Contributing to Pyomo

We welcome all contributions including bug fixes, feature enhancements, and documentation improvements. Pyomo manages source code contributions via GitHub pull requests (PRs).

Contribution Requirements

A PR should be 1 set of related changes. PRs for large-scale non-functional changes (i.e. PEP8, comments) should be separated from functional changes. This simplifies the review process and ensures that functional changes aren’t obscured by large amounts of non-functional changes.

We do not squash and merge PRs so all commits in your branch will appear in the main history. In addition to well-documented PR descriptions, we encourage modular/targeted commits with descriptive commit messages.

Coding Standards

  • Required: black

  • No use of __author__

  • Inside pyomo.contrib: Contact information for the contribution maintainer (such as a Github ID) should be included in the Sphinx documentation

The first step of Pyomo’s GitHub Actions workflow is to run black and a spell-checker to ensure style guide compliance and minimize typos. Before opening a pull request, please run:

 # Auto-apply correct formatting
pip install black
black -S -C <path> --exclude examples/pyomobook/python-ch/
# Find typos in files
conda install typos
typos --config .github/workflows/typos.toml <path>

If the spell-checker returns a failure for a word that is spelled correctly, please add the word to the .github/workflows/typos.toml file.

Online Pyomo documentation is generated using Sphinx with the napoleon extension enabled. For API documentation we use of one of these supported styles for docstrings, but we prefer the NumPy standard. Whichever you choose, we require compliant docstrings for:

  • Modules

  • Public and Private Classes

  • Public and Private Functions

We also encourage you to include examples, especially for new features and contributions to pyomo.contrib.


Pyomo uses unittest, pytest, GitHub Actions, and Jenkins for testing and continuous integration. Submitted code should include tests to establish the validity of its results and/or effects. Unit tests are preferred but we also accept integration tests. We require at least 70% coverage of the lines modified in the PR and prefer coverage closer to 90%. We also require that all tests pass before a PR will be merged.


If you are having issues getting tests to pass on your Pull Request, please tag any of the core developers to ask for help.

The Pyomo main branch provides a Github Actions workflow (configured in the .github/ directory) that will test any changes pushed to a branch with a subset of the complete test harness that includes multiple virtual machines (ubuntu, mac-os, windows) and multiple Python versions. For existing forks, fetch and merge your fork (and branches) with Pyomo’s main. For new forks, you will need to enable GitHub Actions in the ‘Actions’ tab on your fork. This will enable the tests to run automatically with each push to your fork.

At any point in the development cycle, a “work in progress” pull request may be opened by including ‘[WIP]’ at the beginning of the PR title. Any pull requests marked ‘[WIP]’ or draft will not be reviewed or merged by the core development team. However, any ‘[WIP]’ pull request left open for an extended period of time without active development may be marked ‘stale’ and closed.


Draft and WIP Pull Requests will NOT trigger tests. This is an effort to reduce our CI backlog. Please make use of the provided branch test suite for evaluating / testing draft functionality.

Python Version Support

By policy, Pyomo supports and tests the currently supported Python versions, as can be seen on Status of Python Versions. It is expected that tests will pass for all of the supported and tested versions of Python, unless otherwise stated.

At the time of the first Pyomo release after the end-of-life of a minor Python version, we will remove testing and support for that Python version.

This will also result in a bump in the minor Pyomo version.

For example, assume Python 3.A is declared end-of-life while Pyomo is on version 6.3.Y. After the release of Pyomo 6.3.(Y+1), Python 3.A will be removed, and the next Pyomo release will be 6.4.0.

Working on Forks and Branches

All Pyomo development should be done on forks of the Pyomo repository. In order to fork the Pyomo repository, visit, click the “Fork” button in the upper right corner, and follow the instructions.

This section discusses two recommended workflows for contributing pull-requests to Pyomo. The first workflow, labeled Working with my fork and the GitHub Online UI, does not require the use of ‘remotes’, and suggests updating your fork using the GitHub online UI. The second workflow, labeled Working with remotes and the git command-line, outlines a process that defines separate remotes for your fork and the main Pyomo repository.

More information on git can be found at Section 2.5 has information on working with remotes.

Working with my fork and the GitHub Online UI

After creating your fork (per the instructions above), you can then clone your fork of the repository with

git clone<username>/pyomo.git

For new development, we strongly recommend working on feature branches. When you have a new feature to implement, create the branch with the following.

cd pyomo/     # to make sure you are in the folder managed by git
git branch <branch_name>
git checkout <branch_name>

Development can now be performed. When you are ready, commit any changes you make to your local repository. This can be done multiple times with informative commit messages for different tasks in the feature development.

git add <filename>
git status  # to check that you have added the correct files
git commit -m 'informative commit message to describe changes'

In order to push the changes in your local branch to a branch on your fork, use

git push origin <branch_name>

When you have completed all the changes and are ready for a pull request, make sure all the changes have been pushed to the branch <branch_name> on your fork.

  • visit<username>/pyomo.

  • Just above the list of files and directories in the repository, you should see a button that says “Branch: main”. Click on this button, and choose the correct branch.

  • Click the “New pull request” button just to the right of the “Branch: <branch_name>” button.

  • Fill out the pull request template and click the green “Create pull request” button.

At times during your development, you may want to merge changes from the Pyomo main development branch into the feature branch on your fork and in your local clone of the repository.

Using GitHub UI to merge Pyomo main into a branch on your fork

To update your fork, you will actually be merging a pull-request from the head Pyomo repository into your fork.

  • Visit

  • Click on the “New pull request” button just above the list of files and directories.

  • You will see the title “Compare changes” with some small text below it which says “Compare changes across branches, commits, tags, and more below. If you need to, you can also compare across forks.” Click the last part of this: “compare across forks”.

  • You should now see four buttons just below this: “base repository: Pyomo/pyomo”, “base: main”, “head repository: Pyomo/pyomo”, and “compare: main”. Click the leftmost button and choose “<username>/Pyomo”.

  • Then click the button which is second to the left, and choose the branch which you want to merge Pyomo main into. The four buttons should now read: “base repository: <username>/pyomo”, “base: <branch_name>”, “head repository: Pyomo/pyomo”, and “compare: main”. This is setting you up to merge a pull-request from Pyomo’s main branch into your fork’s <branch_name> branch.

  • You should also now see a pull request template. If you fill out the pull request template and click “Create pull request”, this will create a pull request which will update your fork and branch with any changes that have been made to the main branch of Pyomo.

  • You can then merge the pull request by clicking the green “Merge pull request” button from your fork on GitHub.

Working with remotes and the git command-line

After you have created your fork, you can clone the fork and setup git ‘remotes’ that allow you to merge changes from (and to) different remote repositories. Below, we have included a set of recommendations, but, of course, there are other valid GitHub workflows that you can adopt.

The following commands show how to clone your fork and setup two remotes, one for your fork, and one for the head Pyomo repository.

git clone<username>/pyomo.git
git remote rename origin my-fork
git remote add head-pyomo

Note, you can see a list of your remotes with

git remote -v

The commands for creating a local branch and performing local commits are the same as those listed in the previous section above. Below are some common tasks based on this multi-remote setup.

If you have changes that have been committed to a local feature branch (<branch_name>), you can push these changes to the branch on your fork with,

git push my-fork <branch_name>

In order to update a local branch with changes from a branch of the Pyomo repository,

git checkout <branch_to_update>
git fetch head-pyomo
git merge head-pyomo/<branch_to_update_from> --ff-only

The “–ff-only” only allows a merge if the merge can be done by a fast-forward. If you do not require a fast-forward, you can drop this option. The most common concrete example of this would be

git checkout main
git fetch head-pyomo
git merge head-pyomo/main --ff-only

The above commands pull changes from the main branch of the head Pyomo repository into the main branch of your local clone. To push these changes to the main branch on your fork,

git push my-fork main

Setting up your development environment

After cloning your fork, you will want to install Pyomo from source.

Step 1 (recommended): Create a new conda environment.

conda create --name pyomodev

You may change the environment name from pyomodev as you see fit. Then activate the environment:

conda activate pyomodev

Step 2 (optional): Install PyUtilib

The hard dependency on PyUtilib was removed in Pyomo 6.0.0. There is still a soft dependency for any code related to pyomo.dataportal.plugins.sheet.

If your contribution requires PyUtilib, you will likely need the main branch of PyUtilib to contribute. Clone a copy of the repository in a new directory:

git clone

Then in the directory containing the clone of PyUtilib run:

python develop

Step 3: Install Pyomo

Finally, move to the directory containing the clone of your Pyomo fork and run:

python develop

These commands register the cloned code with the active python environment (pyomodev). This way, your changes to the source code for pyomo are automatically used by the active environment. You can create another conda environment to switch to alternate versions of pyomo (e.g., stable).

Review Process

After a PR is opened it will be reviewed by at least two members of the core development team. The core development team consists of anyone with write-access to the Pyomo repository. Pull requests opened by a core developer only require one review. The reviewers will decide if they think a PR should be merged or if more changes are necessary.

Reviewers look for:

  • Outside of pyomo.contrib: Code rigor and standards, edge cases, side effects, etc.

  • Inside of pyomo.contrib: No “glaringly obvious” problems with the code

  • Documentation and tests

The core development team tries to review pull requests in a timely manner but we make no guarantees on review timeframes. In addition, PRs might not be reviewed in the order they are opened in.

Where to put contributed code

In order to contribute to Pyomo, you must first make a fork of the Pyomo git repository. Next, you should create a branch on your fork dedicated to the development of the new feature or bug fix you’re interested in. Once you have this branch checked out, you can start coding. Bug fixes and minor enhancements to existing Pyomo functionality should be made in the appropriate files in the Pyomo code base. New examples, features, and packages built on Pyomo should be placed in pyomo.contrib. Follow the link below to find out if pyomo.contrib is right for your code.


Pyomo uses the pyomo.contrib package to facilitate the inclusion of third-party contributions that enhance Pyomo’s core functionality. The are two ways that pyomo.contrib can be used to integrate third-party packages:

  • pyomo.contrib can provide wrappers for separate Python packages, thereby

    allowing these packages to be imported as subpackages of pyomo.

  • pyomo.contrib can include contributed packages that are developed and

    maintained outside of the Pyomo developer team.

Including contrib packages in the Pyomo source tree provides a convenient mechanism for defining new functionality that can be optionally deployed by users. We expect this mechanism to include Pyomo extensions and experimental modeling capabilities. However, contrib packages are treated as optional packages, which are not maintained by the Pyomo developer team. Thus, it is the responsibility of the code contributor to keep these packages up-to-date.

Contrib package contributions will be considered as pull-requests, which will be reviewed by the Pyomo developer team. Specifically, this review will consider the suitability of the proposed capability, whether tests are available to check the execution of the code, and whether documentation is available to describe the capability. Contrib packages will be tested along with Pyomo. If test failures arise, then these packages will be disabled and an issue will be created to resolve these test failures.

Contrib Packages within Pyomo

Third-party contributions can be included directly within the pyomo.contrib package. The pyomo/contrib/example package provides an example of how this can be done, including a directory for plugins and package tests. For example, this package can be imported as a subpackage of pyomo.contrib:

from pyomo.environ import *
from pyomo.contrib.example import a

# Print the value of 'a' defined by this package

Although pyomo.contrib.example is included in the Pyomo source tree, it is treated as an optional package. Pyomo will attempt to import this package, but if an import failure occurs, Pyomo will silently ignore it. Otherwise, this pyomo package will be treated like any other. Specifically:

  • Plugin classes defined in this package are loaded when pyomo.environ is loaded.

  • Tests in this package are run with other Pyomo tests.