python - Can Deferred.callback() or Deferred.errback() raise an exception to the caller? -


can there condition calling .callback() or .errback() raise exception caller won't captured deferred?

say have the following deferred , callbacks:

from twisted.internet import defer  def bad_callback(result):     raise exception()  def bad_errback(result):     raise exception()  d = defer.deferred() d.addcallbacks(bad_callback, bad_errback)

if call d.callback(none), result in d exception bad_callback(). if call d.errback(exception()), result in d exception raised bad_errback(). but, in either of cases exceptions not raised caller.

now, know of couple conditions calling .callback() or .errback() raise exception caller, conditions violate proper use of deferreds.

  • obviously, if call .callback() or .errback() improper number of arguments, raise typeerror.

  • calling called deferred raise alreadycallederror.

  • calling .callback(defer.deferred()) raise assertionerror.

  • calling .errback() equivalent calling .errback(failure.failure()) raise nocurrentexceptionerror if there no active exception.

really question comes down to: can safely rely on behavior calling .callback(result) or .errback(exception_or_failure) result never raise exception long deferred has not been called , result proper?

i ran example, adding 2 lines bottom:

d.callback(none) print("ok!")

and got output:

unhandled error in deferred: unhandled error traceback (most recent call last):   file "callbacks.py", line 11, in <module>     d.callback(none)   file ".../twisted/internet/defer.py", line 368, in callback     self._startruncallbacks(result)   file ".../twisted/internet/defer.py", line 464, in _startruncallbacks     self._runcallbacks() --- <exception caught here> ---   file ".../twisted/internet/defer.py", line 551, in _runcallbacks     current.result = callback(current.result, *args, **kw)   file "callbacks.py", line 4, in bad_callback     raise exception() exceptions.exception:  ok!

so in specific case (as have determined yourself), no, exception won't re-raised.

in general case, there few places exceptions effectively propagate out; if have memoryerror because totally out of memory, it's deferred implementation allocate little memory somewhere attempting call function or , exception come you.

but risk of programming in python in general; there several exceptions (memoryerror, keyboardinterrupt) may arise no warning. if whole process isn't burning down, no, callback , errback won't raise exceptions except in cases you've outlined.


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