Title:Easy to use object-oriented thread pool framework
Author: Christopher Arndt
Version: 1.3.2
Date: 2015-11-29
License:MIT License


This module is OBSOLETE and is only provided on PyPI to support old projects that still use it. Please DO NOT USE IT FOR NEW PROJECTS!

Use modern alternatives like the multiprocessing module in the standard library or even an asynchroneous approach with asyncio.


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.


This module is regarded as an extended example, not as a finished product. Feel free to adapt it too your needs.

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.


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

API documentation

The documentation is also packaged in the distribution.


You can download the latest version of this module here:

Download directory

or see the colorized source code:

You can also install it from the Python Package Index PyPI via pip:

[sudo] pip install threadpool

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

git clone


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:


Due to the parallel nature of threads, you have to keep some things in mind:

  • Do not use simultaneous threads for tasks were they compete for a single, scarce resource (e.g. a harddisk or stdout). This will probably be slower than taking a serialized approach.
  • If you call ThreadPool.wait() the main thread will block until _all_ results have arrived. If you only want to check for results that are available immediately, use ThreadPool.poll().
  • The results of the work requests are not stored anywhere. You should provide an appropriate callback if you want to do so.


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


2015-11-29 (1.3.2)
  • Added missing to source distribution
2015-10-14 (1.3.1)
  • Minor distribution procedure changes
2015-10-14 (1.3.0)
  • Migrated repository from SVN to Git.
  • Incorporated changes (with minor adjustments) from Lutz Prechelt from to make Threadpool Python 3 compatible. Thanks, Lutz!
  • Fixed some build errors.
2009-10-07 (1.2.7)
  • I made a stupid error and made import from but this module is not installed by Removed import again.
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.
  • Update reference to "Python In A Nutshell" to second edition (suggested by Alex Martelli).
  • Fixed typo in (thanks to Nicholas Bollweg, Aaron Levinson, Rogério Schneider, Grégory Starck for reporting).
  • Fixed missing first argument in call to Queue.get() in (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).
  • '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.
  • 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 and created a proper distribution package
  • added timeout parameter to putRequest()