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# Using Threads in Twisted

## Running code in a thread-safe manner

Most code in Twisted is not thread-safe. For example, writing data to a transport from a protocol is not thread-safe. Therefore, we want a way to schedule methods to be run in the main event loop. This can be done using the function callFromThread:

    from twisted.internet import reactor

    def notThreadSafe(x):
         """do something that isn't thread-safe"""
         # ...

    def threadSafeScheduler():
        """Run in thread-safe manner."""
        reactor.callFromThread(notThreadSafe, 3) # will run 'notThreadSafe(3)'
                                                 # in the event loop
    reactor.run()

## Running code in threads

Sometimes we may want to run methods in threads. For example, in order to access blocking APIs. Twisted provides methods for doing so using the IReactorThreads API. Additional utility functions are provided in twisted.internet.threads. Basically, these methods allow us to queue methods to be run by a thread pool.

For example, to run a method in a thread we can do:

    from twisted.internet import reactor

    def aSillyBlockingMethod(x):
        import time
        time.sleep(2)
        print x

    # run method in thread
    reactor.callInThread(aSillyBlockingMethod, "2 seconds have passed")
    reactor.run()

## Utility Methods

The utility methods are not part of the reactor APIs, but are implemented in threads.

If we have multiple methods to run sequentially within a thread, we can do:

    from twisted.internet import reactor, threads

    def aSillyBlockingMethodOne(x):
        import time
        time.sleep(2)
        print x

    def aSillyBlockingMethodTwo(x):
        print x

    # run both methods sequentially in a thread
    commands = [(aSillyBlockingMethodOne, ["Calling First"], {})]
    commands.append((aSillyBlockingMethodTwo, ["And the second"], {}))
    threads.callMultipleInThread(commands)
    reactor.run()

For functions whose results we wish to get, we can have the result returned as a Deferred:

    from twisted.internet import reactor, threads

    def doLongCalculation():
        # .... do long calculation here ...
        return 3

    def printResult(x):
        print x

    # run method in thread and get result as defer.Deferred
    d = threads.deferToThread(doLongCalculation)
    d.addCallback(printResult)
    reactor.run()

If you wish to call a method in the reactor thread and get its result, you can use blockingCallFromThread:

    from twisted.internet import threads, reactor, defer
    from twisted.web.client import getPage
    from twisted.web.error import Error

    def inThread():
        try:
            result = threads.blockingCallFromThread(
                reactor, getPage, "http://twistedmatrix.com/")
        except Error, exc:
            print exc
        else:
            print result
        reactor.callFromThread(reactor.stop)

    reactor.callInThread(inThread)
    reactor.run()

`blockingCallFromThread` will return the object or raise the exception returned or raised by the function passed to it. If the function passed to it returns a Deferred, it will return the value the Deferred is called back with or raise the exception it is errbacked with.

## Managing the Thread Pool

The thread pool is implemented by ThreadPool.

We may want to modify the size of the thread pool, increasing or decreasing the number of threads in use. We can do this do this quite easily:

    from twisted.internet import reactor

    reactor.suggestThreadPoolSize(30)

The default size of the thread pool depends on the reactor being used; the default reactor uses a minimum size of 5 and a maximum size of 10. Be careful that you understand threads and their resource usage before drastically altering the thread pool sizes.