@Marvin iin prefect 2.18.3 there is a flow decorator tool called on_failure. IS there an equivalent that is run on_start?

It seems like there isn't a specific

on_start

decorator in Prefect 2.18.3, similar to the

on_failure

decorator. However, you can set up custom notifications or actions at the start of a flow run by including the necessary code within the flow itself. If you're looking to implement some functionality when a flow starts, you might consider placing it at the beginning of your flow definition or creating a task that performs the desired action and setting it as the initial task in your flow. For more details, check out the Prefect 2.x documentation on flow concepts and integration examples. Any more details would help if your need is more specific!

@Marvin I have a function precheck() that must be run everytime before the flow gets kicked off. Is it possible to have that precheck() function ran as a precheck before running the actual flow? I dont want to have it in the flow code necessarily, as other people may not be able to use it

To ensure your

precheck()

function runs every time before the actual flow kicks off without embedding it in the flow code, you can achieve this by using Prefect's state handlers or hooks. Here’s a possible approach: 1. State Handlers: You can define a state handler that performs the

precheck()

function when the flow enters a

Pending

state (which is before it runs). Here's a simplified example:

python
from prefect import flow

def precheck(flow, flow_run, state):
    # Your precheck logic here
    print("Running precheck")
    # You can raise an exception or return a failed state if precheck fails

@flow(state_handlers=[precheck])
def my_flow():
    print("Running main flow logic")

my_flow()

2. Custom Execution: Create a wrapper script or function to run

precheck()

before invoking the flow:

python
from prefect import flow

def precheck():
    # Your precheck logic here
    print("Running precheck")
    # Optionally return False if checks fail, or raise an exception

def run_with_precheck(flow_function):
    precheck()
    flow_function()

@flow
def my_flow():
    print("Running main flow logic")

# Usage
run_with_precheck(my_flow)

These methods help maintain code separation and ensure

precheck()

is executed as needed. For more details, check out the Prefect 2.x documentation on state handlers and flow hooks. If you need more customization or have specific constraints, let me know!

@Marvin TypeError: flow() got an unexpected keyword argument 'state_handlers'

Got it. It looks like there's no "state_handlers" argument for the flow decorator. Let's consider other alternatives like using Prefect's lifecycle hooks or adding a custom command in deployment. Another potential approach involves running the

precheck()

function programmatically before executing the flow, especially if hooks don't cover your use case. Here’s an improved implementation using a custom wrapper: ### Custom Wrapper Implementation 1. Define your

precheck

function outside your flow:

python
def precheck():
    # Your precheck logic here
    print("Running precheck")
    # Optionally raise an exception if the check fails

2. Create a function that runs the

precheck

and then executes the flow:

python
from prefect import flow

def run_flow_with_precheck(flow_function, *args, **kwargs):
    precheck()  # Run your precheck logic
    flow_function(*args, **kwargs)  # Execute the actual flow

@flow
def my_flow():
    print("Running main flow logic")

# Example usage
if __name__ == "__main__":
    run_flow_with_precheck(my_flow)

### Using Deployment Configuration If you’re deploying flows using Prefect’s deployment and want to add a precheck step: 1. Create a CLI script to run

precheck

followed by the flow:

python
import sys
from prefect.deployments import run_deployment

def precheck():
    # Your precheck logic here
    print("Running precheck")
    # Optionally raise an exception if the check fails

def main():
    try:
        precheck()  # Run precheck
        run_deployment(name="your-flow-name")  # Deploy and run your flow
    except Exception as e:
        print(f"Precheck failed: {e}")
        sys.exit(1)

if __name__ == "__main__":
    main()

2. Deploy this script as part of your CI/CD pipeline or container orchestration. Does this help address your implementation scenario more effectively? Let me know if you need further customization.