Write and run flows

Flows are the most central Prefect object. A flow is a container for workflow logic as code.

Flows are defined as Python functions. They can take inputs, perform work, and return an output.

You can turn any function into a Prefect flow by adding the @flow decorator to it:

from prefect import flow


@flow
def my_flow():
    return

When a function becomes a flow, its behavior changes, giving it the following capabilities:

All runs of the flow have persistent state. Transitions between states are recorded, allowing you to observe and act on flow execution.
Input arguments can be type validated as workflow parameters.
Retries can be performed on failure.
Timeouts can be enforced to prevent unintentional, long-running workflows.
Metadata about flow runs, such as run time and final state, is automatically tracked.
A flow can be deployed, which exposes an API for interacting with it remotely.

Flows are uniquely identified by name. You can provide a name parameter value for the flow:

@flow(name="My Flow")
def my_flow():
    return

If you don’t provide a name, Prefect uses the flow function name.

Running flows

A flow run is a single execution of a flow.

You can create a flow run by calling a flow by its function name, just as you would a normal Python function. For example, by running a script or importing the function into an interactive session and calling it.

You can also create a flow run by:

Using external schedulers such as cron to invoke a flow function
Triggering a deployment of that flow on Prefect Cloud or a self-hosted Prefect server instance
Starting a flow run for the deployment through a schedule, the Prefect UI, or the Prefect API

However you run your flow, Prefect monitors the flow run, capturing its state for observability.

Logging

You can log a variety of metadata about your flow runs for monitoring, troubleshooting, and auditing purposes.

Example

The script below fetches statistics about the main Prefect repository. (Note that httpx is an HTTP client library and a dependency of Prefect.)

Turn this function into a Prefect flow and run the script:

repo_info.py

import httpx
from prefect import flow


@flow
def get_repo_info():
    url = "https://api.github.com/repos/PrefectHQ/prefect"
    response = httpx.get(url)
    response.raise_for_status()
    repo = response.json()
    print("PrefectHQ/prefect repository statistics 🤓:")
    print(f"Stars 🌠 : {repo['stargazers_count']}")
    print(f"Forks 🍴 : {repo['forks_count']}")

if __name__ == "__main__":
    get_repo_info()

Running this script results in the following output:

12:47:42.792 | INFO | prefect.engine - Created flow run 'ludicrous-warthog' for flow 'get-repo-info'
PrefectHQ/prefect repository statistics 🤓:
Stars 🌠 : 12146
Forks 🍴 : 1245
12:47:45.008 | INFO | Flow run 'ludicrous-warthog' - Finished in state Completed()

Flows can contain arbitrary Python code

As shown above, flow definitions can contain arbitrary Python code.

Specifying parameters

As with any Python function, you can pass arguments to a flow, including both positional and keyword arguments. These arguments defined on your flow function are called parameters. They are stored by the Prefect orchestration engine on the flow run object.

Prefect automatically performs type conversion of inputs using any provided type hints. Type hints provide an easy way to enforce typing on your flow parameters and can be customized with Pydantic. Prefect supports any Pydantic model as a type hint for a flow parameter.

from prefect import flow
from pydantic import BaseModel


class Model(BaseModel):
    a: int
    b: float
    c: str

@flow
def model_validator(model: Model):
    print(model)

For example, to automatically convert an arguement to a datetime:

from prefect import flow
from datetime import datetime

@flow
def what_day_is_it(date: Optional[datetime] = None):
    if date is None:
        date = datetime.now(timezone.utc)
    print(f"It was {date.strftime('%A')} on {date.isoformat()}")

if __name__ == "__main__":
    what_day_is_it("2021-01-01T02:00:19.180906")

When you run this flow, you’ll see the following output:

It was Friday on 2021-01-01T02:00:19.180906

Note that you can provide parameter values to a flow through the API using a deployment. Flow run parameters sent to the API on flow calls are coerced to the appropriate types.

Prefect API requires keyword arguments

When creating flow runs from the Prefect API, you must specify parameter names when overriding defaults. The values passed cannot be positional.

Parameters are validated before a flow is run. If a flow call receives invalid parameters, a flow run is created in a Failed state. If a flow run for a deployment receives invalid parameters, it moves from a Pending state to a Failed state without entering a Running state.

Flow run parameters cannot exceed 512kb in size.

Composing flows

Flows can call tasks, the most granular units of orchestrated work in Prefect workflows:

from prefect import flow, task


@task
def print_hello(name):
    print(f"Hello {name}!")

@flow(name="Hello Flow")
def hello_world(name="world"):
    print_hello(name)

A single flow function can contain all of your workflow’s code. However, if you put all of your workflow logic in a single flow function and any line of code fails, the entire flow fails and must be retried from the beginning. Organizing your workflow code into smaller flows and tasks lets you take advantage of Prefect features such as retries, more granular visibility into runtime state, the ability to determine final state regardless of individual task state, and more.

You may call any number of tasks, other flows, and even regular Python functions within your flow. You can pass parameters to your flow function to use elsewhere in the workflow, and Prefect will report on the progress and final state of any invocation.

We recommend writing atomic tasks. Each task should be a single, discrete piece of work in your workflow, such as calling an API, performing a database operation, or transforming a data point. The more granular you make your tasks, the better your workflows can recover from failures and the easier you can find and fix issues. Prefect tasks are well suited for parallel or distributed execution using distributed computation frameworks such as Dask or Ray.

Nesting flows

In addition to calling tasks within a flow, you can also call other flows. A nested flow run is created when a flow function is called by another flow. When one flow calls another, the calling flow run is the “parent” run, the called flow run is the “child” run.

Nesting flows is a great way to organize your workflows and offer more visibility within the UI. In the UI, each child flow run is linked to its parent and can be individually observed.

For most purposes, nested flow runs behave just as all flow runs do. There is a full representation of the nested flow run in the backend as if it had been called separately. Nested flow runs differ from normal flow runs in that they resolve any passed task futures into data. This allows data to be passed from the parent flow run to the child run easily.

When a nested flow run starts, it creates a new task runner for any tasks it contains. When the nested flow run completes, the task runner shuts down. Nested flow runs block execution of the parent flow run until completion. However, asynchronous nested flows can run concurrently with AnyIO task groups or asyncio.gather.

The relationship between nested runs is recorded through a special task run in the parent flow run that represents the child flow run. The state_details feild of the task run representing the child flow run includes a child_flow_run_id. The state_details feild of the nested flow run includes a parent_task_run_id.

You can define multiple flows within the same file. Whether running locally or through a deployment, you must indicate which flow is the entrypoint for a flow run.

Cancelling nested flow runs

A nested flow run cannot be cancelled without cancelling its parent flow run. If you need to be able to cancel a nested flow run independent of its parent flow run, we recommend deploying it separately and starting it with the run_deployment method.

You can also define flows or tasks in separate modules and import them for use:

from prefect import flow, task


@flow(name="Subflow")
def my_subflow(msg):
    print(f"Subflow says: {msg}")

Here’s a parent flow that imports and uses my_subflow() as a subflow:

from prefect import flow, task
from subflow import my_subflow


@task(name="Print Hello")
def print_hello(name):
    msg = f"Hello {name}!"
    print(msg)
    return msg

@flow(name="Hello Flow")
def hello_world(name="world"):
    message = print_hello(name)
    my_subflow(message)

if __name__=="__main__":
    hello_world("Marvin")

Running the hello_world() flow (in this example from the file hello.py) creates a flow run like this:

$ python hello.py
15:19:21.651 | INFO    | prefect.engine - Created flow run 'daft-cougar' for flow 'Hello Flow'
15:19:21.945 | INFO    | Flow run 'daft-cougar' - Created task run 'Print Hello-84f0fe0e-0' for task 'Print Hello'
Hello Marvin!
15:19:22.055 | INFO    | Task run 'Print Hello-84f0fe0e-0' - Finished in state Completed()
15:19:22.107 | INFO    | Flow run 'daft-cougar' - Created subflow run 'ninja-duck' for flow 'Subflow'
Subflow says: Hello Marvin!
15:19:22.794 | INFO    | Flow run 'ninja-duck' - Finished in state Completed()
15:19:23.215 | INFO    | Flow run 'daft-cougar' - Finished in state Completed('All states completed.')

Here are some scenarios where you should choose a nested flow rather than calling tasks individually:

Observability: Nested flows, like any other flow run, have first-class observability within the Prefect UI and Prefect Cloud. You’ll see nested flows’ status in the Runs dashboard rather than having to dig down into the tasks within a specific flow run. See Final state determination for examples of using task state within flows.
Conditional flows: If you have a group of tasks that run only under certain conditions, you can group them within a nested flow and conditionally run the nested flow rather than each task individually.
Parameters: Flows have first-class support for parameterization, making it easy to run the same group of tasks in different use cases by simply passing different parameters to the nested flow in which they run.
Task runners: Nested flows enable you to specify the task runner used for tasks within the flow. For example, to optimize parallel execution of certain tasks with Dask, group them in a nested flow that uses the Dask task runner. You can use a different task runner for each nested flow.

Supported functions

Almost any standard Python function can be turned into a Prefect flow by adding the @flow decorator.

Flows are always executed in the main thread by default to facilitate native Python debugging and profiling.

Synchronous functions

The simplest Prefect flow is a synchronous Python function. Here’s an example of a synchronous flow that prints a message:

from prefect import flow

@flow
def print_message():
    print("Hello, I'm a flow")

print_message()

Asynchronous functions

Prefect also supports asynchronous functions. The resulting flows are coroutines that can be awaited or run concurrently, following the standard rules of async Python.

import asyncio

from prefect import task, flow

@task
async def print_values(values):
    for value in values:
        await asyncio.sleep(1)
        print(value, end=" ")

@flow
async def async_flow():
    print("Hello, I'm an async flow")

    # runs immediately
    await print_values([1, 2])  

    # runs concurrently
    coros = [print_values("abcd"), print_values("6789")]
    await asyncio.gather(*coros)

asyncio.run(async_flow())

Class methods

Prefect supports synchronous and asynchronous class methods as flows, including instance methods, class methods, and static methods. For class methods and static methods, you must apply the appropriate method decorator above the @flow decorator:

from prefect import flow


class MyClass:

    @flow
    def my_instance_method(self):
        pass

    @classmethod
    @flow
    def my_class_method(cls):
        pass

    @staticmethod
    @flow
    def my_static_method():
        pass

MyClass().my_instance_method()
MyClass.my_class_method()
MyClass.my_static_method()

Generators

Prefect supports synchronous and asynchronous generators as flows. The flow is considered to be Running as long as the generator is yielding values. When the generator is exhausted, the flow is considered Completed. Any values yielded by the generator can be consumed by other flows or tasks.

from prefect import flow


@flow
def generator():
    for i in range(10):
        yield i

@flow
def consumer(x):
    print(x)

for val in generator():
    consumer(val)

Generator functions are consumed when returned from flows

The result of a completed flow must be serializable, but generators cannot be serialized. Therefore, if you return a generator from a flow, the generator will be fully consumed and its yielded values will be returned as a list. This can lead to unexpected behavior or blocking if the generator is infinite or very large.

Here is an example of proactive generator consumption:

from prefect import flow

def gen():
    yield from [1, 2, 3]
    print('Generator consumed!')

@flow
def f():
    return gen()
    
f()  # prints 'Generator consumed!'

If you need to return a generator without consuming it, you can yield it instead of using return. Values yielded from generator flows are not considered final results and do not face the same serialization constraints:

from prefect import flow

def gen():
    yield from [1, 2, 3]
    print('Generator consumed!')

@flow
def f():
    yield gen
    
generator = next(f())
list(generator) # prints 'Generator consumed!'

Parameters

As with any Python function, you can pass arguments to a flow including both positional and keyword arguments. These arguments defined on your flow function are called parameters. They are stored by the Prefect orchestration engine on the flow run object.

Prefect automatically performs type conversion of inputs using any provided type hints. Type hints provide an easy way to enforce typing on your flow parameters and can be greatly enhanced with Pydantic. Prefect supports any Pydantic model as a type hint within a flow is coerced automatically into the relevant object type:

from prefect import flow
from pydantic import BaseModel

class Model(BaseModel):
    a: int
    b: float
    c: str

@flow
def model_validator(model: Model):
    print(model)

For example, to automatically convert something to a datetime:

from prefect import flow
from datetime import datetime

@flow
def what_day_is_it(date: Optional[datetime] = None):
    if date is None:
        date = datetime.now(timezone.utc)
    print(f"It was {date.strftime('%A')} on {date.isoformat()}")

if __name__ == "__main__":
    what_day_is_it("2021-01-01T02:00:19.180906")

When you run this flow, you’ll see the following output:

It was Friday on 2021-01-01T02:00:19.180906

Note that you can provide parameter values to a flow through the API using a deployment. Flow run parameters sent to the API on flow calls are coerced to the appropriate types.

Prefect API requires keyword arguments

When creating flow runs from the Prefect API, you must specify parameter names when overriding defaults. They cannot be positional.

Flow run parameters cannot exceed 512kb in size.

Flow runs

A flow run represents a single execution of the flow.

You can create a flow run by calling the flow manually. For example, by running a Python script or importing the flow into an interactive session and calling it.

You can also create a flow run by:

Using external schedulers such as cron to invoke a flow function
Creating a deployment on Prefect Cloud or a locally run Prefect server
Creating a flow run for the deployment through a schedule, the Prefect UI, or the Prefect API

However you run the flow, the Prefect API monitors the flow run, capturing flow run state for observability.

Logging

Prefect enables you to log a variety of useful information about your flow and task runs. You can capture information about your workflows for purposes such as monitoring, troubleshooting, and auditing. Check out Logging for more information.

When you run a flow that contains tasks or additional flows, Prefect tracks the relationship of each child run to the parent flow run.

Retries

Unexpected errors may occur. For example the GitHub API may be temporarily unavailable or rate limited. Check out Transactions to learn how to make your flows more resilient.

Writing flows

The @flow decorator is used to designate a flow:

from prefect import flow

@flow
def my_flow():
    return

There are no rigid rules for what code you include within a flow definition. All valid Python is acceptable.

Flows are uniquely identified by name. You can provide a name parameter value for the flow. If you don’t provide a name, Prefect uses the flow function name.

@flow(name="My Flow")
def my_flow():
    return

Flows can call tasks to allow Prefect to orchestrate and track more granular units of work:

from prefect import flow, task

@task
def print_hello(name):
    print(f"Hello {name}!")

@flow(name="Hello Flow")
def hello_world(name="world"):
    print_hello(name)

Flows and tasks

There’s nothing stopping you from putting all of your code in a single flow function.

However, organizing your workflow code into smaller flow and task units lets you take advantage of Prefect features like retries, more granular visibility into runtime state, the ability to determine final state regardless of individual task state, and more.

In addition, if you put all of your workflow logic in a single flow function and any line of code fails, the entire flow fails and must be retried from the beginning. You can avoid this by breaking up the code into multiple tasks.

You may call any number of other tasks, subflows, and even regular Python functions within your flow. You can pass parameters to your flow function to use elsewhere in the workflow, and Prefect will report on the progress and final state of any invocation.

Prefect encourages “small tasks.” Each one should represent a single logical step of your workflow. This allows Prefect to better contain task failures.

Subflows

In addition to calling tasks within a flow, you can also call other flows. Child flows are called subflows and allow you to efficiently manage, track, and version common multi-task logic.

Subflows are a great way to organize your workflows and offer more visibility within the UI.

Add a flow decorator to the get_open_issues function:

@flow
def get_open_issues(repo_name: str, open_issues_count: int, per_page: int = 100):
    issues = []
    pages = range(1, -(open_issues_count // -per_page) + 1)
    for page in pages:
        issues.append(
            get_url.submit(
                f"https://api.github.com/repos/{repo_name}/issues",
                params={"page": page, "per_page": per_page, "state": "open"},
            )
        )
    return [i for p in issues for i in p.result()]

Whenever you run the parent flow, the subflow is called and runs. In the UI, each subflow run is linked to its parent and can be individually inspected.

Flow settings

Flows can be configured by passing arguments to the decorator. Flows accept the following optional settings:

Argument	Description
`description`	An optional string description for the flow. If not provided, the description is pulled from the docstring for the decorated function.
`name`	An optional name for the flow. If not provided, the name is inferred from the function.
`retries`	An optional number of times to retry on flow run failure.
`retry_delay_seconds`	An optional number of seconds to wait before retrying the flow after failure. This is only applicable if `retries` is nonzero.
`flow_run_name`	An optional name to distinguish runs of this flow; this name can be provided as a string template with the flow’s parameters as variables; you can also provide this name as a function that returns a string.
`task_runner`	An optional task runner to use for task execution within the flow when you `.submit()` tasks. If not provided and you `.submit()` tasks, the `ThreadPoolTaskRunner` is used.
`timeout_seconds`	An optional number of seconds indicating a maximum runtime for the flow. If the flow exceeds this runtime, it is marked as failed. Flow execution may continue until the next task is called.
`validate_parameters`	Boolean indicating whether parameters passed to flows are validated by Pydantic. Default is `True`.
`version`	An optional version string for the flow. If not provided, we will attempt to create a version string as a hash of the file containing the wrapped function. If the file cannot be located, the version will be null.

For example, you can provide a name value for the flow. Here is the optional description argument and a non-default task runner.

from prefect import flow

@flow(
    name="My Flow",
    description="My flow with retries and linear backoff",
    retries=3,
    retry_delay_seconds=[10, 20, 30],
)
def my_flow():
    return

You can also provide the description as the docstring on the flow function.

@flow(
    name="My Flow",
    retries=3,
    retry_delay_seconds=[10, 20, 30],
)
def my_flow():
    """My flow with retries and linear backoff"""
    return

You can distinguish runs of this flow by providing a flow_run_name. This setting accepts a string that can optionally contain templated references to the parameters of your flow. The name is formatted using Python’s standard string formatting syntax:

import datetime
from prefect import flow

@flow(flow_run_name="{name}-on-{date:%A}")
def my_flow(name: str, date: datetime.datetime):
    pass

# creates a flow run called 'marvin-on-Thursday'
my_flow(name="marvin", date=datetime.datetime.now(datetime.timezone.utc))

Additionally this setting also accepts a function that returns a string for the flow run name:

import datetime
from prefect import flow

def generate_flow_run_name():
    date = datetime.datetime.now(datetime.timezone.utc)

    return f"{date:%A}-is-a-nice-day"

@flow(flow_run_name=generate_flow_run_name)
def my_flow(name: str):
    pass

# creates a flow run called 'Thursday-is-a-nice-day'
if __name__ == "__main__":
    my_flow(name="marvin")

If you need access to information about the flow, use the prefect.runtime module. For example:

from prefect import flow
from prefect.runtime import flow_run

def generate_flow_run_name():
    flow_name = flow_run.flow_name

    parameters = flow_run.parameters
    name = parameters["name"]
    limit = parameters["limit"]

    return f"{flow_name}-with-{name}-and-{limit}"

@flow(flow_run_name=generate_flow_run_name)
def my_flow(name: str, limit: int = 100):
    pass

# creates a flow run called 'my-flow-with-marvin-and-100'
if __name__ == "__main__":
    my_flow(name="marvin")

Note that validate_parameters check that input values conform to the annotated types on the function. Where possible, values are coerced into the correct type. For example, if a parameter is defined as x: int and “5” is passed, it resolves to 5. If set to False, no validation is performed on flow parameters.

Final state determination

States are a record of the status of a particular task or flow run. See the manage states page for more information.

The final state of the flow is determined by its return value. The following rules apply:

If an exception is raised directly in the flow function, the flow run is marked as failed.
If the flow does not return a value (or returns None), its state is determined by the states of all of the tasks and subflows within it.
- If any task run or subflow run failed, then the final flow run state is marked as FAILED.
- If any task run was cancelled, then the final flow run state is marked as CANCELLED.
If a flow returns a manually created state, it is used as the state of the final flow run. This allows for manual determination of final state.
If the flow run returns any other object, then it is marked as completed.

The following examples illustrate each of these cases:

Raise an exception

If an exception is raised within the flow function, the flow is immediately marked as failed.

from prefect import flow

@flow
def always_fails_flow():
    raise ValueError("This flow immediately fails")

if __name__ == "__main__":
    always_fails_flow()

Running this flow produces the following result:

22:22:36.864 | INFO    | prefect.engine - Created flow run 'acrid-tuatara' for flow 'always-fails-flow'
22:22:37.060 | ERROR   | Flow run 'acrid-tuatara' - Encountered exception during execution:
Traceback (most recent call last):...
ValueError: This flow immediately fails

Return `none`

A flow with no return statement is determined by the state of all of its task runs.

from prefect import flow, task

@task
def always_fails_task():
    raise ValueError("I fail successfully")

@task
def always_succeeds_task():
    print("I'm fail safe!")
    return "success"

@flow
def always_fails_flow():
    always_fails_task.submit().result(raise_on_failure=False)
    always_succeeds_task()

if __name__ == "__main__":
    always_fails_flow()

Running this flow produces the following result:

18:32:05.345 | INFO    | prefect.engine - Created flow run 'auburn-lionfish' for flow 'always-fails-flow'
18:32:05.582 | INFO    | Flow run 'auburn-lionfish' - Created task run 'always_fails_task-96e4be14-0' for task 'always_fails_task'
18:32:05.582 | INFO    | Flow run 'auburn-lionfish' - Submitted task run 'always_fails_task-96e4be14-0' for execution.
18:32:05.610 | ERROR   | Task run 'always_fails_task-96e4be14-0' - Encountered exception during execution:
Traceback (most recent call last):
  ...
ValueError: I fail successfully
18:32:05.638 | ERROR   | Task run 'always_fails_task-96e4be14-0' - Finished in state Failed('Task run encountered an exception.')
18:32:05.658 | INFO    | Flow run 'auburn-lionfish' - Created task run 'always_succeeds_task-9c27db32-0' for task 'always_succeeds_task'
18:32:05.659 | INFO    | Flow run 'auburn-lionfish' - Executing 'always_succeeds_task-9c27db32-0' immediately...
I'm fail safe!
18:32:05.703 | INFO    | Task run 'always_succeeds_task-9c27db32-0' - Finished in state Completed()
18:32:05.730 | ERROR   | Flow run 'auburn-lionfish' - Finished in state Failed('1/2 states failed.')
Traceback (most recent call last):
  ...
ValueError: I fail successfully

Return a future

If a flow returns one or more futures, the final state is determined based on the underlying states.

from prefect import flow, task

@task
def always_fails_task():
    raise ValueError("I fail successfully")

@task
def always_succeeds_task():
    print("I'm fail safe!")
    return "success"

@flow
def always_succeeds_flow():
    x = always_fails_task.submit().result(raise_on_failure=False)
    y = always_succeeds_task.submit(wait_for=[x])
    return y

if __name__ == "__main__":
    always_succeeds_flow()

Running this flow produces the following result—it succeeds because it returns the future of the task that succeeds:

18:35:24.965 | INFO    | prefect.engine - Created flow run 'whispering-guan' for flow 'always-succeeds-flow'
18:35:25.204 | INFO    | Flow run 'whispering-guan' - Created task run 'always_fails_task-96e4be14-0' for task 'always_fails_task'
18:35:25.205 | INFO    | Flow run 'whispering-guan' - Submitted task run 'always_fails_task-96e4be14-0' for execution.
18:35:25.232 | ERROR   | Task run 'always_fails_task-96e4be14-0' - Encountered exception during execution:
Traceback (most recent call last):
  ...
ValueError: I fail successfully
18:35:25.265 | ERROR   | Task run 'always_fails_task-96e4be14-0' - Finished in state Failed('Task run encountered an exception.')
18:35:25.289 | INFO    | Flow run 'whispering-guan' - Created task run 'always_succeeds_task-9c27db32-0' for task 'always_succeeds_task'
18:35:25.289 | INFO    | Flow run 'whispering-guan' - Submitted task run 'always_succeeds_task-9c27db32-0' for execution.
I'm fail safe!
18:35:25.335 | INFO    | Task run 'always_succeeds_task-9c27db32-0' - Finished in state Completed()
18:35:25.362 | INFO    | Flow run 'whispering-guan' - Finished in state Completed('All states completed.')

Return multiple states or futures

If a flow returns a mix of futures and states, the final state is determined by resolving all futures to states, then determining if any of the states are not COMPLETED.

from prefect import task, flow

@task
def always_fails_task():
    raise ValueError("I am bad task")

@task
def always_succeeds_task():
    return "foo"

@flow
def always_succeeds_flow():
    return "bar"

@flow
def always_fails_flow():
    x = always_fails_task()
    y = always_succeeds_task()
    z = always_succeeds_flow()
    return x, y, z

Running this flow produces the following result. It fails because one of the three returned futures failed. Note that the final state is Failed, but the states of each of the returned futures is included in the flow state:

20:57:51.547 | INFO    | prefect.engine - Created flow run 'impartial-gorilla' for flow 'always-fails-flow'
20:57:51.645 | INFO    | Flow run 'impartial-gorilla' - Created task run 'always_fails_task-58ea43a6-0' for task 'always_fails_task'
20:57:51.686 | INFO    | Flow run 'impartial-gorilla' - Created task run 'always_succeeds_task-c9014725-0' for task 'always_succeeds_task'
20:57:51.727 | ERROR   | Task run 'always_fails_task-58ea43a6-0' - Encountered exception during execution:
Traceback (most recent call last):...
ValueError: I am bad task
20:57:51.787 | INFO    | Task run 'always_succeeds_task-c9014725-0' - Finished in state Completed()
20:57:51.808 | INFO    | Flow run 'impartial-gorilla' - Created subflow run 'unbiased-firefly' for flow 'always-succeeds-flow'
20:57:51.884 | ERROR   | Task run 'always_fails_task-58ea43a6-0' - Finished in state Failed('Task run encountered an exception.')
20:57:52.438 | INFO    | Flow run 'unbiased-firefly' - Finished in state Completed()
20:57:52.811 | ERROR   | Flow run 'impartial-gorilla' - Finished in state Failed('1/3 states failed.')
Failed(message='1/3 states failed.', type=FAILED, result=(Failed(message='Task run encountered an exception.', type=FAILED, result=ValueError('I am bad task'), task_run_id=5fd4c697-7c4c-440d-8ebc-dd9c5bbf2245), Completed(message=None, type=COMPLETED, result='foo', task_run_id=df9b6256-f8ac-457c-ba69-0638ac9b9367), Completed(message=None, type=COMPLETED, result='bar', task_run_id=cfdbf4f1-dccd-4816-8d0f-128750017d0c)), flow_run_id=6d2ec094-001a-4cb0-a24e-d2051db6318d)

Returning multiple states

When returning multiple states, they must be contained in a set, list, or tuple. If using other collection types, the result of the contained states are checked.

Return a manual state

If a flow returns a manually created state, the final state is determined based on the return value.

from prefect import task, flow
from prefect.states import Completed, Failed

@task
def always_fails_task():
    raise ValueError("I fail successfully")

@task
def always_succeeds_task():
    print("I'm fail safe!")
    return "success"

@flow
def always_succeeds_flow():
    x = always_fails_task.submit()
    y = always_succeeds_task.submit()
    if y.result() == "success":
        return Completed(message="I am happy with this result")
    else:
        return Failed(message="How did this happen!?")

if __name__ == "__main__":
    always_succeeds_flow()

Running this flow produces the following result.

18:37:42.844 | INFO    | prefect.engine - Created flow run 'lavender-elk' for flow 'always-succeeds-flow'
18:37:43.125 | INFO    | Flow run 'lavender-elk' - Created task run 'always_fails_task-96e4be14-0' for task 'always_fails_task'
18:37:43.126 | INFO    | Flow run 'lavender-elk' - Submitted task run 'always_fails_task-96e4be14-0' for execution.
18:37:43.162 | INFO    | Flow run 'lavender-elk' - Created task run 'always_succeeds_task-9c27db32-0' for task 'always_succeeds_task'
18:37:43.163 | INFO    | Flow run 'lavender-elk' - Submitted task run 'always_succeeds_task-9c27db32-0' for execution.
18:37:43.175 | ERROR   | Task run 'always_fails_task-96e4be14-0' - Encountered exception during execution:
Traceback (most recent call last):
  ...
ValueError: I fail successfully
I'm fail safe!
18:37:43.217 | ERROR   | Task run 'always_fails_task-96e4be14-0' - Finished in state Failed('Task run encountered an exception.')
18:37:43.236 | INFO    | Task run 'always_succeeds_task-9c27db32-0' - Finished in state Completed()
18:37:43.264 | INFO    | Flow run 'lavender-elk' - Finished in state Completed('I am happy with this result')

Return an object

If the flow run returns any other object, then it is marked as completed.

from prefect import task, flow

@task
def always_fails_task():
    raise ValueError("I fail successfully")

@flow
def always_succeeds_flow():
    always_fails_task().submit()
    return "foo"

if __name__ == "__main__":
    always_succeeds_flow()

Running this flow produces the following result.

21:02:45.715 | INFO    | prefect.engine - Created flow run 'sparkling-pony' for flow 'always-succeeds-flow'
21:02:45.816 | INFO    | Flow run 'sparkling-pony' - Created task run 'always_fails_task-58ea43a6-0' for task 'always_fails_task'
21:02:45.853 | ERROR   | Task run 'always_fails_task-58ea43a6-0' - Encountered exception during execution:
Traceback (most recent call last):...
ValueError: I am bad task
21:02:45.879 | ERROR   | Task run 'always_fails_task-58ea43a6-0' - Finished in state Failed('Task run encountered an exception.')
21:02:46.593 | INFO    | Flow run 'sparkling-pony' - Finished in state Completed()
Completed(message=None, type=COMPLETED, result='foo', flow_run_id=7240e6f5-f0a8-4e00-9440-a7b33fb51153)

Get started

Develop

Deploy

Automate

Cloud and Server

Resources

Write and run flows

Running flows

Example

Specifying parameters

Composing flows

Nesting flows

Supported functions

Synchronous functions

Asynchronous functions

Class methods

Generators

Parameters

Flow runs

Writing flows

Subflows

Flow settings

Final state determination

Raise an exception

Return `none`

Return a future

Return multiple states or futures

Return a manual state

Return an object

See also

Get started

Develop

Deploy

Automate

Cloud and Server

Resources

​Running flows

​Example

​Specifying parameters

​Composing flows

​Nesting flows

​Supported functions

​Synchronous functions

​Asynchronous functions

​Class methods

​Generators

​Parameters

​Flow runs

​Writing flows

​Subflows

​Flow settings

​Final state determination

​Raise an exception

​Return none

​Return a future

​Return multiple states or futures

​Return a manual state

​Return an object

​See also

Running flows

Example

Specifying parameters

Composing flows

Nesting flows

Supported functions

Synchronous functions

Asynchronous functions

Class methods

Generators

Parameters

Flow runs

Writing flows

Subflows

Flow settings

Final state determination

Raise an exception

Return `none`

Return a future

Return multiple states or futures

Return a manual state

Return an object

See also