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<title>Twisted Documentation: Using Threads in Twisted</title>

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<h1 class="title">Using Threads in Twisted</h1>

<div class="toc"><ol><li><a href="#auto0">Running code in a thread-safe manner</a></li><li><a href="#auto1">Running code in threads</a></li><li><a href="#auto2">Utility Methods</a></li><li><a href="#auto3">Managing the Thread Pool</a></li></ol></div>

<h2>Running code in a thread-safe manner<a name="auto0"/></h2>

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 <code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.internet.interfaces.IReactorThreads.callFromThread.html" title="twisted.internet.interfaces.IReactorThreads.callFromThread">twisted.internet.interfaces.IReactorThreads.callFromThread</a></code>:

<pre class="python"> 1

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()

</pre>

<h2>Running code in threads<a name="auto1"/></h2>

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 (<code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.internet.interfaces.IReactorThreads.html" title="twisted.internet.interfaces.IReactorThreads">twisted.internet.interfaces.IReactorThreads</a></code>).

Additional utility functions are provided in <code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.internet.threads.html" title="twisted.internet.threads">twisted.internet.threads</a></code>. 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:

<pre class="python"> 1

def aSillyBlockingMethod(x):

import time

time.sleep(2)

print x

# run method in thread

reactor.callInThread(aSillyBlockingMethod, "2 seconds have passed")

reactor.run()

</pre>

<h2>Utility Methods<a name="auto2"/></h2>

The utility methods are not part of the <code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.internet.reactor.html" title="twisted.internet.reactor">twisted.internet.reactor</a></code> APIs, but are implemented

in <code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.internet.threads.html" title="twisted.internet.threads">twisted.internet.threads</a></code>.

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

we can do:

<pre class="python"> 1

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def aSillyBlockingMethodOne(x):

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import time

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time.sleep(2)

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print x

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def aSillyBlockingMethodTwo(x):

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print x

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# run both methods sequentially in a thread

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commands = [(aSillyBlockingMethodOne, ["Calling First"], {})]

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commands.append((aSillyBlockingMethodTwo, ["And the second"], {}))

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threads.callMultipleInThread(commands)

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reactor.run()

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</pre>

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For functions whose results we wish to get, we can have the

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result returned as a Deferred:

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<pre class="python"> 1

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def doLongCalculation():

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# .... do long calculation here ...

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return 3

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def printResult(x):

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print x

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# run method in thread and get result as defer.Deferred

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d = threads.deferToThread(doLongCalculation)

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d.addCallback(printResult)

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reactor.run()

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</pre>

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

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you can use <code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.internet.threads.blockingCallFromThread.html" title="twisted.internet.threads.blockingCallFromThread">blockingCallFromThread</a></code>:

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<pre class="python"> 1

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from twisted.web.client import getPage

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from twisted.web.error import Error

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def inThread():

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try:

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result = threads.blockingCallFromThread(

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reactor, getPage, "http://twistedmatrix.com/")

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except Error, exc:

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print exc

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else:

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print result

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reactor.callFromThread(reactor.stop)

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reactor.callInThread(inThread)

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reactor.run()

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</pre>

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<code>blockingCallFromThread</code> will return the object or raise

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the exception returned or raised by the function passed to it. If the

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function passed to it returns a Deferred, it will return the value the

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Deferred is called back with or raise the exception it is errbacked

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with.

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<h2>Managing the Thread Pool<a name="auto3"/></h2>

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The thread pool is implemented by <code class="API"><a href="http://twistedmatrix.com/documents/10.0.0/api/twisted.python.threadpool.ThreadPool.html" title="twisted.python.threadpool.ThreadPool">twisted.python.threadpool.ThreadPool</a></code>.

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We may want to modify the size of the threadpool, increasing

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or decreasing the number of threads in use. We can do this

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do this quite easily:

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<pre class="python">1

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reactor.suggestThreadPoolSize(30)

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</pre>

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The default size of the thread pool depends on the reactor being used;

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the default reactor uses a minimum size of 5 and a maximum size of 10. Be

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careful that you understand threads and their resource usage before

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drastically altering the thread pool sizes.

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</div>

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