When the twisted reactor is running and an exception occurs within a deferred that isn't caught, "Unhandled Error" is printed to the terminal along with a traceback and the exception. Is it possible to handle/intercept these exceptions (e.g., set a callback or override a method)?
EDIT: I'm aware that I can catch a failure by adding an errback to a deferrerd. What I want to know is if there is a way to intercept an unhandled failure/exception that has traversed its way up the chain to the reactor.
EDIT: Essentially, I'm wondering if the twisted reactor has a global error handler or something that can be accessed. I wonder because it prints the traceback and error from the failure.
Example:
Unhandled Error
Traceback (most recent call last):
File "/var/projects/python/server.py", line 359, in run_server
return server.run()
File "/var/projects/python/server.py", line 881, in run
reactor.run()
File "/usr/local/lib/python2.6/dist-packages/Twisted-11.0.0-py2.6-linux-x86_64.egg/twisted/internet/base.py", line 1162, in run
self.mainLoop()
File "/usr/local/lib/python2.6/dist-packages/Twisted-11.0.0-py2.6-linux-x86_64.egg/twisted/internet/base.py", line 1171, in mainLoop
self.runUntilCurrent()
--- <exception caught here> ---
File "/usr/local/lib/python2.6/dist-packages/Twisted-11.0.0-py2.6-linux-x86_64.egg/twisted/internet/base.py", line 793, in runUntilCurrent
call.func(*call.args, **call.kw)
File "/var/projects/python/server.py", line 524, in monitor
elapsed = time.time() - info.last
exceptions.NameError: global name 'info' is not defined
Because these tracebacks are written using a call to twisted.python.log.deferr()
(in Twisted 10.2 anyway), it is possible to redirect them using a log observer. This is the most common thing to do with these stack traces. I can't find any base class for log observers (surprisingly) but there are a couple built in:
twisted.python.log.PythonLoggingObserver
- Anything logged goes to the standard Python logging
module. (I use this in my application.)
twisted.python.log.FileLogObserver
- Anything logged goes to a file.
Both of these will catch stack traces reported by the reactor. All you have to do is construct the log observer (no arguments) and then call the object's start()
method.
(Side note: there's also a StdioOnnaStick
class that you can construct and assign to sys.stdout
or sys.stderr
if you want. Then anything you print
goes to the Twisted log.)
To really, truly intercept these calls, so the stack traces never get logged at all, you could either:
twisted.internet.SelectReactor
and override its runUntilCurrent()
method. That is what logs the stack traces. You would need to study the source of twisted.internet.base.ReactorBase
before doing this.twisted.*
imports, set twisted.python.log.deferr
to a function of your choosing, that is compatible with the prototype def err(_stuff=None, _why=None, **kw)
.