Package twisted :: Package flow :: Module stage :: Class Callback

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## Class Callback

    Instruction --+
                  |
              Stage --+
                      |
                     Callback

Known Subclasses:
  `_Protocol`

    Converts a single-thread push interface into a pull interface.

    Once this stage is constructed, its result, errback, and
    finish member variables may be called by a producer.   The
    results of which can be obtained by yielding the Callback and
    then calling next().   For example:

        source = flow.Callback()
        reactor.callLater(0, lambda: source.result("one"))
        reactor.callLater(.5, lambda: source.result("two"))
        reactor.callLater(1, lambda: source.finish())
        printFlow(source)

Method Summary
| `__init__(self, *trap)`
| ``__call__`(self, result)` called by the producer to indicate a successful result
| ``_yield`(self)` executed during a yield statement by previous stage
| ``errback`(self, fail)` called by the producer in case of Failure
| ``finish`(self)` called by producer to indicate successful stream completion
| ``result`(self, result)` called by the producer to indicate a successful result
Inherited from Stage
| ``__iter__`(self)`
| ``next`(self)` return current result

Class Variable Summary
`class` | `Instruction` = `twisted.flow.stage.Instruction`

Method Details

### __call__(self, result)
(Call operator)

called by the producer to indicate a successful result

### _yield(self)

executed during a yield statement by previous stage

This 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.

  Overrides:
    `twisted.flow.base.Stage._yield` (inherited documentation)

### errback(self, fail)

called by the producer in case of Failure

### finish(self)

called by producer to indicate successful stream completion

### result(self, result)

called by the producer to indicate a successful result

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