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Event discrete, process based simulation for Python.

Project description


SimPy is a process-based discrete-event simulation framework based on standard Python. Processes in SimPy are defined by Python generator functions and can, for example, be used to model active components like customers, vehicles or agents. SimPy also provides various types of shared resources to model limited capacity congestion points (like servers, checkout counters and tunnels).

Simulations can be performed “as fast as possible”, in real time (wall clock time) or by manually stepping through the events.

Though it is theoretically possible to do continuous simulations with SimPy, it has no features that help you with that. Also, SimPy is not really required for simulations with a fixed step size and where your processes don’t interact with each other or with shared resources.

The documentation contains a tutorial, several guides explaining key concepts, a number of examples and the API reference.

SimPy is released under the MIT License. Simulation model developers are encouraged to share their SimPy modeling techniques with the SimPy community. Please post a message to the SimPy mailing list.

There is an introductory talk that explains SimPy’s concepts and provides some examples: watch the video or get the slides.

A Simple Example

One of SimPy’s main goals is to be easy to use. Here is an example for a simple SimPy simulation: a clock process that prints the current simulation time at each step:

>>> import simpy
>>> def clock(env, name, tick):
...     while True:
...         print(name,
...         yield env.timeout(tick)
>>> env = simpy.Environment()
>>> env.process(clock(env, 'fast', 0.5))
<Process(clock) object at 0x...>
>>> env.process(clock(env, 'slow', 1))
<Process(clock) object at 0x...>
fast 0
slow 0
fast 0.5
slow 1
fast 1.0
fast 1.5


SimPy requires Python >= 3.6, both CPython and PyPy3 are known to work.

You can install SimPy easily via pip:

$ pip install -U simpy

You can also download and install SimPy manually:

$ cd where/you/put/simpy/
$ python install

To run SimPy’s test suite on your installation, execute:

$ py.test --pyargs simpy

Getting started

If you’ve never used SimPy before, the SimPy tutorial is a good starting point for you. You can also try out some of the Examples shipped with SimPy.

Documentation and Help

You can find a tutorial, examples, topical guides and an API reference, as well as some information about SimPy and its history in our online documentation. For more help, contact the SimPy mailing list. SimPy users are pretty helpful. You can, of course, also dig through the source code.

If you find any bugs, please post them on our issue tracker.

Enjoy simulation programming in SimPy!

Ports and comparable libraries

Re-implementations of SimPy and libraries similar to SimPy are available in the following languages:

Changelog for SimPy

4.0.2 - 2023-07-30

  • [CHANGE] Tested with Python 3.9, 3.10, and 3.11

  • [CHANGE] Improved docs w.r.t. triggered and processed events

  • [CHANGE] Improved gas station example

  • [FIX] ClassVar annotations in BaseResource

  • [FIX] Documentation typos

  • [FIX] Help static analyzers find exported symbols

  • [FIX] license_file deprecation in setup.cfg

  • [FIX] Do not re-annotate type of __path__

  • [FIX] Annotate ConditionValue.__init__() return value

  • [FIX] Unbreak docs build by updating to Sphinx 6.2.1

  • [FIX] Workaround Sphinx circular import problem

4.0.1 - 2020-04-15

  • [FIX] Typing repair for Get and Put as ContextManagers

4.0.0 - 2020-04-06

  • [BREAKING] Python 3.6 is the minimum supported version

  • [BREAKING] BaseEnvironment is eliminated. Inherit Environment instead.

  • [BREAKING] Environment.exit() is eliminated. Use return instead.

  • [NEW] “Porting from SimPy 3 to 4” topical guide in docs

  • [NEW] SimPy is now fully type annotated (PEP-483, PEP-484)

  • [NEW] PEP-517/PEP-518 compatible build system

3.0.13 - 2020-04-05

  • [FIX] Repair several minor typos in documentation

  • [FIX] Possible AttributeError in Process._resume()

  • [CHANGE] Use setuptools_scm in distribution build

3.0.12 - 2020-03-12

  • [FIX] Fix URLs for and re-release

3.0.11 - 2018-07-13

  • [FIX] Repair Environment.exit() to support PEP-479 and Python 3.7.

  • [FIX] Fix wrong usage_since calculation in preemptions

  • [NEW] Add “Time and Scheduling” section to docs

  • [CHANGE] Move Interrupt from events to exceptions

  • [FIX] Various minor documentation improvements

3.0.10 – 2016-08-26

  • [FIX] Conditions no longer leak callbacks on events (thanks to Peter Grayson).

3.0.9 – 2016-06-12

  • [NEW] PriorityStore resource and performance benchmarks were implemented by Peter Grayson.

  • [FIX] Support for identifying nested preemptions was added by Cristian Klein.

3.0.8 – 2015-06-23

  • [NEW] Added a monitoring guide to the documentation.

  • [FIX] Improved packaging (thanks to Larissa Reis).

  • [FIX] Fixed and improved various test cases.

3.0.7 – 2015-03-01

  • [FIX] State of resources and requests were inconsistent before the request has been processed (issue #62).

  • [FIX] Empty conditions were never triggered (regression in 3.0.6, issue #63).

  • [FIX] will fail if the until event does not get triggered (issue #64).

  • [FIX] Callback modification during event processing is now prohibited (thanks to Andreas Beham).

3.0.6 - 2015-01-30

  • [NEW] Guide to SimPy resources.

  • [CHANGE] Improve performance of condition events.

  • [CHANGE] Improve performance of filter store (thanks to Christoph Körner).

  • [CHANGE] Exception tracebacks are now more compact.

  • [FIX] AllOf conditions handle already processed events correctly (issue #52).

  • [FIX] Add sync() to RealtimeEnvironment to reset its internal wall-clock reference time (issue #42).

  • [FIX] Only send copies of exceptions into processes to prevent traceback modifications.

  • [FIX] Documentation improvements.

3.0.5 – 2014-05-14

  • [CHANGE] Move interruption and all of the safety checks into a new event (pull request #30)

  • [FIX] FilterStore.get() now behaves correctly (issue #49).

  • [FIX] Documentation improvements.

3.0.4 – 2014-04-07

  • [NEW] Verified, that SimPy works on Python 3.4.

  • [NEW] Guide to SimPy events

  • [CHANGE] The result dictionary for condition events (AllOF / & and AnyOf / |) now is an OrderedDict sorted in the same way as the original events list.

  • [CHANGE] Condition events now also except processed events.

  • [FIX] Resource.request() directly after Resource.release() no longer successful. The process now has to wait as supposed to.

  • [FIX] now accept all exceptions derived from BaseException instead of only Exception.

3.0.3 – 2014-03-06

  • [NEW] Guide to SimPy basics.

  • [NEW] Guide to SimPy Environments.

  • [FIX] Timing problems with real time simulation on Windows (issue #46).

  • [FIX] Installation problems on Windows due to Unicode errors (issue #41).

  • [FIX] Minor documentation issues.

3.0.2 – 2013-10-24

  • [FIX] The default capacity for Container and FilterStore is now also inf.

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


SimPy was originally created by Klaus G. Müller and Tony Vignaux in 2002.

In 2008, Ontje Lünsdorf and Stefan Scherfke started to contribute to SimPy and became active maintainers in 2011.

In 2011, Karen Turner came on board to generally help with all the bits and pieces that may get forgotten :-)

We’d also like to thank:

  • Johannes Koomer

  • Steven Kennedy

  • Matthew Grogan

  • Sean Reed

  • Christoph Körner

  • Andreas Beham

  • Larissa Reis

  • Peter Grayson

  • Cristian Klein

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