[Twisted-Python] Q: PyQt and error handling in Deferreds

[Egor Cheshkov]

Hello!

I'm writing client application using Twisted and PyQt. I want to handle 
connection errors by showing the message box that describes it, so my errback  
looks like this:

    def _ebSessionRequest(self, err):
        QMessageBox.critical(self, "Warder", "Can't connect to server.")
        self.setEnabled(1)
        self.editAccesCode.setFocus()

It works nice but when the errback fired i get two tracebacks:

Traceback (most recent call last):
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 215, 
in errback
    self._startRunCallbacks(fail, 1)
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 249, 
in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 262, 
in _runCallbacks
    self.result = callback(self.result, *args, **kw)
  File "main.py", line 185, in _ebSessionRequest
    QMessageBox.critical(self, "Warder", "Can't connect to server.")
--- <exception caught here> ---
  File "/usr/lib/python2.2/site-packages/twisted/internet/qtreactor.py", line 
74, in read
    why = w.doRead()
  File "/usr/lib/python2.2/site-packages/twisted/internet/tcp.py", line 378, 
in doConnect
    self.failIfNotConnected(error.getConnectError((connectResult, 
os.strerror(connectResult))))
  File "/usr/lib/python2.2/site-packages/twisted/internet/tcp.py", line 315, 
in failIfNotConnected
    self.connector.connectionFailed(failure.Failure(err))
  File "/usr/lib/python2.2/site-packages/twisted/internet/base.py", line 527, 
in connectionFailed
    self.factory.clientConnectionFailed(self, reason)
  File "/usr/lib/python2.2/site-packages/twisted/web/xmlrpc.py", line 209, in 
clientConnectionLost
    self.deferred.errback(reason)
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 215, 
in errback
    self._startRunCallbacks(fail, 1)
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 240, 
in _startRunCallbacks
    raise AlreadyCalledError()
twisted.internet.defer.AlreadyCalledError:
Traceback (most recent call last):
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 215, 
in errback
    self._startRunCallbacks(fail, 1)
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 249, 
in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 262, 
in _runCallbacks
    self.result = callback(self.result, *args, **kw)
  File "main.py", line 185, in _ebSessionRequest
    QMessageBox.critical(self, "Warder", "Can't connect to server.")
--- <exception caught here> ---
  File "/usr/lib/python2.2/site-packages/twisted/internet/qtreactor.py", line 
83, in read
    w.connectionLost(failure.Failure(why))
  File "/usr/lib/python2.2/site-packages/twisted/internet/tcp.py", line 396, 
in connectionLost
    self.failIfNotConnected(error.ConnectError())
  File "/usr/lib/python2.2/site-packages/twisted/internet/tcp.py", line 315, 
in failIfNotConnected
    self.connector.connectionFailed(failure.Failure(err))
  File "/usr/lib/python2.2/site-packages/twisted/internet/base.py", line 527, 
in connectionFailed
    self.factory.clientConnectionFailed(self, reason)
  File "/usr/lib/python2.2/site-packages/twisted/web/xmlrpc.py", line 209, in 
clientConnectionLost
    self.deferred.errback(reason)
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 215, 
in errback
    self._startRunCallbacks(fail, 1)
  File "/usr/lib/python2.2/site-packages/twisted/internet/defer.py", line 240, 
in _startRunCallbacks
    raise AlreadyCalledError()
twisted.internet.defer.AlreadyCalledError:

My question is: is it normal? How can I avoid such tracebacks?


-- 
Egor Cheshkov <egor at iplus.ru>