expects each of the three
methods below to be defined and to return an object with a particular
meaning. First, getMailFrom :
def getMailFrom(self):
result = self.mailFrom
self.mailFrom = None
return result
This method is called to determine the reverse-path ,
otherwise known as the envelope from , of the message. This
value will be used when sending the MAIL FROM SMTP
command. The method must return a string which conforms to the RFC 2821 definition
of a reverse-path . In simpler terms, it should be a string
like "alice@example.com" . Only one envelope from is allowed by the SMTP protocol, so it cannot be a list of
strings or a comma separated list of addresses. Our implementation
of getMailFrom does a little bit more than just return a
string; we’ll get back to this in a little bit.
The next method is getMailTo :
def getMailTo(self):
return [self.mailTo]
getMailTo is similar to getMailFrom . It
returns one or more RFC 2821 addresses (this time a
forward-path , or envelope to ). Since SMTP allows
multiple recipients, getMailTo returns a list of these
addresses. The list must contain at least one address, and even if
there is exactly one recipient, it must still be in a list.
The final callback we will define to provide information to
Twisted is getMailData :
def getMailData(self):
return StringIO.StringIO(self.mailData)
This one is quite simple as well: it returns a file or a file-like
object which contains the message contents. In our case, we return
a StringIO since we already have a string containing our
message. If the contents of the file returned
by getMailData span multiple lines (as email messages
often do), the lines should be \n delimited (as they
would be when opening a text file in the "rt" mode):
necessary newline translation will be performed
by SMTPClient automatically.
There is one more new callback method defined in smtpclient-7.tac. This one isn’t for providing information about the messages to Twisted, but for Twisted to provide information about the success or failure of the message transmission to the application:
def sentMail(self, code, resp, numOk, addresses, log):
print('Sent', numOk, 'messages')
Each of the arguments to sentMail provides some
information about the success or failure of the message transmission
transaction. code is the response code from the ultimate
command. For successful transactions, it will be 250. For transient
failures (those which should be retried), it will be between 400 and
499, inclusive. For permanent failures (this which will never work,
no matter how many times you retry them), it will be between 500 and
599.
Thus far we have succeeded in creating a Twisted client application
which starts up, connects to a (possibly) remote host, transmits some
data, and disconnects. Notably missing, however, is application
shutdown. Hitting ^C is fine during development, but it’s not exactly
a long-term solution. Fortunately, programmatic shutdown is extremely
simple. smtpclient-8.tac
extends sentMail with these two lines:
from twisted.internet import reactor
reactor.stop()
The stop method of the reactor causes the main event
loop to exit, allowing a Twisted server to shut down. With this
version of the example, we see that the program actually terminates
after sending the message, without user-intervention:
exarkun@boson:~/doc/mail/tutorial/smtpclient$ twistd -ny smtpclient-8.tac
19:52 EST [-] Log opened.
19:52 EST [-] twistd SVN-Trunk (/usr/bin/python2.4 2.4.1) starting up
19:52 EST [-] reactor class: twisted.internet.selectreactor.SelectReactor
19:52 EST [-] Loading smtpclient-8.tac...
19:52 EST [-] Loaded.
19:52 EST [-] Starting factory <__builtin__.SMTPClientFactory instance
at 0xb791beec>
19:52 EST [-] Enabling Multithreading.
19:52 EST [SMTPTutorialClient,client] Sent 1 messages
19:52 EST [SMTPTutorialClient,client] Stopping factory
<__builtin__.SMTPClientFactory instance at 0xb791beec>
19:52 EST [-] Main loop terminated.
19:52 EST [-] Server Shut Down.
exarkun@boson:~/doc/mail/tutorial/smtpclient$
One task remains to be completed in this tutorial SMTP client: instead of always sending mail through a well-known host, we will look up the mail exchange server for the recipient address and try to deliver the message to that host.
In smtpclient-9.tac , we’ll take the
first step towards this feature by defining a function which returns
the mail exchange host for a particular domain:
def getMailExchange(host):
return 'localhost'
Obviously this doesn’t return the correct mail exchange host yet
(in fact, it returns the exact same host we have been using all
along), but pulling out the logic for determining which host to
connect to into a function like this is the first step towards our
ultimate goal. Now that we have getMailExchange , we’ll
call it when constructing our TCPClient service:
smtpClientService = internet.TCPClient(
getMailExchange('example.net'), 25, smtpClientFactory)
We’ll expand on the definition of getMailExchange in
the next example.
In the previous example we defined getMailExchange to
return a string representing the mail exchange host for a particular
domain. While this was a step in the right direction, it turns out
not to be a very big one. Determining the mail exchange host for a
particular domain is going to involve network traffic (specifically,
some DNS requests). These might take an arbitrarily large amount of
time, so we need to introduce a Deferred to represent the
result of getMailExchange . smtpclient-10.tac redefines it
thusly:
def getMailExchange(host):
return defer.succeed('localhost')
defer.succeed is a function which creates a
new Deferred which already has a result, in this
case 'localhost' . Now we need to adjust
our TCPClient -constructing code to expect and properly
handle this Deferred :
def cbMailExchange(exchange):
smtpClientFactory = SMTPClientFactory()
smtpClientService = internet.TCPClient(exchange, 25, smtpClientFactory)
smtpClientService.setServiceParent(application)
getMailExchange('example.net').addCallback(cbMailExchange)
An in-depth exploration of Deferred s is beyond the
scope of this document. For such a look, see
the Deferred Reference TCPClient until the Deferred
returned by getMailExchange fires. Once it does, we
proceed normally through the creation of
our SMTPClientFactory and TCPClient , as well
as set the TCPClient ‘s service parent, just as we did in
the previous examples.
At last we’re ready to perform the mail exchange lookup. We do
this by calling on an object provided specifically for this
task, twisted.mail.relaymanager.MXCalculator :
def getMailExchange(host):
def cbMX(mxRecord):
return str(mxRecord.name)
return relaymanager.MXCalculator().getMX(host).addCallback(cbMX)
Because getMX returns a Record_MX object
rather than a string, we do a little bit of post-processing to get the
results we want. We have already converted the rest of the tutorial
application to expect a Deferred
from getMailExchange , so no further changes are
required. smtpclient-11.tac completes
this tutorial by being able to both look up the mail exchange host for
the recipient domain, connect to it, complete an SMTP transaction,
report its results, and finally shut down the reactor.