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#ask-ai
Title
# ask-ai
m

Martin Laurent

02/15/2024, 8:41 AM
during an asset execution, how do I get the path where the pickled output will be written
s

Scout

02/15/2024, 8:41 AM
During asset execution, the path where the pickled output will be written is determined by the configuration of the IO manager that you are using. If you are using a built-in IO manager like the
FilesystemIOManager
or a custom IO manager from a package such as
dagster-aws
or
dagster-azure
, the path is typically constructed using the base directory, asset key, and possibly other components such as the run ID, step key, and output name. For example, with the
FilesystemIOManager
, the documentation provides guidance on how the base directory is determined and how the path is constructed: - The base directory can be specified in the IO manager’s “base_dir” configuration value. - If not specified, it may fall back to a “storage/” directory under the “local_artifact_storage” in your
dagster.yaml
file, or the
DAGSTER_HOME
environment variable, or a temporary directory. - The path for an asset's output is then constructed as
<base_dir>/<asset_key>
. If the asset key has multiple components, the final component is used as the name of the file, and the preceding components as parent directories under the
base_dir
. For example, if you have an asset with key
AssetKey(["one", "two", "three"])
and a base directory of
/my/base/path
, the output would be stored in a file called
three
in the directory
/my/base/path/one/two/
. If you need to access the path programmatically during asset execution, you would typically rely on the context provided to your asset's function. However, the exact method to retrieve the path depends on the IO manager's implementation and is not explicitly detailed in the documentation. If you are using a custom IO manager or need to access this path, you may need to refer to the specific IO manager's documentation or source code to understand how to retrieve the path within the asset's execution context.
m

Martin Laurent

02/15/2024, 8:44 AM
@Scout how do I access the IO manager object using the asset's context?
s

Scout

02/15/2024, 8:44 AM
To access the IO manager object using the asset's context, you can use the
context.resources
attribute within your asset's computation function. The
context.resources
is a dictionary-like object that contains all the resources available to the asset, including the IO manager. Here's a step-by-step guide on how to access the IO manager from the asset's context: 1. Define your IO manager and make sure it is configured in your Dagster repository. 2. When defining your asset, specify the IO manager key if it's not the default IO manager. 3. Inside your asset's computation function, you can access the IO manager using the
context.resources
attribute followed by the key of the IO manager. Here's an example of how you might do this:
Copy code
python from dagster import asset, IOManager, ResourceDefinition  class MyIOManager(IOManager):     # Implementation of your IO manager     ...  # Define your IO manager resource my_io_manager_resource = ResourceDefinition.resource(MyIOManager())  # Define your asset and specify the IO manager key if necessary @asset(required_resource_keys={'my_io_manager_key': my_io_manager_resource}) def my_asset(context):     # Access the IO manager from the context     io_manager = context.resources.my_io_manager_key     # Now you can use the io_manager object as needed     ...
In this example,
my_io_manager_key
is the key that you use to access the IO manager from the
context.resources
. If you are using the default IO manager, you can access it without specifying a key, as it will be available under
context.resources.io_manager
. Please note that the exact key to use when accessing the IO manager from
context.resources
depends on how you have configured your resources and what key you have associated with your IO manager in the resource definitions.
m

Martin Laurent

02/15/2024, 8:50 AM
@Scout It seems the IO manager needs a OutputContext to compute the output path. Is it possible to provide one from an AssetExecutionContext?
s

Scout

02/15/2024, 8:50 AM
In Dagster, the
OutputContext
is typically constructed internally by the Dagster framework during the execution of an asset or op. It is not common practice to manually construct an
OutputContext
within an
AssetExecutionContext
because the
OutputContext
is meant to be used within the IO manager's
handle_output
method, which is called by the framework when an output is being handled. However, if you need to construct an
OutputContext
for testing purposes or to manually invoke an IO manager's methods, you can use the
build_output_context
utility function provided by Dagster. This function allows you to create an
OutputContext
with the necessary parameters for testing or other non-standard use cases. Here's an example of how you might use `build_output_context`:
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python from dagster import build_output_context  # Construct an OutputContext for testing or other purposes output_context = build_output_context(     # You can specify various parameters here as needed     asset_key=AssetKey(["my_asset"]),     # ... other parameters )  # Now you can use the output_context with your IO manager io_manager = MyIOManager() io_manager.handle_output(output_context, value)
Please note that this is not a typical pattern for asset execution in Dagster. The
build_output_context
function is primarily intended for testing IO managers outside of the normal execution flow. In a standard Dagster asset, you would not manually create an
OutputContext
or directly call
handle_output
on an IO manager. Instead, you would let the Dagster framework manage the execution and IO management process. If you find yourself needing to manually construct an
OutputContext
within an asset's execution context, it might be worth reconsidering the design of your pipeline to ensure it aligns with Dagster's execution model. If you have a specific use case that requires this pattern, it would be helpful to provide more context so that we can offer more targeted advice.
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