Making a project dg-compatible

note

This guide is only relevant if you are starting from an existing Dagster project. This setup is unnecessary if you used dg scaffold project to create your project.

Let's begin with a Dagster project that has some assets:

tree

.
├── README.md
├── my_existing_project
│   ├── __init__.py
│   ├── assets.py
│   └── definitions.py
└── pyproject.toml

2 directories, 5 files

Install dependencies

Install the `dg` command line tool

We'll need to install the dg command line tool, which is used to scaffold components. We recommend installing dg globally using the uv package manager; it can also be installed using pip.

uv tool install dagster-dg

Install `dagster-components`

Next, we'll need to install the dagster-components package.

Though this is optional, we generally recommend using a separate virtual environment for each project, which can be accomplished using uv:

Using uv virtual environment
Using pip

uv venv

Then, we can use uv sync to install the dependencies from our pyproject.toml, and then install the dagster-components package:

uv sync && uv add dagster-components

pip install dagster-components

Update project structure

Update `pyproject.toml`

Add a tool.dg section to your pyproject.toml file. This will tell the dg command line tool that this project is a valid Dagster project.

pyproject.toml
...
[tool.dg]
directory_type = "project"

[tool.dg.project]
root_module = "my_existing_project"
defs_module = "my_existing_project.defs"

[tool.dagster]
module_name = "my_existing_project.definitions"
code_location_name = "my_existing_project"
...

Create a `defs` directory

Next, you'll want to create a directory to contain any new definitions you add to your project. By convention, this directory is named defs, and exists at the top level of your project's Python module.

mkdir my_existing_project/defs

Modify top-level definitions

dagster-components provides a utility to create a Definitions object from your defs directory. Because you're working with an existing project, you'll want to combine your existing definitions with the ones from your defs directory.

To do so, you'll need to modify your definitions.py file, or whichever file contains your top-level Definitions object.

You can manually construct a set of definitions using load_defs, then merge them with your existing definitions using Definitions.merge. You point load_defs at the module you just created.

Before
After

# isort: skip_file
from my_existing_project import assets

import dagster as dg

all_assets = dg.load_assets_from_modules([assets])

defs = dg.Definitions(
    assets=all_assets,
)

import dagster_components as dg_components
import my_existing_project.defs
from my_existing_project import assets

import dagster as dg

all_assets = dg.load_assets_from_modules([assets])

defs = dg.Definitions.merge(
    dg.Definitions(assets=all_assets),
    dg_components.load_defs(my_existing_project.defs),
)

Now, your project is fully compatible with dg! dg can be used to scaffold new definitions directly into the existing project.

Install dependencies​

Install the dg command line tool​

Install dagster-components​

Update project structure​

Update pyproject.toml​

Create a defs directory​

Modify top-level definitions​

Next steps​