Part of twisted.flow.base View Source View In Hierarchy
Known subclasses: twisted.flow.controller.Block, twisted.flow.pipe.Pipe, twisted.flow.stage.Callback, twisted.flow.stage.Concurrent, twisted.flow.stage.Map, twisted.flow.stage.Merge, twisted.flow.threads.Threaded, twisted.flow.wrap._Deferred, twisted.flow.wrap._Iterable, twisted.flow.wrap._List, twisted.flow.wrap._String
Abstract base defining protocol for iterator/generators in a flowThis is the primary component in the flow system, it is an iterable object which must be passed to a yield statement before each call to next(). Usage:
iterable = DerivedStage( ... , SpamError, EggsError)) yield iterable for result in iterable: # handle good result, or SpamError or EggsError yield iterableAlternatively, when inside a generator, the next() method can be used directly. In this case, if no results are available, StopIteration is raised, and if left uncaught, will nicely end the generator. Of course, unexpected failures are raised. This technique is especially useful when pulling from more than one stage at a time. For example:
def someGenerator(): iterable = SomeStage( ... , SpamError, EggsError) while True: yield iterable result = iterable.next() # handle good result or SpamError or EggsErrorFor many generators, the results become available in chunks of rows. While the default value is to get one row at a time, there is a 'chunked' property which allows them to be returned via the next() method as many rows rather than row by row. For example:
iterable = DerivedStage(...) iterable.chunked = True for results in iterable: for result in results: # handle good result yield iterableFor those wishing more control at the cost of a painful experience, the following member variables can be used to great effect:
- results: This is a list of results produced by the generator, they can be fetched one by one using next() or in a group together. If no results were produced, then this is an empty list. These results should be removed from the list after they are read; or, after reading all of the results set to an empty list - stop: This is true if the underlying generator has finished execution (raised a StopIteration or returned). Note that several results may exist, and stop may be true. - failure: If the generator produced an exception, then it is wrapped as a Failure object and put here. Note that several results may have been produced before the failure. To ensure that the failure isn't accidently reported twice, it is adviseable to set stop to True.The order in which these member variables is used is *critical* for proper adherance to the flow protocol. First, all successful results should be handled. Second, the iterable should be checked to see if it is finished. Third, a failure should be checked; while handling a failure, either the loop should be exited, or the iterable's stop member should be set. For example:
iterable = SomeStage(...) while True: yield iterable if iterable.results: for result in iterable.results: # handle good result iterable.results =  if iterable.stop: break if iterable.failure: iterable.stop = True # handle iterable.failure break
|Method||next||return current result|
|Method||_yield||executed during a yield statement by previous stage|
return current resultThis is the primary function to be called to retrieve the current result. It complies with the iterator protocol by raising StopIteration when the stage is complete. It also raises an exception if it is called before the stage is yielded.
executed during a yield statement by previous stageThis method is private within the scope of the flow module, it is used by one stage in the flow to ask a subsequent stage to produce its value. The result of the yield is then stored in self.result and is an instance of Failure if a problem occurred.