# Deferred Reference

This document is a guide to the behaviour of the `twisted.internet.defer.Deferred` object, and to various ways you can use them when they are returned by functions.

This document assumes that you are familiar with the basic principle that the Twisted framework is structured around: asynchronous, callback-based programming, where instead of having blocking code in your program or using threads to run blocking code, you have functions that return immediately and then begin a callback chain when data is available.

After reading this document, the reader should expect to be able to deal with most simple APIs in Twisted and Twisted-using code that return Deferreds.

- what sorts of things you can do when you get a Deferred from a function call; and
- how you can write your code to robustly handle errors in Deferred code.

## Deferreds

Twisted uses the `Deferred` object to manage the callback sequence. The client application attaches a series of functions to the deferred to be called in order when the results of the asychronous request are available (this series of functions is known as a series of callbacks, or a callback chain), together with a series of functions to be called if there is an error in the asychronous request (known as a series of errbacks or an errback chain). The asychronous library code calls the first callback when the result is available, or the first errback when an error occurs, and the `Deferred` object then hands the results of each callback or errback function to the next function in the chain.

## Callbacks

A `twisted.internet.defer.Deferred` is a promise that a function will at some point have a result. We can attach callback functions to a Deferred, and once it gets a result these callbacks will be called. In addition Deferreds allow the developer to register a callback for an error, with the default behavior of logging the error. The deferred mechanism standardizes the application programmer's interface with all sorts of blocking or delayed operations.

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    from twisted.internet import reactor, defer

    def getDummyData(x):
        """
        This function is a dummy which simulates a delayed result and
        returns a Deferred which will fire with that result. Don't try too
        hard to understand this.
        """
        d = defer.Deferred()
        # simulate a delayed result by asking the reactor to fire the
        # Deferred in 2 seconds time with the result x * 3
        reactor.callLater(2, d.callback, x * 3)
        return d

    def printData(d):
        """
        Data handling function to be added as a callback: handles the
        data by printing the result
        """
        print d

    d = getDummyData(3)
    d.addCallback(printData)

    # manually set up the end of the process by asking the reactor to
    # stop itself in 4 seconds time
    reactor.callLater(4, reactor.stop)
    # start up the Twisted reactor (event loop handler) manually
    reactor.run()

### Multiple callbacks

Multiple callbacks can be added to a Deferred. The first callback in the Deferred's callback chain will be called with the result, the second with the result of the first callback, and so on. Why do we need this? Well, consider a Deferred returned by twisted.enterprise.adbapi - the result of a SQL query. A web widget might add a callback that converts this result into HTML, and pass the Deferred onwards, where the callback will be used by twisted to return the result to the HTTP client. The callback chain will be bypassed in case of errors or exceptions.

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    from twisted.internet import reactor, defer

    class Getter:
        def gotResults(self, x):
            """
            The Deferred mechanism provides a mechanism to signal error
            conditions.  In this case, odd numbers are bad.

            This function demonstrates a more complex way of starting
            the callback chain by checking for expected results and
            choosing whether to fire the callback or errback chain
            """
            if self.d is None:
                print "Nowhere to put results"
                return

            d = self.d
            self.d = None
            if x % 2 == 0:
                d.callback(x*3)
            else:
                d.errback(ValueError("You used an odd number!"))

        def _toHTML(self, r):
            """
            This function converts r to HTML.

            It is added to the callback chain by getDummyData in
            order to demonstrate how a callback passes its own result
            to the next callback
            """
            return "Result: %s" % r

        def getDummyData(self, x):
            """
            The Deferred mechanism allows for chained callbacks.
            In this example, the output of gotResults is first
            passed through _toHTML on its way to printData.

            Again this function is a dummy, simulating a delayed result
            using callLater, rather than using a real asynchronous
            setup.
            """
            self.d = defer.Deferred()
            # simulate a delayed result by asking the reactor to schedule
            # gotResults in 2 seconds time
            reactor.callLater(2, self.gotResults, x)
            self.d.addCallback(self._toHTML)
            return self.d

    def printData(d):
        print d

    def printError(failure):
        import sys
        sys.stderr.write(str(failure))

    # this series of callbacks and errbacks will print an error message
    g = Getter()
    d = g.getDummyData(3)
    d.addCallback(printData)
    d.addErrback(printError)

    # this series of callbacks and errbacks will print "Result: 12"
    g = Getter()
    d = g.getDummyData(4)
    d.addCallback(printData)
    d.addErrback(printError)

    reactor.callLater(4, reactor.stop)
    reactor.run()

Note: Pay particular attention to the handling of `self.d` in the `gotResults` method. Before the `Deferred` is fired with a result or an error, the attribute is set to `None` so that the `Getter` instance no longer has a reference to the `Deferred` about to be fired. This has several benefits. First, it avoids any chance `Getter.gotResults` will accidentally fire the same `Deferred` more than once (which would result in an `AlreadyCalledError` exception). Second, it allows a callback on that `Deferred` to call `Getter.getDummyData` (which sets a new value for the `d` attribute) without causing problems. Third, it makes the Python garbage collector's job easier by eliminating a reference cycle.

### Visual Explanation

1. Requesting method (data sink) requests data, gets Deferred object.
2. Requesting method attaches callbacks to Deferred object.

1. When the result is ready, give it to the Deferred object. `.callback(result)` if the operation succeeded, `.errback(failure)` if it failed. Note that `failure` is typically an instance of a `twisted.python.failure.Failure` instance.
2. Deferred object triggers previously-added (call/err)back with the `result` or `failure`. Execution then follows the following rules, going down the chain of callbacks to be processed.
   - Result of the callback is always passed as the first argument to the next callback, creating a chain of processors.
   - If a callback raises an exception, switch to errback.
   - An unhandled failure gets passed down the line of errbacks, this creating an asynchronous analog to a series to a series of `except:` statements.
   - If an errback doesn't raise an exception or return a `twisted.python.failure.Failure` instance, switch to callback.

## Errbacks

Deferred's error handling is modeled after Python's exception handling. In the case that no errors occur, all the callbacks run, one after the other, as described above.

If the errback is called instead of the callback (e.g. because a DB query raised an error), then a `twisted.python.failure.Failure` is passed into the first errback (you can add multiple errbacks, just like with callbacks). You can think of your errbacks as being like `except` blocks of ordinary Python code.

Unless you explicitly `raise` an error in an except block, the `Exception` is caught and stops propagating, and normal execution continues. The same thing happens with errbacks: unless you explicitly `return` a `Failure` or (re-)raise an exception, the error stops propagating, and normal callbacks continue executing from that point (using the value returned from the errback). If the errback does return a `Failure` or raise an exception, then that is passed to the next errback, and so on.

Note: If an errback doesn't return anything, then it effectively returns `None`, meaning that callbacks will continue to be executed after this errback. This may not be what you expect to happen, so be careful. Make sure your errbacks return a `Failure` (probably the one that was passed to it), or a meaningful return value for the next callback.

Also, `twisted.python.failure.Failure` instances have a useful method called trap, allowing you to effectively do the equivalent of:

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    try:
        # code that may throw an exception
        cookSpamAndEggs()
    except (SpamException, EggException):
        # Handle SpamExceptions and EggExceptions
        ...

You do this by:

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    def errorHandler(failure):
        failure.trap(SpamException, EggException)
        # Handle SpamExceptions and EggExceptions

    d.addCallback(cookSpamAndEggs)
    d.addErrback(errorHandler)

If none of arguments passed to `failure.trap` match the error encapsulated in that `Failure`, then it re-raises the error.

There's another potential gotcha here. There's a method `twisted.internet.defer.Deferred.addCallbacks` which is similar to, but not exactly the same as, `addCallback` followed by `addErrback`. In particular, consider these two cases:

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    # Case 1
    d = getDeferredFromSomewhere()
    d.addCallback(callback1)       # A
    d.addErrback(errback1)         # B
    d.addCallback(callback2)
    d.addErrback(errback2)

    # Case 2
    d = getDeferredFromSomewhere()
    d.addCallbacks(callback1, errback1)  # C
    d.addCallbacks(callback2, errback2)

If an error occurs in `callback1`, then for Case 1 `errback1` will be called with the failure. For Case 2, `errback2` will be called. Be careful with your callbacks and errbacks.

What this means in a practical sense is in Case 1, the callback in line A will handle a success condition from `getDeferredFromSomewhere`, and the errback in line B will handle any errors that occur from either the upstream source, or that occur in A. In Case 2, the errback in line C will only handle an error condition raised by `getDeferredFromSomewhere`, it will not do any handling of errors raised in `callback1`.

### Unhandled Errors

If a Deferred is garbage-collected with an unhandled error (i.e. it would call the next errback if there was one), then Twisted will write the error's traceback to the log file. This means that you can typically get away with not adding errbacks and still get errors logged. Be careful though; if you keep a reference to the Deferred around, preventing it from being garbage-collected, then you may never see the error (and your callbacks will mysteriously seem to have never been called). If unsure, you should explicitly add an errback after your callbacks, even if all you do is:

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    # Make sure errors get logged
    from twisted.python import log
    d.addErrback(log.err)

## Handling either synchronous or asynchronous results

In some applications, there are functions that might be either asynchronous or synchronous. For example, a user authentication function might be able to check in memory whether a user is authenticated, allowing the authentication function to return an immediate result, or it may need to wait on network data, in which case it should return a Deferred to be fired when that data arrives. However, a function that wants to check if a user is authenticated will then need to accept both immediate results and Deferreds.

In this example, the library function `authenticateUser` uses the application function `isValidUser` to authenticate a user:

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    def authenticateUser(isValidUser, user):
        if isValidUser(user):
            print "User is authenticated"
        else:
            print "User is not authenticated"

However, it assumes that `isValidUser` returns immediately, whereas `isValidUser` may actually authenticate the user asynchronously and return a Deferred. It is possible to adapt this trivial user authentication code to accept either a synchronous `isValidUser` or an asynchronous `isValidUser`, allowing the library to handle either type of function. It is, however, also possible to adapt synchronous functions to return Deferreds. This section describes both alternatives: handling functions that might be synchronous or asynchronous in the library function (`authenticateUser`) or in the application code.

### Handling possible Deferreds in the library code

Here is an example of a synchronous user authentication function that might be passed to `authenticateUser`:

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    def synchronousIsValidUser(user):
        '''
        Return true if user is a valid user, false otherwise
        '''
        return user in ["Alice", "Angus", "Agnes"]

Source listing - listings/deferred/synch-validation.py

However, here's an `asynchronousIsValidUser` function that returns a Deferred:

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    from twisted.internet import reactor, defer

    def asynchronousIsValidUser(user):
        d = defer.Deferred()
        reactor.callLater(2, d.callback, user in ["Alice", "Angus", "Agnes"])
        return d

Our original implementation of `authenticateUser` expected `isValidUser` to be synchronous, but now we need to change it to handle both synchronous and asynchronous implementations of `isValidUser`. For this, we use `maybeDeferred` to call `isValidUser`, ensuring that the result of `isValidUser` is a Deferred, even if `isValidUser` is a synchronous function:

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    from twisted.internet import defer

    def printResult(result):
        if result:
            print "User is authenticated"
        else:
            print "User is not authenticated"

    def authenticateUser(isValidUser, user):
        d = defer.maybeDeferred(isValidUser, user)
        d.addCallback(printResult)

Now `isValidUser` could be either `synchronousIsValidUser` or `asynchronousIsValidUser`.

It is also possible to modify `synchronousIsValidUser` to return a Deferred, see Generating Deferreds for more information.

## DeferredList

Sometimes you want to be notified after several different events have all happened, rather than waiting for each one individually. For example, you may want to wait for all the connections in a list to close. `twisted.internet.defer.DeferredList` is the way to do this.

To create a DeferredList from multiple Deferreds, you simply pass a list of the Deferreds you want it to wait for:

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    # Creates a DeferredList
    dl = defer.DeferredList([deferred1, deferred2, deferred3])

You can now treat the DeferredList like an ordinary Deferred; you can call `addCallbacks` and so on. The DeferredList will call its callback when all the deferreds have completed. The callback will be called with a list of the results of the Deferreds it contains, like so:

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    # A callback that unpacks and prints the results of a DeferredList
    def printResult(result):
        for (success, value) in result:
            if success:
                print 'Success:', value
            else:
                print 'Failure:', value.getErrorMessage()

    # Create three deferreds.
    deferred1 = defer.Deferred()
    deferred2 = defer.Deferred()
    deferred3 = defer.Deferred()

    # Pack them into a DeferredList
    dl = defer.DeferredList([deferred1, deferred2, deferred3], consumeErrors=True)

    # Add our callback
    dl.addCallback(printResult)

    # Fire our three deferreds with various values.
    deferred1.callback('one')
    deferred2.errback(Exception('bang!'))
    deferred3.callback('three')

    # At this point, dl will fire its callback, printing:
    #    Success: one
    #    Failure: bang!
    #    Success: three
    # (note that defer.SUCCESS == True, and defer.FAILURE == False)

A standard DeferredList will never call errback, but failures in Deferreds passed to a DeferredList will still errback unless `consumeErrors` is passed `True`. See below for more details about this and other flags which modify the behavior of DeferredList.

Note:

If you want to apply callbacks to the individual Deferreds that go into the DeferredList, you should be careful about when those callbacks are added. The act of adding a Deferred to a DeferredList inserts a callback into that Deferred (when that callback is run, it checks to see if the DeferredList has been completed yet). The important thing to remember is that it is this callback which records the value that goes into the result list handed to the DeferredList's callback.

Therefore, if you add a callback to the Deferred after adding the Deferred to the DeferredList, the value returned by that callback will not be given to the DeferredList's callback. To avoid confusion, we recommend not adding callbacks to a Deferred once it has been used in a DeferredList.

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    def printResult(result):
        print result
    def addTen(result):
        return result + " ten"

    # Deferred gets callback before DeferredList is created
    deferred1 = defer.Deferred()
    deferred2 = defer.Deferred()
    deferred1.addCallback(addTen)
    dl = defer.DeferredList([deferred1, deferred2])
    dl.addCallback(printResult)
    deferred1.callback("one") # fires addTen, checks DeferredList, stores "one ten"
    deferred2.callback("two")
    # At this point, dl will fire its callback, printing:
    #     [(1, 'one ten'), (1, 'two')]

    # Deferred gets callback after DeferredList is created
    deferred1 = defer.Deferred()
    deferred2 = defer.Deferred()
    dl = defer.DeferredList([deferred1, deferred2])
    deferred1.addCallback(addTen) # will run *after* DeferredList gets its value
    dl.addCallback(printResult)
    deferred1.callback("one") # checks DeferredList, stores "one", fires addTen
    deferred2.callback("two")
    # At this point, dl will fire its callback, printing:
    #     [(1, 'one), (1, 'two')]

### Other behaviours

DeferredList accepts three keyword arguments that modify its behaviour: `fireOnOneCallback`, `fireOnOneErrback` and `consumeErrors`. If `fireOnOneCallback` is set, the DeferredList will immediately call its callback as soon as any of its Deferreds call their callback. Similarly, `fireOnOneErrback` will call errback as soon as any of the Deferreds call their errback. Note that DeferredList is still one-shot, like ordinary Deferreds, so after a callback or errback has been called the DeferredList will do nothing further (it will just silently ignore any other results from its Deferreds).

The `fireOnOneErrback` option is particularly useful when you want to wait for all the results if everything succeeds, but also want to know immediately if something fails.

The `consumeErrors` argument will stop the DeferredList from propagating any errors along the callback chains of any Deferreds it contains (usually creating a DeferredList has no effect on the results passed along the callbacks and errbacks of their Deferreds). Stopping errors at the DeferredList with this option will prevent Unhandled error in Deferred warnings from the Deferreds it contains without needing to add extra errbacks1. Passing a true value for the `consumeErrors` parameter will not change the behavior of `fireOnOneCallback` or `fireOnOneErrback`.

### gatherResults

A common use for DeferredList is to "join" a number of parallel asynchronous operations, finishing successfully if all of the operations were successful, or failing if any one of the operations fails. In this case, `twisted.internet.defer.gatherResults` is a useful shortcut:

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    from twisted.internet import defer
    d1 = defer.Deferred()
    d2 = defer.Deferred()
    d = defer.gatherResults([d1, d2], consumeErrors=True)
    def printResult(result):
        print result
    d.addCallback(printResult)
    d1.callback("one")
    # nothing is printed yet; d is still awaiting completion of d2
    d2.callback("two")
    # printResult prints ["one", "two"]

The `consumeErrors` argument has the same meaning as it does for `DeferredList`: if true, it causes `gatherResults` to consume any errors in the passed-in Deferreds. Always use this argument unless you are adding further callbacks or errbacks to the passed-in Deferreds, or unless you know that they will not fail. Otherwise, a failure will result in an unhandled error being logged by Twisted. This argument is available since Twisted 11.1.0.

## Class Overview

This is an overview API reference for Deferred from the point of using a Deferred returned by a function. It is not meant to be a substitute for the docstrings in the Deferred class, but can provide guidelines for its use.

There is a parallel overview of functions used by the Deferred's creator in Generating Deferreds.

### Basic Callback Functions

- `addCallbacks(self, callback[, errback, callbackArgs, callbackKeywords, errbackArgs, errbackKeywords])`

  This is the method you will use to interact with Deferred. It adds a pair of callbacks parallel to each other (see diagram above) in the list of callbacks made when the Deferred is called back to. The signature of a method added using addCallbacks should be `myMethod(result, *methodArgs, **methodKeywords)`. If your method is passed in the callback slot, for example, all arguments in the tuple `callbackArgs` will be passed as `*methodArgs` to your method.

  There are various convenience methods that are derivative of addCallbacks. I will not cover them in detail here, but it is important to know about them in order to create concise code.

  - `addCallback(callback, *callbackArgs, **callbackKeywords)`

    Adds your callback at the next point in the processing chain, while adding an errback that will re-raise its first argument, not affecting further processing in the error case.

    Note that, while addCallbacks (plural) requires the arguments to be passed in a tuple, addCallback (singular) takes all its remaining arguments as things to be passed to the callback function. The reason is obvious: addCallbacks (plural) cannot tell whether the arguments are meant for the callback or the errback, so they must be specifically marked by putting them into a tuple. addCallback (singular) knows that everything is destined to go to the callback, so it can use Python's * and ** syntax to collect the remaining arguments.

  - `addErrback(errback, *errbackArgs, **errbackKeywords)`

    Adds your errback at the next point in the processing chain, while adding a callback that will return its first argument, not affecting further processing in the success case.

  - `addBoth(callbackOrErrback, *callbackOrErrbackArgs, **callbackOrErrbackKeywords)`

    This method adds the same callback into both sides of the processing chain at both points. Keep in mind that the type of the first argument is indeterminate if you use this method! Use it for `finally:` style blocks.

### Chaining Deferreds

If you need one Deferred to wait on another, all you need to do is return a Deferred from a method added to addCallbacks. Specifically, if you return Deferred B from a method added to Deferred A using A.addCallbacks, Deferred A's processing chain will stop until Deferred B's .callback() method is called; at that point, the next callback in A will be passed the result of the last callback in Deferred B's processing chain at the time.

If this seems confusing, don't worry about it right now -- when you run into a situation where you need this behavior, you will probably recognize it immediately and realize why this happens. If you want to chain deferreds manually, there is also a convenience method to help you.

- `chainDeferred(otherDeferred)`

  Add `otherDeferred` to the end of this Deferred's processing chain. When self.callback is called, the result of my processing chain up to this point will be passed to `otherDeferred.callback`. Further additions to my callback chain do not affect `otherDeferred`

  This is the same as `self.addCallbacks(otherDeferred.callback, otherDeferred.errback)`

## See also

1. Generating Deferreds, an introduction to writing asynchronous functions that return Deferreds.

## Footnotes

1. Unless of course a later callback starts a fresh error — but as we've already noted, adding callbacks to a Deferred after its used in a DeferredList is confusing and usually avoided.

Index

Version: 13.0.0