Develop on Prefect

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Make a code contribution

We welcome all forms of contributions to Prefect, whether it’s small typo fixes in our documentation, bug fixes or feature enhancements! If this is your first time making an open source contribution we will be glad to work with you and help you get up to speed.

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Fork the repository

All contributions to Prefect need to start on a fork of the repository. Once you have successfully forked the Prefect repo, create a branch with an informative name:

git checkout -b fix-for-issue-NUM

After committing your changes to this branch, you can then open a pull request from your fork that we will review with you.

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Install Prefect for development

Once you have cloned your fork of the repo:

# Clone the repository
git clone https://github.com/GITHUB-USERNAME/prefect.git
cd prefect

you can install an editable version of Prefect for quick iteration. We recommend using a virtual environment such as venv:

To ensure your changes comply with our linting policies, you can optionally set up pre-commit hooks to run with every commit:

pre-commit install

To manually run the pre-commit hooks against all files, you can run:

pre-commit run --all-files

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Write tests

Prefect relies on unit testing to ensure proposed changes don’t negatively impact any functionality. For all code changes, including bug fixes, we ask that you write at least one corresponding test. One rule of thumb - especially for bug fixes - is that you should write a test that fails prior to your changes and passes with your changes. This ensures the test will fail and prevent the bug from resurfacing if other changes are made in the future. All tests can be found in the tests/ directory of the repository.

You can run the test suite with pytest:

# run all tests
pytest tests

# run a specific file
pytest tests/test_flows.py

# run all tests that match a pattern
pytest tests/test_tasks.py -k cache_policy

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Working with a development UI

If you plan to use the UI during development, you will need to build a development version of the UI first.

Using the Prefect UI in development requires installation of npm. We recommend using nvm to manage Node.js versions. Once installed, run nvm use from the root of the Prefect repository to initialize the proper version of npm and node.

Start a development UI that reloads on code changes:

prefect dev ui

This command is most useful if you are working directly on the UI codebase.

Alternatively, you can build a static UI that will be served when running prefect server start:

prefect dev build-ui

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Working with a development server

The Prefect CLI provides several helpful commands to aid development of server-side changes.

You can start all services with hot-reloading on code changes (note that this requires installation of UI dependencies):

prefect dev start

Start a Prefect API that reloads on code changes:

prefect dev api

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Add database migrations

If your code changes necessitate modifications to a database table, first update the SQLAlchemy model in src/prefect/server/database/orm_models.py.

For example, to add a new column to the flow_run table, add a new column to the FlowRun model:

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

class FlowRun(Run):
    """SQLAlchemy model of a flow run."""
    ...
    new_column: Mapped[Union[str, None]] = mapped_column(sa.String, nullable=True) # <-- add this line

Next, generate new migration files. Generate a new migration file for each database type.

Migrations are generated for whichever database type PREFECT_API_DATABASE_CONNECTION_URL is set to. See how to set the database connection URL for each database type.

To generate a new migration file, run:

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

Make the 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 automatically generates a migration file based on the changes to the models.

The new migration is 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 inspecting the migration file, apply the migration to the database by running:

prefect server database upgrade -y

After successfully creating migrations for all database types, update MIGRATION-NOTES.md to document the changes.