Execute a task from Python

Walkthrough

This page shows to instantiate a task from Python, set its inputs, execute it and retrieve its outputs. It can be useful for debugging a single task for example.

To illustrate it, we will use the SumTask below that adds two numbers a and b, and stores the result in result:

class SumTask(
    Task, input_names=["a"], optional_input_names=["b"], output_names=["result"]
):
    def run(self):
        result = self.inputs.a
        if self.inputs.b:
            result += self.inputs.b
        self.outputs.result = result

Tip

This task can be imported from ewokscore.tests.examples.tasks.sumtask

Task inputs need to be set via the inputs argument when creating the task before executing via execute:

>>> task = SumTask(inputs={"a": 5, "b": 10})
>>> task.execute()

The results can then be retrieved from outputs or using get_output_value:

>>> task.outputs["result"]
15
>>> task.get_output_value('result')
15

Limitations

Inputs cannot be modified once the task is instantiated

>>> task = SumTask(inputs={"a": 5, "b": 10})
>>> task.inputs.a = 10
...
ewokscore.variable.ReadOnlyVariableError: a

A new task instance needs to be created to execute it with different inputs

>>> task = SumTask(inputs={"a": 10, "b": 10})
>>> task.execute()
>>> task.get_output_value("result")
20

Outputs will be missing before the task is executed. More specifically, the outputs are set to MISSING_DATA until they are set by the execution:

>>> task = SumTask(inputs={"a": 5, "b": 10})
>>> task.get_output_value("result")
<MISSING_DATA>

Full code

from ewokscore.tests.examples.tasks.sumtask import SumTask

task = SumTask(inputs={"a": 5, "b": 10})
task.execute()
print(task.get_output_value("result"))