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LeWIS - Let's Write Intricate Simulators!

Project Description


LeWIS - Let’s Write Intricate Simulators.

Lewis is a Python framework for simulating hardware devices. It is compatible with both Python 2 and 3.

Lewis can be run directly using Python 2.7 or >= 3.4, or using a prepackaged Docker image that includes all dependencies. See relevant usage sections for details.

Resources: GitHubDockerHubDockerfile

Lewis started out under the name “plankton”, but due to a package with the same name on PyPI we decided to rename the project.

Purpose and Use Cases

Lewis is being developed in the context of instrument control at the ESS, but it is general enough to be used in many other contexts that require detailed, stateful software simulations of hardware devices.

We consider a detailed device simulation to be one that can communicate using the same protocol as the real device, and that can very closely approximate real device behaviour in terms of what is seen through this protocol. This includes gradual processes, side-effects and error conditions.

The purpose of Lewis is to provide a common framework to facilitate the development of such simulators. The framework provides a common set of tools and abstracts away protocol adapters, which helps minimize code replication and allows the developer of a simulated device to focus on capturing device behaviour.

Potential use cases for detailed device simulators include:

  • Replacing the physical device when developing and testing software that interfaces with the device
  • Testing failure conditions without risking damage to the physical device
  • Automated system and unit tests of software that communicates with the device
  • Perform “dry runs” against test scripts that are to be run on the real device

Using a simulation for the above has the added benefit that, unlike most real devices, a simulation may be sped up / fast-forwarded past any lengthy delays or processes that occur in the device.


Brief Terminology

Devices and Interfaces are two independent concepts in Lewis. The Device is model for the device behaviour and internal memory. A Device can be represented using a StateMachine, but it does not have to be. A Device does not include anything specific to the communication protocol with the Device. An Interface provides a protocol binding to a Device. The Device and Interface are created as part of a Simulation that provides a “heart beat” and other environmental aspects.

What Can You Do With Lewis?

  • Create new Devices to closely imitate the internal behaviour and memory of something
  • Optionally make a Device work as a StateMachine via StateMachineDevice to give rich behaviours
  • Create one or more Interfaces over your Device to expose it as an EPICS IOC, a TCP listener, or on any other bespoke protocol you like
  • Access and control the Device while it is running via a “back door”
  • Access and control the Simulation while it is running via a “back door”

Additional Documentation

Documentation can be found at It can also be generated from the sources:

$ git clone
$ cd lewis
$ pip install -r requirements-dev.txt
$ sphinx-build -b html docs/ docs/_build/html

The docs/_build/html directory will then contain the full documentation in HTML format.

Release History

This version
History Node


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