Task Dependencies¶

In many workflows, tasks depend on the results of other tasks. Pravaha allows you to define task dependencies using the depends_on attribute.

Defining dependencies ensures that tasks are executed in the correct order and prevents failures caused by missing prerequisites.

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Defining Dependencies¶

Pravaha supports two types of dependencies:

OR-based dependencies
AND-based dependencies

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OR-based Dependencies¶

An OR-based dependency means that a task will execute if at least one of its dependent tasks completes successfully.

Example¶

from pravaha.core.task import Task
from pravaha.core.executor import TaskExecutor
from pravaha.dependency.dependency import Dependency

# Dependency(type="OR | AND", dependencies=list[str])
@Task("a", depends_on=Dependency("OR", dependencies=["b", "c"]))
def a():
    print("a")

@Task("b")
def b():
    pass

@Task("c")
def c():
    pass

TaskExecutor.execute()

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AND-based Dependencies¶

An AND-based dependency means that a task will execute only if all of its dependent tasks complete successfully.

AND-based dependencies can be defined in two ways.

### 1. Implicit AND dependency using a list

By default, passing a list to depends_on is treated as an AND dependency.

from pravaha.core.task import Task
from pravaha.core.executor import TaskExecutor

@Task(name="task1", depends_on=["task2"])
def task1():
    print("Task1 executed")

@Task(name="task2")
def task2():
    print("Task2 executed")

if __name__ == "__main__":
    TaskExecutor.execute()

Expected Output¶

Task2 executed
Task1 executed

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### 2. Explicit AND dependency using Dependency

You can also explicitly define an AND-based dependency using the Dependency object.

from pravaha.core.task import Task
from pravaha.core.executor import TaskExecutor
from pravaha.dependency.dependency import Dependency

# Dependency(type="OR | AND", dependencies=list[str])
@Task("a", depends_on=Dependency("AND", dependencies=["b", "c"]))
def a():
    print("a")

@Task("b")
def b():
    pass

@Task("c")
def c():
    pass

TaskExecutor.execute()

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Notes¶

Task registration order does not matter when dependencies are defined.
The executor automatically determines the correct execution order based on the dependency graph.
Circular dependencies are not allowed. Ensure that your workflow graph is acyclic (DAG).