Contributing

Thank you for considering contributing to Prefect. These instructions are for working with Prefect 2. For Prefect 3 see the Prefect 3 documentation.

Setting up a development environment

First, download the source code and install an editable version of the Python package:

# Clone the repository's 2.x branch
git clone https://github.com/PrefectHQ/prefect.git
cd prefect
git checkout -b new-branch-2.x 2.x

# We recommend using a virtual environment

python -m venv .venv
source .venv/bin/activate

# Install the package with development dependencies

pip install -e ".[dev]"

# Set up pre-commit hooks for required formatting

pre-commit install

If you don't want to install the pre-commit hooks, you can manually install the formatting dependencies with:

pip install $(./scripts/precommit-versions.py)

You'll need to run black and ruff before a contribution can be accepted.

After installation, you can run the test suite with pytest:

# Run all the tests
pytest tests

# Run a subset of tests

pytest tests/test_flows.py

Building the Prefect UI

If you intend to run a local Prefect server during development, you must first build the UI. See UI development for instructions.

Prefect Code of Conduct

Our Pledge

In the interest of fostering an open and welcoming environment, we as contributors and maintainers pledge to making participation in our project and our community a harassment-free experience for everyone, regardless of age, body size, disability, ethnicity, sex characteristics, gender identity and expression, level of experience, education, socio-economic status, nationality, personal appearance, race, religion, or sexual identity and orientation.

Our Standards

Examples of behavior that contributes to creating a positive environment include:

• Using welcoming and inclusive language
• Being respectful of differing viewpoints and experiences
• Gracefully accepting constructive criticism
• Focusing on what is best for the community
• Showing empathy towards other community members

Examples of unacceptable behavior by participants include:

• The use of sexualized language or imagery and unwelcome sexual attention or advances
• Trolling, insulting/derogatory comments, and personal or political attacks
• Public or private harassment
• Publishing others' private information, such as a physical or electronic address, without explicit permission
• Other conduct which could reasonably be considered inappropriate in a professional setting

Our Responsibilities

Project maintainers are responsible for clarifying the standards of acceptable behavior and are expected to take appropriate and fair corrective action in response to any instances of unacceptable behavior.

Project maintainers have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, or to ban temporarily or permanently any contributor for other behaviors that they deem inappropriate, threatening, offensive, or harmful.

Scope

This Code of Conduct applies within all project spaces, and it also applies when an individual is representing the project or its community in public spaces. Examples of representing a project or community include using an official project e-mail address, posting via an official social media account, or acting as an appointed representative at an online or offline event. Representation of a project may be further defined and clarified by project maintainers.

Enforcement

Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting Chris White at chris@prefect.io. All complaints will be reviewed and investigated and will result in a response that is deemed necessary and appropriate to the circumstances. The project team is obligated to maintain confidentiality with regard to the reporter of an incident. Further details of specific enforcement policies may be posted separately.

Project maintainers who do not follow or enforce the Code of Conduct in good faith may face temporary or permanent repercussions as determined by other members of the project's leadership.

Attribution

This Code of Conduct is adapted from the Contributor Covenant, version 1.4, available at https://www.contributor-covenant.org/version/1/4/code-of-conduct.html

For answers to common questions about this code of conduct, see https://www.contributor-covenant.org/faq

Developer tooling

The Prefect CLI provides several helpful commands to aid development.

Start all services with hot-reloading on code changes (requires UI dependencies to be installed):

prefect dev start

Start a Prefect API that reloads on code changes:

prefect dev api

Start a Prefect worker that reloads on code changes:

prefect dev agent

UI development

Developing the Prefect UI requires that npm is installed.

Start a development UI that reloads on code changes:

prefect dev ui

Build the static UI (the UI served by prefect server start):

prefect dev build-ui

Docs Development

Prefect uses mkdocs for the docs website and the mkdocs-material theme. While we use mkdocs-material-insiders for production, builds can still happen without the extra plugins. Deploy previews are available on pull requests, so you'll be able to browse the final look of your changes before merging.

To build the docs:

mkdocs build

To serve the docs locally at http://127.0.0.1:8000/:

mkdocs serve

For additional mkdocs help and options:

mkdocs --help

We use the mkdocs-material theme. To add additional JavaScript or CSS to the docs, please see the theme documentation here.

Internal developers can install the production theme by running:

pip install -e git+https://github.com/PrefectHQ/mkdocs-material-insiders.git#egg=mkdocs-material
mkdocs build # or mkdocs build --config-file mkdocs.insiders.yml if needed

Kubernetes development

Generate a manifest to deploy a development API to a local kubernetes cluster:

prefect dev kubernetes-manifest

To access the Prefect UI running in a Kubernetes cluster, use the kubectl port-forward command to forward a port on your local machine to an open port within the cluster. For example:

kubectl port-forward deployment/prefect-dev 4200:4200

This forwards port 4200 on the default internal loop IP for localhost to the Prefect server deployment.

To tell the local prefect command how to communicate with the Prefect API running in Kubernetes, set the PREFECT_API_URL environment variable:

export PREFECT_API_URL=http://localhost:4200/api

Since you previously configured port forwarding for the localhost port to the Kubernetes environment, you’ll be able to interact with the Prefect API running in Kubernetes when using local Prefect CLI commands.

Adding database migrations

To make changes to a table, first update the SQLAlchemy model in src/prefect/server/database/orm_models.py. For example, if you wanted to add a new column to the flow_run table, you would add a new column to the FlowRun model:

# src/prefect/server/database/orm_models.py

@declarative_mixin
class ORMFlowRun(ORMRun):
    """SQLAlchemy model of a flow run."""
    ...
    new_column = Column(String, nullable=True) # <-- add this line

Next, you will need to generate new migration files. You must generate a new migration file for each database type. Migrations will be generated for whatever database type PREFECT_API_DATABASE_CONNECTION_URL is set to. See here for how to set the database connection URL for each database type.

To generate a new migration file, run the following command:

prefect server database revision --autogenerate -m "<migration name>"

Try to make your migration name brief but descriptive. For example:

• add_flow_run_new_column
• add_flow_run_new_column_idx
• rename_flow_run_old_column_to_new_column

The --autogenerate flag will automatically generate a migration file based on the changes to the models.

Always inspect the output of --autogenerate

--autogenerate will generate a migration file based on the changes to the models. However, it is not perfect. Be sure to check the file to make sure it only includes the changes you want to make. Additionally, you may need to remove extra statements that were included and not related to your change.

The new migration can be found in the src/prefect/server/database/migrations/versions/ directory. Each database type has its own subdirectory. For example, the SQLite migrations are stored in src/prefect/server/database/migrations/versions/sqlite/.

After you have inspected the migration file, you can apply the migration to your database by running the following command:

prefect server database upgrade -y

Once you have successfully created and applied migrations for all database types, make sure to update MIGRATION-NOTES.md to document your additions.