PySD

PySD runs system dynamics models in Python.^1,2 System dynamics describes a system as stocks, quantities that build up or drain away over time, linked by feedback loops.^3,5 PySD reads a model file made in Vensim (the .mdl format) or an XMILE file exported from Stella or other programs.⁵ It translates the model into Python code that runs without the original program.³ It does not replace those programs, since models are still drawn in a visual modelling tool.³ Not every Vensim and XMILE function is supported, so some models run only partly, though the project says most run properly.¹

James P Houghton created it in 2014 to translate Vensim models to Python.³ Alex Prey added the XMILE translator in 2017, and Eneko Martin-Martinez restructured the translation in 2022.³ The paper to cite is Martin-Martinez, Samsó, Houghton and Solé, "PySD: System Dynamics Modeling in Python", Journal of Open Source Software, 2022.¹ It is maintained by its community under the MIT licence.^1,2 The latest release is 3.14.3, from March 2025; in September 2026 the code gained a builder that writes models out as standalone Julia files.^2,6,7

A translated model runs with one command and returns a table (a pandas DataFrame) with the value of every part of the model at each time step.⁴ Parameters can be changed between runs, output can be sampled at the times when real measurements were taken, and results can be saved as CSV or netCDF files.^4,5 A separate PySD Cookbook of notebooks shows model fitting, replacing parts of a model with machine-learning regressions, and comparing a model's predictions across geographic scales.² An R package, pysd2r, makes it usable from R.²

The project's stated reason for existing is to bring system dynamics into the tools of data science instead of rebuilding those tools inside system dynamics software.^1,2 Someone who has built a stock-and-flow model of a town's housing or a watershed's water budget in Vensim or Stella can use PySD to fit it to measured data and run it across many settings. En-ROADS is an example of a large system dynamics model on this site.