SimPy History & Change Log

SimPy was originally based on ideas from Simula and Simscript but uses standard Python. It combines two previous packages, SiPy, in Simula-Style (Klaus Müller) and SimPy, in Simscript style (Tony Vignaux and Chang Chui).

SimPy was based on efficient implementation of co-routines using Python’s generators capability.

SimPy 3 introduced a completely new and easier-to-use API, but still relied on Python’s generators as they proved to work very well.

The package has been hosted on since September 15th, 2002. In June 2012, the project moved to

3.0.1 – 2013-10-24

  • [FIX] Documentation and default parameters of Store didn’t match. Its default capacity is now inf.

3.0 – 2013-10-11

SimPy 3 has been completely rewritten from scratch. Our main goals were to simplify the API and code base as well as making SimPy more flexible and extensible. Some of the most important changes are:

  • Stronger focus on events. Processes yield event instances and are suspended until the event is triggered. An example for an event is a timeout (formerly known as hold), but even processes are now events, too (you can wait until a process terminates).
  • Events can be combined with & (and) and | (or) to create condition events.
  • Process can now be defined by any generator function. You don’t have to subclass Process anymore.
  • No more global simulation state. Every simulation stores its state in an environment which is comparable to the old Simulation class.
  • Improved resource system with newly added resource types.
  • Removed plotting and GUI capabilities. Pyside and matplotlib are much better with this.
  • Greatly improved test suite. Its cleaner, and the tests are shorter and more numerous.
  • Completely overhauled documentation.

There is a guide for porting from SimPy 2 to SimPy 3. If you want to stick to SimPy 2 for a while, change your requirements to 'SimPy>=2.3,<3'.

All in all, SimPy has become a framework for asynchronous programming based on coroutines. It brings more than ten years of experience and scientific know-how in the field of event-discrete simulation to the world of asynchronous programming and should thus be a solid foundation for everything based on an event loop.

You can find information about older versions on the history page

2.3.1 – 2012-01-28

  • [NEW] More improvements on the documentation.
  • [FIX] Syntax error in when installing on Py3.2.
  • [FIX] Added mock to the dep. list in SimPy.test().

2.3 – 2011-12-24

  • [NEW] Support for Python 3.2. Support for Python <= 2.5 has been dropped.
  • [NEW] SimPy.test() method to run the tests on the installed version of SimPy.
  • [NEW] Tutorials/examples were integrated into the test suite.
  • [CHANGE] Even more code clean-up (e.g., removed prints throughout the code, removed if-main-blocks, ...).
  • [CHANGE] Many documentation improvements.

2.2 – 2011-09-27

  • [CHANGE] Restructured package layout to be conform to the Hitchhiker’s Guide to packaging
  • [CHANGE] Tests have been ported to pytest.
  • [CHANGE] Documentation improvements and clean-ups.
  • [FIX] Fixed incorrect behavior of Store._put, thanks to Johannes Koomer for the fix.

2.1 – 2010-06-03

  • [NEW] A function step has been added to the API. When called, it executes the next scheduled event. (step is actually a method of Simulation.)
  • [NEW] Another new function is peek. It returns the time of the next event. By using peek and step together, one can easily write e.g. an interactive program to step through a simulation event by event.
  • [NEW] A simple interactive debugger has been added. It allows stepping through a simulation, with options to skip to a certain time, skip to the next event of a given process, or viewing the event list.
  • [NEW] Versions of the Bank tutorials (documents and programs) using the advanced- [NEW] object-oriented API have been added.
  • [NEW] A new document describes tools for gaining insight into and debugging SimPy models.
  • [CHANGE] Major re-structuring of SimPy code, resulting in much less SimPy code – great for the maintainers.
  • [CHANGE] Checks have been added which test whether entities belong to the same Simulation instance.
  • [CHANGE] The Monitor and Tally methods timeAverage and timeVariance now calculate only with the observed time-series. No value is assumed for the period prior to the first observation.
  • [CHANGE] Changed class Lister so that circular references between objects no longer lead to stack overflow and crash.
  • [FIX] Functions allEventNotices and allEventTimes are working again.
  • [FIX] Error messages for methods in SimPy.Lib work again.

2.0.1 – 2009-04-06

  • [NEW] Tests for real time behavior ( and in folder SimPy).
  • [FIX] Repaired a number of coding errors in several models in the SimPyModels folder.
  • [FIX] Repaired bug introduced by recoding for the OO API.
  • [FIX] Repaired errors in sample programs in documents:
    • Simulation with SimPy - In Depth Manual
    • SimPy’s Object Oriented API Manual
    • Simulation With Real Time Synchronization Manual
    • SimPlot Manual
    • Publication-quality Plot Production With Matplotlib Manual

2.0.0 – 2009-01-26

This is a major release with changes to the SimPy application programming interface (API) and the formatting of the documentation.

API changes

In addition to its existing API, SimPy now also has an object oriented API. The additional API

  • allows running SimPy in parallel on multiple processors or multi-core CPUs,
  • supports better structuring of SimPy programs,
  • allows subclassing of class Simulation and thus provides users with the capability of creating new simulation modes/libraries like SimulationTrace, and
  • reduces the total amount of SimPy code, thereby making it easier to maintain.

Note that the OO API is in addition to the old API. SimPy 2.0 is fully backward compatible.

Documentation format changes

SimPy’s documentation has been restructured and processed by the Sphinx documentation generation tool. This has generated one coherent, well structured document which can be easily browsed. A seach capability is included.

March 2008: Version 1.9.1

This is a bug-fix release which cures the following bugs:

  • Excessive production of circular garbage, due to a circular reference between Process instances and event notices. This led to large memory requirements.
  • Runtime error for preempts of proceeses holding multiple Resource objects.

It also adds a Short Manual, describing only the basic facilities of SimPy.

December 2007: Version 1.9

This is a major release with added functionality/new user API calls and bug fixes.

Major changes

  • The event list handling has been changed to improve the runtime performance of large SimPy models (models with thousands of processes). The use of dictionaries for timestamps has been stopped. Thanks are due to Prof. Norm Matloff and a team of his students who did a study on improving SimPy performance. This was one of their recommendations. Thanks, Norm and guys! Furthermore, in version 1.9 the ‘heapq’ sorting package replaces ‘bisect’. Finally, cancelling events no longer removes them, but rather marks them. When their event time comes, they are ignored. This was Tony Vignaux’ idea!
  • The Manual has been edited and given an easier-to-read layout.
  • The Bank2 tutorial has been extended by models which use more advanced SimPy commands/constructs.

Bug fixes

  • The tracing of ‘activate’ statements has been enabled.


  • A method returning the time-weighted variance of observations has been added to classes Monitor and Tally.
  • A shortcut activation method called “start” has been added to class Process.

January 2007: Version 1.8

Major Changes

  • SimPy 1.8 and future releases will not run under the obsolete Python 2.2 version. They require Python 2.3 or later.
  • The Manual has been thoroughly edited, restructured and rewritten. It is now also provided in PDF format.
  • The Cheatsheet has been totally rewritten in a tabular format. It is provided in both XLS (MS Excel spreadsheet) and PDF format.
  • The version of SimPy.Simulation(RT/Trace/Step) is now accessible by the variable ‘version’.
  • The __str__ method of Histogram was changed to return a table format.

Bug fixes

  • Repaired a bug in yield waituntil runtime code.
  • Introduced check for capacity parameter of a Level or a Store being a number > 0.
  • Added code so that self.eventsFired gets set correctly after an event fires in a compound yield get/put with a waitevent clause (reneging case).
  • Repaired a bug in prettyprinting of Store objects.


  • New compound yield statements support time-out or event-based reneging in get and put operations on Store and Level instances.
  • yield get on a Store instance can now have a filter function.
  • All Monitor and Tally instances are automatically registered in list allMonitors and allTallies, respectively.
  • The new function startCollection allows activation of Monitors and Tallies at a specified time.
  • A printHistogram method was added to Tally and Monitor which generates a table-form histogram.
  • In SimPy.SimulationRT: A function for allowing changing the ratio wall clock time to simulation time has been added.

June 2006: Version 1.7.1

This is a maintenance release. The API has not been changed/added to.

  • Repair of a bug in the _get methods of Store and Level which could lead to synchronization problems (blocking of producer processes, despite space being available in the buffer).
  • Repair of Level __init__ method to allow initialBuffered to be of either float or int type.
  • Addition of type test for Level get parameter ‘nrToGet’ to limit it to positive int or float.
  • To improve pretty-printed output of ‘Level’ objects, changed attribute ‘_nrBuffered’ to ‘nrBuffered’ (synonym for ‘amount’ property).
  • To improve pretty-printed output of ‘Store’ objects, added attribute ‘buffered’ (which refers to ‘_theBuffer’ attribute).

February 2006: Version 1.7

This is a major release.

  • Addition of an abstract class Buffer, with two sub-classes Store and Level Buffers are used for modelling inter-process synchronization in producer/ consumer and multi-process cooperation scenarios.
  • Addition of two new yield statements:
    • yield put for putting items into a buffer, and
    • yield get for getting items from a buffer.
  • The Manual has undergone a major re-write/edit.
  • All scripts have been restructured for compatibility with IronPython 1 beta2. This was doen by moving all import statements to the beginning of the scripts. After the removal of the first (shebang) line, all scripts (with the exception of plotting and GUI scripts) can run successfully under this new Python implementation.

September 2005: Version 1.6.1

This is a minor release.

  • Addition of Tally data collection class as alternative to Monitor. It is intended for collecting very large data sets more efficiently in storage space and time than Monitor.
  • Change of Resource to work with Tally (new Resource API is backwards-compatible with 1.6).
  • Addition of function setHistogram to class Monitor for initializing histograms.
  • New function allEventNotices() for debugging/teaching purposes. It returns a prettyprinted string with event times and names of process instances.
  • Addition of function allEventTimes (returns event times of all scheduled events).

15 June 2005: Version 1.6

  • Addition of two compound yield statement forms to support the modelling of processes reneging from resource queues.
  • Addition of two test/demo files showing the use of the new reneging statements.
  • Addition of test for prior simulation initialization in method activate().
  • Repair of bug in monitoring thw waitQ of a resource when preemption occurs.
  • Major restructuring/editing to Manual and Cheatsheet.

1 February 2005: Version 1.5.1


    Starting with this version 1.5.1, SimPy is being release under the GNU Lesser General Public License (LGPL), instead of the GNU GPL. This change has been made to encourage commercial firms to use SimPy in for-profit work.

  • Minor re-release

  • No additional/changed functionality

  • Includes unit test file’’ which had been accidentally deleted from 1.5

  • Provides updated version of ‘Bank.html’ tutorial.

  • Provides an additional tutorial (‘Bank2.html’) which shows how to use the new synchronization constructs introduced in SimPy 1.5.

  • More logical, cleaner version numbering in files.

1 December 2004: Version 1.5

  • No new functionality/API changes relative to 1.5 alpha
  • Repaired bug related to waiting/queuing for multiple events
  • SimulationRT: Improved synchronization with wallclock time on Unix/Linux

25 September 2004: Version 1.5alpha

  • New functionality/API additions

    • SimEvents and signalling synchronization constructs, with ‘yield waitevent’ and ‘yield queueevent’ commands.
    • A general “wait until” synchronization construct, with the ‘yield waituntil’ command.
  • No changes to 1.4.x API, i.e., existing code will work as before.

19 May 2004: Version 1.4.2

  • Sub-release to repair two bugs:

    • The unittest for monitored Resource queues does not fail anymore.
    • SimulationTrace now works correctly with “yield hold,self” form.
  • No functional or API changes

29 February 2004: Version 1.4.1

  • Sub-release to repair two bugs:

    • The (optional) monitoring of the activeQ in Resource now works correctly.
    • The “” example is now implemented correctly.
  • No functional or API changes

1 February 2004: Version 1.4

  • Released on

22 December 2003: Version 1.4 alpha

  • New functionality/API changes

    • All classes in the SimPy API are now new style classes, i.e., they inherit from object either directly or indirectly.
    • Module has been merged into module and all modules. Import of Simulation or any SimulationXXX module now also imports Monitor.
    • Some Monitor methods/attributes have changed. See Manual!
    • Monitor now inherits from list.
    • A class Histogram has been added to and all modules.
    • A module SimulationRT has been added which allows synchronization between simulated and wallclock time.
    • A moduleSimulationStep which allows the execution of a simulation model event-by-event, with the facility to execute application code after each event.
    • A Tk/Tkinter-based module SimGUI has been added which provides a SimPy GUI framework.
    • A Tk/Tkinter-based module SimPlot has been added which provides for plot output from SimPy programs.

22 June 2003: Version 1.3

  • No functional or API changes
  • Reduction of sourcecode linelength in to <= 80 characters

June 2003: Version 1.3 alpha

  • Significantly improved performance

  • Significant increase in number of quasi-parallel processes SimPy can handle

  • New functionality/API changes:

    • Addition of SimulationTrace, an event trace utility
    • Addition of Lister, a prettyprinter for instance attributes
    • No API changes
  • Internal changes:

    • Implementation of a proposal by Simon Frost: storing the keys of the event set dictionary in a binary search tree using bisect. Thank you, Simon! SimPy 1.3 is dedicated to you!
  • Update of Manual to address tracing.

  • Update of Interfacing doc to address output visualization using Scientific Python gplt package.

29 April 2003: Version 1.2

  • No changes in API.

  • Internal changes:
    • Defined “True” and “False” in to support Python 2.2.

22 October 2002

  • Re-release of 0.5 Beta on to replace corrupted file
  • No code changes whatever!

18 October 2002

  • Version 0.5 Beta-release, intended to get testing by application developers and system integrators in preparation of first full (production) release. Released on on 20 October 2002.

  • More models

  • Documentation enhanced by a manual, a tutorial (“The Bank”) and installation instructions.

  • Major changes to the API:

    • Introduced ‘simulate(until=0)’ instead of ‘scheduler(till=0)’. Left ‘scheduler()’ in for backward compatibility, but marked as deprecated.

    • Added attribute “name” to class Process. Process constructor is now:

      def __init__(self,name="a_process")

      Backward compatible if keyword parameters used.

    • Changed Resource constructor to:

      def __init__(self,capacity=1,name="a_resource",unitName="units")

      Backward compatible if keyword parameters used.

27 September 2002

  • Version 0.2 Alpha-release, intended to attract feedback from users
  • Extended list of models
  • Upodated documentation

17 September 2002

  • Version 0.1.2 published on SourceForge; fully working, pre-alpha code
  • Implements simulation, shared resources with queuing (FIFO), and monitors for data gathering/analysis.
  • Contains basic documentation (cheatsheet) and simulation models for test and demonstration.