@Marvin How can I create dependencies between tasks within a flow? For example, I have four tasks:

connect

,

extract

,

load

, and

transform

. Each task should wait to execute until the previous task is complete.

connect

takes in multiple arguments from the flow and returns multiple variables.

extract

and takes in multiple arguments including one variable from

connect

and returns multiple variables.

load

takes in multiple arguments including one variable from

connect

and

extract

, but doesn't return any variables.

transform

takes in multiple arguments including one variable from

connect

and

extract

, but doesn't return any variables.

To create dependencies between tasks within a flow, you can define the tasks and then execute them in the desired order within a flow function. Here's a basic example demonstrating this concept using Prefect:

python
from prefect import flow, task

@task
def connect(arg1, arg2):
    # logic for connecting
    return result1, result2

@task
def extract(arg1, arg2, connect_result):
    # logic for extracting
    return extract_result1, extract_result2

@task
def load(arg1, arg2, connect_result, extract_result):
    # logic for loading
    pass

@task
def transform(arg1, arg2, connect_result, extract_result):
    # logic for transforming
    pass

@flow
def my_flow(arg1, arg2):
    conn_result1, conn_result2 = connect(arg1, arg2)
    extract_result1, extract_result2 = extract(arg1, arg2, conn_result2)
    load(arg1, arg2, conn_result1, extract_result1)
    transform(arg1, arg2, conn_result1, extract_result1)

if __name__ == "__main__":
    my_flow(arg1, arg2)

In this example: -

connect

is the first task, taking in multiple arguments and returning multiple values. -

extract

depends on the output of

connect

. -

load

and

transform

both depend on the outputs of

connect

and

extract

. Source: Code Example

@Marvin What about using the

wait_for

keyword?

The

wait_for

keyword can be used to create task dependencies within a flow. This keyword allows tasks to wait for the completion of other tasks before starting execution. Here is how you can use it: ### Code Example using

wait_for

python
from prefect import task, flow

@task
def connect(arg1, arg2):
    return "connect_result"

@task
def extract(arg1, arg2, connect_result):
    return "extract_result"

@task
def load(arg1, arg2, connect_result, extract_result):
    print("Loading...")

@task
def transform(arg1, arg2, connect_result, extract_result):
    print("Transforming...")

@flow
def my_flow(arg1, arg2):
    connect_result = connect(arg1, arg2)
    extract_result = extract(arg1, arg2, wait_for=[connect_result])
    load(arg1, arg2, connect_result, wait_for=[connect_result, extract_result])
    transform(arg1, arg2, connect_result, wait_for=[connect_result, extract_result])

if __name__ == "__main__":
    my_flow("arg1_value", "arg2_value")

### Quick Explanation -

connect

runs first. -

extract

will only start after

connect

completes. -

load

and

transform

will only start after both

connect

and

extract

complete. Source: GitHub Pull Request - wait_for Keyword

@Marvin I'm getting this error message:

prefect.exceptions.ParameterBindError: Error binding parameters for function 'extract': missing a required argument: 'connect_result'.
Function 'extract' has signature 'arg1, arg2, connect_result' but received args: ('arg1_value', 'arg2_value') and kwargs: [].

The error you're encountering is due to an argument mismatch when calling the

extract

task. You need to properly pass the results of the previous tasks as arguments. Here is a corrected example demonstrating how to create dependencies between tasks using

wait_for

and properly passing arguments: ### Corrected Code Example

python
from prefect import flow, task

@task
def connect(arg1, arg2):
    # logic for connecting
    return "connect_result"

@task
def extract(arg1, arg2, connect_result):
    # logic for extracting
    return "extract_result"

@task
def load(arg1, arg2, connect_result, extract_result):
    # logic for loading
    pass

@task
def transform(arg1, arg2, connect_result, extract_result):
    # logic for transforming
    pass

@flow
def my_flow(arg1, arg2):
    conn_result = connect(arg1, arg2)
    extract_result = extract(arg1, arg2, conn_result)
    load(arg1, arg2, conn_result, extract_result)
    transform(arg1, arg2, conn_result, extract_result)

if __name__ == "__main__":
    my_flow("arg1_value", "arg2_value")

### Explanation -

connect

runs first and returns

conn_result

. -

extract

waits for

connect

to complete and then uses its result. -

load

and

transform

wait for both

connect

and

extract

to complete before running. Source: Basic Task Example

@Marvin But this doesn't include the

wait_for

keyword.

I apologize for that oversight. Let's integrate the

wait_for

keyword correctly to create the task dependencies: ### Proper

wait_for

Usage Example

python
from prefect import task, flow

@task
def connect(arg1, arg2):
    return "connect_result"

@task
def extract(arg1, arg2, connect_result):
    return "extract_result"

@task
def load(arg1, arg2, connect_result, extract_result):
    print("Loading...")

@task
def transform(arg1, arg2, connect_result, extract_result):
    print("Transforming...")

@flow
def my_flow(arg1, arg2):
    connect_result = connect(arg1, arg2)
    extract_result = extract(arg1, arg2, connect_result, wait_for=[connect_result])
    load(arg1, arg2, connect_result, extract_result, wait_for=[connect_result, extract_result])
    transform(arg1, arg2, connect_result, extract_result, wait_for=[connect_result, extract_result])

if __name__ == "__main__":
    my_flow("arg1_value", "arg2_value")

### Explanation 1.

connect

runs first. 2.

extract

waits for

connect

to complete, specified by

wait_for=[connect_result]

. 3.

load

and

transform

wait for both

connect

and

extract

to complete, specified by

wait_for=[connect_result, extract_result]

. This ensures that the tasks run in sequence, adhering to the dependencies you've outlined. ### Reference To learn more about using Prefect and the

wait_for

keyword efficiently, please take a look at the Prefect documentation and the GitHub Pull Request about wait_for.