Description

A thread pool is an object that maintains a pool of worker threads to perform time consuming operations in parallel. It assigns jobs to the threads by putting them in a work request queue, where they are picked up by the next available thread. This then performs the requested operation in the background and puts the results in another queue.

The thread pool object can then collect the results from all threads from this queue as soon as they become available or after all threads have finished their work. It's then possible to define callbacks to handle each result as it comes in.

Basic usage

>>> pool = ThreadPool(poolsize)
>>> requests = makeRequests(some_callable, list_of_args, callback)
>>> [pool.putRequest(req) for req in requests]
>>> pool.wait()

See the end of the module source code for a longer, annotated usage example.

Documentation

You can view the API documentation, generated by epydoc, here:

API documentation

The documentation is also packaged in the distribution.

Download

You can download the latest version of this module here:

Download directory

or see the colorized source code:

threadpool.py

You can also install it from the Cheeseshop via easy_install:

[sudo] easy:install threadpool

Or you can check out the latest development version from the Subversion repository:

svn co svn://svn.chrisarndt.de/projects/threadpool/trunk threadpool

Discussion

The basic concept and some code was taken from the book "Python in a Nutshell" by Alex Martelli, copyright O'Reilly 2003, ISBN 0-596-00188-6, from section 14.5 "Threaded Program Architecture". I wrapped the main program logic in the ThreadPool class, added the WorkRequest class and the callback system and tweaked the code here and there.

There are some other recipes in the Python Cookbook, that serve a similar purpose. This one distinguishes itself by the following characteristics:

• Object-oriented, reusable design
• Provides callback mechanism to process results as they are returned from the worker threads.
• WorkRequest objects wrap the tasks assigned to the worker threads and allow for easy passing of arbitrary data to the callbacks.
• The use of the Queue class solves most locking issues.
• All worker threads are daemonic, so they exit when the main programm exits, no need for joining.
• Threads start running as soon as you create them. No need to start or stop them. You can increase or decrease the pool size at any time, superfluous threads will just exit when they finish their current task.
• You don't need to keep a reference to a thread after you have assigned the last task to it. You just tell it: "don't come back looking for work, when you're done!"
• Threads don't eat up cycles while waiting to be assigned a task, they just block when the task queue is empty (though they wake up every few seconds to check whether they are dismissd).

References

There are several other recipes similar to this module in the Python Cookbook, for example:

• http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/203871
• http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/196618
• http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/302746

News

2009-10-06 (1.2.6)

• Due to some mix up up the I got the bugfix for the 'timeout' parameter to ThreadPool.putRequest exactly the wrong way round (or I "fixed" it twice). It now defaults to None as its should, so putRequest blocks by default, if the requests queue is full. Thanks for Guillaume Taglang for reporting the issue.
• Rename NEWS.txt to CHANGELOG.txt (this file).
• Add SVN checkout instructions to README.

2008-11-19

• Update reference to "Python In A Nutshell" to second edition (suggested by Alex Martelli).
• Fixed typo in WorkerThread.run() (thanks to Nicholas Bollweg, Aaron Levinson, Rogério Schneider, Grégory Starck for reporting).
• Fixed missing first argument in call to Queue.get() in WorkerThread.run() (thanks to Aaron Levinson for report).
• added new argument 'do_join' to ThreadPool.dismissWorkers(). When True, the method will perform Thread.join() on each thread after dismissing it.
• Added joinAllDismissedWorkers method to ThreadPool to join dismissed threads at a later time (thanks to Aaron Levinson for patch for these two changes).

2008-05-04

• 'timeout' parameter of ThreadPool.putRequest now correctly defaults to 0 instead of None (thanks to Mads Sülau Jørgensen for bug report).
• Added default exception handler callback (thanks to Moshe Cohen for the patch).
• Fixed locking issue that prevented worker threads from being dismissed when no work requests are in the requests queue (thanks to Guillaume Pratte for the bug report).
• Add option for results queue size to ThreadPool (thanks to Krzysztof Jakubczyk for the idea).
• Changed name of reuquestQueue and resultsQueue attributes in WorkerThread and ThreadPool to _requests_queue and _results_queue to be more consistent and compliant with PEP 8 and properly indicate private nature.
• Moved repository to Subversion.

2008-05-03

• Updated homepage and download URL
• Updated README
• Enable packaging as an eggs with the use of setuptools
• License changes to MIT License (Python license is only for code licensed by the PSF)

2006-06-23 1.2.3 (never announced)

• fixed typo in ThreadPool.putRequest() (reported by Jérôme Schneider)

2006-05-19 1.2.2 (first release as a package)

• fixed wrong usage of isinstance in makeRequests() Thanks to anonymous for bug report in comment on ASPN
• added setup.py and created a proper distribution package
• added timeout parameter to putRequest()