What's the Definitions object?

Whenever you define an asset, resource, or schedule in your Dagster deployment, it needs to be part of a Definitions object for the code location you’re working in. Each code location can only have a single Definitions object. This object maps to one code location. With code locations, users isolate multiple Dagster projects from each other without requiring multiple deployments. You’ll learn more about code locations a bit later in this lesson.

Let’s use our cookie example to demonstrate. In this case, our cookie assets - like our dry and wet ingredients, cookie dough, chocolate chip cookie dough, and eventually, chocolate chip cookies - can all be thought of as (cookie) definitions:

In Dagster, the Definitions object is located in the definitions.py file. If we wanted to create a Definitions object for the cookie definitions it would look something like this:

import dagster_cookies.defs as defs

defs = dg.components.load_defs(defs)

We do not need to explicitly list all the assets in our project. Dagster can automatically load most objects (we will later see there are other object types besides assets) by importing the contents of the defs module.

Remember that using dg scaffold has been placing all our asset files within the defs directory.

.
└── dagster_essentials
    └── defs
        └── assets
            ├── __init__.py
            ├── constants.py
            ├── metrics.py
            └── trips.py

You’ll only have one code location (and therefore one Definitions object) in this course, but as your project grows, you’ll need to update the Definitions object to include new definitions.

How Dagster uses the Definitions object

Now that we’ve discussed what the Definitions object is, let’s go into how Dagster uses it.

When running dg dev, Dagster looks to the pyproject.toml file. This file contains all of the Python dependencies necessary to run Dagster as well as Dagster specific settings on where the Definitions object should be loaded.

[tool.dg]
directory_type = "project"

[tool.dg.project]
root_module = "dagster_essentials"
code_location_target_module = "dagster_essentials.definitions"

In your project, open the dagster_university/dagster_essentials/definitions.py file. It should look like the following code:

import dagster_essentials.defs as defs

defs = dg.components.load_defs(defs)