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In the section on connections between threads, you learned that the connect call automatically creates queued connections between objects living in different threads. All QObject instances know which thread they belong to they are said to have thread affinity. There are a few restrictions that apply to the QObject and threads: The child of a QObject must belong to the same thread as the QObject itself. Event-driven objects can be used in only one single thread. All QObjects must be deleted before the QThread that they belong to is deleted. The first rule means that the QThread itself never should be used as a parent because it was created in another thread. The second rule applies to mechanisms such as timers and network sockets. You can t start a timer or make a socket connection in a thread other than the timer s or socket s thread because each thread has a event loop of its own. If you plan to use events in a thread, you must call the QThread::exec method to start the thread s local event loop. The third rule is easy to manage: Let all objects that you create have a parent (or grandparent) on the stack in the thread s run method. It is important to understand that a QObject can be used from several threads at once but most objects provided by Qt are designed to be used from a single thread, so your mileage may vary.

There are several different exceptions that might be raised while you re programming with smtplib. They are: socket.gaierror for errors looking up address information socket.error for general I/O and communication problems socket.herror for other addressing errors smtplib.SMTPException or a subclass of it for SMTP conversation problems

There are some parts of Qt that are easy to use from a single thread. This doesn t mean that they can t be used from a QThread object or that they are incompatible with threaded applications; it s just best to keep all such objects within a single thread. If interaction with other threads is needed, it can be performed using signals, slots, and methods of the thread managing the object in question. The object types to keep in one thread include the entire SQL module and the QTimer, QTcpSocket, QUdpSocket, QHttp, QFtp, and QProcess objects.

The first three errors are covered in more detail in 3; they are passed straight through the smtplib module and up to your program. But so long as the underlying TCP socket works, all problems that actually involve the SMTP e-mail conversation will result in an smtplib.SMTPException. The smtplib module also provides a way to get a series of detailed messages about the steps it takes to send an e-mail. To enable that level of detail, you can call smtpobj.set_debuglevel(1) With this option, you should be able to track down any problems. Take a look at Listing 13 2 for an example program that provides basic error handling and debugging.

<field name="commentRss" type="string" isArray="false"/> <field name="publishedDate" type="date" isArray="false"/> <field name="mediaLink" type="url" isArray="false"/> <field name="mediaType" type="string" isArray="false"/> <field name="latitude" type="latitude" isArray="false"/> <field name="longtitude" type="longitude" isArray="false"/> </object> </objects> </block>

An example of misbehaving is to create a QFtp object from one thread and then interact with it from another thread. This process might work, but it could cause mysterious and hardto-debug problems. To avoid having to hunt these ghost bugs, be careful when using threads.

Listing 13 2. A More Cautious SMTP Client #!/usr/bin/env python # SMTP transmission with debugging - 13 - debug.py import sys, smtplib, socket if len(sys.argv) < 4: print "usage: %s server fromaddr toaddr [toaddr...]" % sys.argv[0] sys.exit(2) server, fromaddr, toaddrs = sys.argv[1], sys.argv[2], sys.argv[3:] message = """To: %s From: %s Subject: Test Message from simple.py Hello, This is a test message sent to you from the debug.py program in Foundations of Python Network Programming. """ % (', '.join(toaddrs), fromaddr) try: s = smtplib.SMTP(server) s.set_debuglevel(1) s.sendmail(fromaddr, toaddrs, message) except (socket.gaierror, socket.error, socket.herror, smtplib.SMTPException), e: print " *** Your message may not have been sent!" print e sys.exit(1) else: print "Message successfully sent to %d recipient(s)" % len(toaddrs) This program looks similar to the last one. However, the output will be very different; take a look at Listing 13 3 for an example. Listing 13 3. Debugging Output from smtplib $ ./debug.py localhost foo@example.com jgoerzen@complete.org send: 'ehlo localhost\r\n' reply: '250-localhost\r\n' reply: '250-PIPELINING\r\n' reply: '250-SIZE 20480000\r\n' reply: '250-VRFY\r\n' reply: '250-ETRN\r\n' reply: '250-STARTTLS\r\n' reply: '250-XVERP\r\n' reply: '250 8BITMIME\r\n' reply: retcode (250); Msg: localhost PIPELINING SIZE 20480000 VRFY

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