Hi. I am new to Prefect 2.0. I need some assistanc...
# best-practices
d
Hi. I am new to Prefect 2.0. I need some assistance from the community here. I am deploying some pipelines that is scheduled to run on a daily basis. How do I pass in the runtime
date
into my flow as I would need to use that as a parameter for my pipeline? E.g runs on Wed 19Oct 2022 23:00. the date parameter
19Oct2022
is passed a parameter into the flow and will be used in the ETL script
1
r
Hi Daniel, A couple of options come to mind. If you just need the date when the flow run commences, would a parameter with a default value work? For example, would something like this work for you?:
Copy code
import pendulum
from prefect import flow, get_run_logger


@flow(name="my flow")
def my_flow(start_date=f"{pendulum.today('UTC'):%d%b%Y}"):
    logger = get_run_logger()
    <http://logger.info|logger.info>(f"Running on {start_date}")


if __name__ == "__main__":
    my_flow()
If you run that, you'll see output similar to:
Copy code
06:59:35.716 | INFO    | prefect.engine - Created flow run 'agate-marten' for flow 'my flow'
06:59:35.813 | INFO    | Flow run 'agate-marten' - Running on 19Oct2022
06:59:35.830 | INFO    | Flow run 'agate-marten' - Finished in state Completed()
Pendulum is one of Prefect's dependencies, so you wouldn't need to install anything extra. You can also do the same thing with Python's built-in
datetime
, but it would be a bit more verbose. Using a parameter with a default value this way gives you the ability to override the parameter if you ever need to, but provides a sensible default that should (hopefully) give what you need most of the time. You could then pass the date into tasks and subflows as needed for your ETL pipeline. You can also read both the expected and actual start dates from the flow run context, like so:
Copy code
from prefect import flow, get_run_logger
from prefect.context import get_run_context


@flow(name="my flow")
def my_flow():
    logger = get_run_logger()
    context = get_run_context()
    expected_start_date = f"{context.flow_run.expected_start_time:%d%b%Y}"
    actual_start_date = f"{context.flow_run.start_time:%d%b%Y}"
    <http://logger.info|logger.info>(f"Expected to run on {expected_start_date}")
    <http://logger.info|logger.info>(f"Actually running on {actual_start_date}")


if __name__ == "__main__":
    my_flow()
Which results in output like:
Copy code
07:13:32.949 | INFO    | prefect.engine - Created flow run 'bald-heron' for flow 'my flow'
07:13:33.059 | INFO    | Flow run 'bald-heron' - Expected to run on 19Oct2022
07:13:33.059 | INFO    | Flow run 'bald-heron' - Actually running on 19Oct2022
07:13:33.075 | INFO    | Flow run 'bald-heron' - Finished in state Completed()
Will either of these help you accomplish your goal?
🙌 3
🙏 1
d
yes that works for me!
@Ryan Peden now I have another question here. when i was trying to get the agent to retrieve the job i got this error
the thing is my script is as such
Copy code
if __name__ == "__main__":
    <i pass in my parameters from a config file>
    today_date_str = kwargs.get("today_date_str")
    query_parameters = get_query_parameters(config, **kwargs)
    <execute flow here>
in that case should I still pass in these parameters to prefect flow?
r
Yes, you should. If you don't provide default values for the params, you'll typically need to pass them when calling the flow in Python, or if you create a deployment you will need to set up parameters in the deployment. If necessary, you can disable flow parameter checks when creating your flow:
Copy code
@flow(validate_parameters=False)
def my_flow(today_date_str: str, query_parameters: dict[str,str]):
    ...
Or if you'd like to keep the validation but mark a params as optional to keep Pydantic happy:
Copy code
from typing import Optional

@flow(validate_parameters=False)
def my_flow(today_date_str: Optional[str], query_parameters: Optional[dict[str,str]]):
    ...
That way, you're not forced to use the parameters but still get validation, so if for example you tried to call the flow with an
int
as your
today_date_str
, validation would fail.