The @asset decorator is used to define a software-defined asset - a combination of an asset key, a compute function, and upstream dependencies.
Signature
@asset(
name: Optional[str] = None,
key_prefix: Optional[Union[str, Sequence[str]]] = None,
ins: Optional[Mapping[str, AssetIn]] = None,
deps: Optional[Iterable[CoercibleToAssetDep]] = None,
metadata: Optional[Mapping[str, Any]] = None,
tags: Optional[Mapping[str, str]] = None,
description: Optional[str] = None,
config_schema: Optional[UserConfigSchema] = None,
required_resource_keys: Optional[AbstractSet[str]] = None,
resource_defs: Optional[Mapping[str, object]] = None,
hooks: Optional[AbstractSet[HookDefinition]] = None,
io_manager_def: Optional[object] = None,
io_manager_key: Optional[str] = None,
dagster_type: Optional[DagsterType] = None,
partitions_def: Optional[PartitionsDefinition] = None,
op_tags: Optional[Mapping[str, Any]] = None,
group_name: Optional[str] = None,
output_required: bool = True,
automation_condition: Optional[AutomationCondition] = None,
backfill_policy: Optional[BackfillPolicy] = None,
retry_policy: Optional[RetryPolicy] = None,
code_version: Optional[str] = None,
key: Optional[CoercibleToAssetKey] = None,
check_specs: Optional[Sequence[AssetCheckSpec]] = None,
owners: Optional[Sequence[str]] = None,
kinds: Optional[AbstractSet[str]] = None,
pool: Optional[str] = None,
) -> AssetsDefinition
Parameters
The name of the asset. If not provided, defaults to the name of the decorated function. Must be a valid Dagster name (only contains letters, numbers, and underscore).
key_prefix
Optional[Union[str, Sequence[str]]]
If provided, the asset’s key is the concatenation of the key_prefix and the asset’s name. Each item must be a valid Dagster name.
ins
Optional[Mapping[str, AssetIn]]
A dictionary that maps input names to information about the input.
deps
Optional[Iterable[CoercibleToAssetDep]]
The assets that are upstream dependencies, but do not correspond to a parameter of the decorated function. Can include AssetDep, AssetsDefinition, SourceAsset, AssetKey, or str.
A dict of metadata entries for the asset.
tags
Optional[Mapping[str, str]]
Tags for filtering and organizing. These tags are not attached to runs of the asset.
A human-readable description of the asset.
The configuration schema for the asset’s underlying op. If set, Dagster will check that config provided matches this schema.
Set of resource handles required by the op.
resource_defs
Optional[Mapping[str, object]]
A mapping of resource keys to resources. These resources will be initialized during execution and can be accessed from the context.
hooks
Optional[AbstractSet[HookDefinition]]
A set of hooks to attach to the asset. These hooks will be executed when the asset is materialized.
The IOManager used for storing the output of the op as an asset. Only one of io_manager_def and io_manager_key can be provided.
The resource key of the IOManager used for storing the output. Defaults to “io_manager”.
Allows specifying type validation functions that will be executed on the output after it runs.
partitions_def
Optional[PartitionsDefinition]
Defines the set of partition keys that compose the asset.
A dictionary of tags for the op that computes the asset. Values that are not strings will be JSON encoded.
A string name used to organize multiple assets into groups. If not provided, defaults to “default”.
Whether the decorated function will always materialize an asset. If False, the function can conditionally not yield a result.
automation_condition
Optional[AutomationCondition]
A condition describing when Dagster should materialize this asset.
Configure Dagster to backfill this asset according to its BackfillPolicy.
The retry policy for the op that computes the asset.
Version of the code that generates this asset. Should be set only for code that deterministically produces the same output when given the same inputs.
key
Optional[CoercibleToAssetKey]
The key for this asset. If provided, cannot specify key_prefix or name.
check_specs
Optional[Sequence[AssetCheckSpec]]
Specs for asset checks that execute in the decorated function after materializing the asset.
A list of strings representing owners of the asset. Each string can be a user’s email address, or a team name prefixed with team:, e.g. team:finops.
A list of strings representing the kinds of the asset. These will be made visible in the Dagster UI.
A string that identifies the concurrency pool that governs this asset’s execution.
Returns
Type: AssetsDefinition
An assets definition object representing the software-defined asset.
Examples
Basic Asset
from dagster import asset
@asset
def my_asset():
return [1, 2, 3]
Asset with Dependencies
import pandas as pd
from dagster import asset
@asset
def upstream_asset():
return pd.DataFrame({"column": [1, 2, 3]})
@asset
def downstream_asset(upstream_asset: pd.DataFrame) -> pd.DataFrame:
# upstream_asset is automatically loaded and passed as an argument
return upstream_asset * 2
Asset with Non-Argument Dependencies
from dagster import asset
@asset
def source_data():
return {"data": [1, 2, 3]}
@asset(deps=[source_data]) # depends on source_data but doesn't load it
def processed_data():
# Load the data yourself if needed
return {"processed": True}
Asset with Configuration
from dagster import asset, Config
from pydantic import Field
class MyAssetConfig(Config):
api_key: str = Field(description="API key for external service")
num_records: int = Field(default=100)
@asset
def configurable_asset(config: MyAssetConfig):
# Access config.api_key and config.num_records
return f"Fetched {config.num_records} records"
Conditional Asset Materialization
from dagster import asset, Output
should_materialize = True
@asset(output_required=False)
def conditional_asset():
if should_materialize:
yield Output(5) # must use yield, not return
# Otherwise, nothing is materialized
@asset
def downstream_asset(conditional_asset):
# Only runs if conditional_asset materialized
return conditional_asset + 1
from dagster import asset, MetadataValue
@asset(
metadata={
"owner": "data-team@company.com",
"priority": "high",
},
tags={"domain": "finance", "pii": "true"},
group_name="finance_assets",
owners=["team:data-platform"],
)
def important_asset():
return calculate_metrics()
Partitioned Asset
from dagster import asset, DailyPartitionsDefinition
@asset(
partitions_def=DailyPartitionsDefinition(start_date="2024-01-01")
)
def daily_sales(context):
partition_date = context.partition_key
return load_sales_for_date(partition_date)
