Access the Active Stack within Steps
How to use step fixtures to access the active ZenML stack from within a step
In general, when defining steps, you usually can only supply inputs that have been output by previous steps. However, there are two exceptions:
  • An object which is a subclass of BaseStepConfig: This object is used to pass run-time parameters to a pipeline run. It can be used to send parameters to a step that are not artifacts. You learned about this one already in the section on Runtime Configuration.
  • A Step Context object: This object gives access to the active stack, materializers, and special integration-specific libraries.
These two types of special parameters are comparable to Pytest fixtures, hence we call them Step Fixtures at ZenML.
To use step fixtures in your steps, just pass a parameter with the right type hint and ZenML will automatically recognize it.
from zenml.steps import step, BaseStepConfig, StepContext
class SubClassBaseStepConfig(BaseStepConfig):
...
@step
def my_step(
config: SubClassBaseStepConfig, # must be subclass of `BaseStepConfig`
context: StepContext, # must be of class `StepContext`
artifact: str, # other parameters are assumed to be outputs of other steps
):
...
The name of the argument can be anything, only the type hint is important. I.e., you don't necessarily need to call your fixtures config or context.

Step Contexts

The StepContext provides additional context inside a step function. It can be used to access artifacts, materializers, and stack components directly from within the step.

Defining Steps with Step Contexts

Unlike BaseStepConfig, you do not need to create a StepContext object yourself and pass it when creating the step. As long as you specify a parameter of type StepContext in the signature of your step function or class, ZenML will automatically create the StepContext and take care of passing it to your step at runtime.
When using a StepContext inside a step, ZenML disables caching for this step by default as the context provides access to external resources which might influence the result of your step execution. To enable caching anyway, explicitly enable it in the @step decorator with @step(enable_cache=True) or when initializing your custom step class.

Using Step Contexts

Within a step, there are many things that you can use the StepContext object for. For example, to access materializers, artifact locations, etc:
from zenml.steps import step, StepContext
@step
def my_step(context: StepContext):
context.get_output_materializer() # Get materializer for a given output.
context.get_output_artifact_uri() # Get URI for a given output.
context.metadata_store # Get access to the metadata store.
See the API Docs for more information on which attributes and methods the StepContext provides.

Fetching previous runs during pipeline execution

One of the most common usecases of the StepContext is to query the metadata store for a list of all previous pipeline runs, e.g., in order to compare a newly trained model to models trained in previous runs, or to fetch artifacts created in different pipelines.
As an example, see the following step that uses the StepContext to query the metadata store while running a step, which we use to find out whether the current run produced a better model than all previous runs:
from zenml.steps import step, StepContext
@step
def find_best_model(context: StepContext, test_acc: float) -> bool:
"""Step that finds if this run produced the highest `test_acc` ever"""
highest_acc = 0
# Inspect all past runs of `best_model_pipeline`.
try:
metadata_store = context.metadata_store
prior_runs = metadata_store.get_pipeline("best_model_pipeline").runs
except KeyError:
print("No prior runs found.")
prior_runs = []
# Find the highest accuracy produced in a prior run.
for run in prior_runs:
# get the output of the second step
prior_test_acc = run.get_step("evaluator").output.read()
if prior_test_acc > highest_acc:
highest_acc = prior_test_acc
# Check whether the current run produced the highest accuracy or not.
if test_acc >= highest_acc:
print(f"This run produced the highest test accuracy: {test_acc}.")
return True
print(
f"This run produced test accuracy {test_acc}, which is not the highest. "
f"The highest previous test accuracy was {highest_acc}."
)
return False

Full Code Example

Code Example for Fetching Historic Runs