# Local Debugging

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Whether you're running Prefect locally with Flow.run() or experimenting with your ideas before putting them into production with Prefect Cloud, Prefect provides you with a wealth of tools for debugging your flows and diagnosing issues.

# Use a FlowRunner for stateless execution

If your problem is related to retries, or if you want to run your flow off-schedule, you might first consider rerunning your flow with run_on_schedule=False. This can be accomplished via environment variable (PREFECT__FLOWS__RUN_ON_SCHEDULE=false) or keyword (flow.run(run_on_schedule=False)). If you instead want to implement a single Flow run yourself consider using a FlowRunner directly:

from prefect.engine.flow_runner import FlowRunner

# ... your flow construction

runner = FlowRunner(flow=my_flow)
flow_state = runner.run(return_tasks=my_flow.tasks)

This will execute your flow immediately, regardless of its schedule. Note that by default a FlowRunner does not provide any individual task states, so if you want that information you must request it with the return_tasks keyword argument.

# Choice of Executor

# LocalExecutor

Your choice of executor is very important for how easily it will be to debug your flow. The DaskExecutor relies on multiprocessing and multithreading, which can be tricky to navigate. Things like shared state and print statements that disappear into the void can be a real nuisance when trying to figure out what's wrong. Luckily, Prefect provides a LocalExecutor that is the most stripped down executor possible - all tasks are executed immediately (in the appropriate order of course) and in your main process.

To swap out executors, follow the schematic:

from prefect.executors import LocalExecutor

# ... your flow construction

state = flow.run(executor=LocalExecutor()) # <-- the executor needs to be initialized

and you're set! The executor keyword can also be provided to the FlowRunner.run method.

# LocalDaskExecutor

If you want to upgrade to a more powerful executor but still maintain an easily debuggable environment, we recommend the LocalDaskExecutor. This executor does defer computation using dask, but avoids any parallelism, making for an execution pipeline which is easier to reason about. You can turn parallelism on by providing either scheduler="threads" or scheduler="processes" when initializing this executor.

Prefect defaults

You can set the LocalDaskExecutor to be the default executor on your local machine. To change your Prefect settings (including the default executor), you can either:

  • modify your ~/.prefect/config.toml file
  • update your OS environment variables; every value in the config file can be overridden by setting PREFECT__SECTION__SUBSECTION__KEY. For example, to change the default executor, you can set PREFECT__ENGINE__EXECUTOR__DEFAULT_CLASS="prefect.executors.LocalExecutor"

# DaskExecutor

Lastly, if your issue is actually related to parallelism, you'll need to use the DaskExecutor. By default prefect silences the Dask logs, to reenable them, pass debug=True.

# Raising Exceptions in realtime

Sometimes, you don't want Prefect's robust error-handling mechanisms to trap exceptions -- you'd rather they were raised so you could immediately debug them yourself! Use the raise_on_exception context manager to raise errors the moment they happen:

from prefect import Flow, task
from prefect.utilities.debug import raise_on_exception

def div(x):
    return 1 / x

with Flow("My Flow") as f:
    val = div(0)

with raise_on_exception():


ZeroDivisionError                         Traceback (most recent call last)

<ipython-input-1-82c40dd24406> in <module>()
     12 with raise_on_exception():
---> 13     f.run()

... # the full traceback is long

<ipython-input-1-82c40dd24406> in div(x)
      5 @task
      6 def div(x):
----> 7     return 1 / x
      9 with Flow("My Flow") as f:

ZeroDivisionError: division by zero

You can now use your favorite debugger to drop into the traceback and proceed as you normally would.


Note that this utility doesn't require you know anything about where the error occurred.

# Re-raising Execeptions post-hoc

Suppose you want to let the full pipeline run and don't want to raise the trapped error at runtime. Assuming your error was trapped and placed in a Failed state, the full exception is stored in the result attribute of the task state. Knowing this, you can re-raise it locally and debug from there!

To demonstrate:

from prefect import Flow, task

def gotcha():
    tup = ('a', ['b'])
        tup[1] += ['c']
    except TypeError:
        assert len(tup[1]) == 1

flow = Flow(name="tuples", tasks=[gotcha])

state = flow.run()
state.result # {<Task: gotcha>: Failed("Unexpected error: AssertionError()")}

failed_state = state.result[gotcha]
raise failed_state.result


TypeError                                 Traceback (most recent call last)

<ipython-input-50-8efcdf8dacda> in gotcha()
      7     try:
----> 8         tup[1] += ['c']
      9     except TypeError:

TypeError: 'tuple' object does not support item assignment

During handling of the above exception, another exception occurred:

AssertionError                            Traceback (most recent call last)
<ipython-input-1-f0f986d2f159> in <module>
     23 failed_state = state.result[gotcha]
---> 24 raise failed_state.result

~/Developer/prefect/src/prefect/engine/runner.py in inner(self, state, *args, **kwargs)
     59         try:
---> 60             new_state = method(self, state, *args, **kwargs)
     61         except ENDRUN as exc:
     62             raise_end_run = True

~/Developer/prefect/src/prefect/engine/task_runner.py in get_task_run_state(self, state, inputs, timeout_handler)
    697             self.logger.info("Running task...")
    698             timeout_handler = timeout_handler or main_thread_timeout
--> 699             result = timeout_handler(self.task.run, timeout=self.task.timeout, **inputs)
    701         # inform user of timeout

~/Developer/prefect/src/prefect/utilities/executors.py in multiprocessing_timeout(fn, timeout, *args, **kwargs)
     69     if timeout is None:
---> 70         return fn(*args, **kwargs)
     71     else:
     72         timeout_length = timeout.total_seconds()

<ipython-input-1-f0f986d2f159> in gotcha()
      9         tup[1] += ['c']
     10     except TypeError:
---> 11         assert len(tup[1]) == 1


%debug # using the IPython magic method to start a pdb session
ipdb> tup
('a', ['b', 'c'])
ipdb> exit

Ah ha! That's what we get for placing mutable objects inside a tuple!

# Fixing your broken Flows

So far we've touched on how to identify your bugs, but how do you fix your broken flow? One way is to re-run the logic that creates the flow from scratch. In production, having a single source of truth for how to create your flow is probably a great idea. However, when iterating quickly or trying to build a proof-of-concept, this can be frustrating.

There are two flow methods that come in handy when trying to update your flow:

  • flow.get_tasks for retrieving the tasks that satisfy certain conditions
  • flow.replace once you're ready to replace a task with an updated / fixed version

Using our silly flow from above, let's define a new task and swap it out using these methods:

def fixed():
    tup = ('a', ('b',))
        tup[1] += ('c',)
    except TypeError:
        assert len(tup[1]) == 1

broken = flow.get_tasks(name="gotcha")[0]
flow.replace(broken, fixed)

flow.run() # Success("All reference tasks succeeded.")

If we have a complex dependency graph, flow.replace can be a real timesaver for quickly swapping out tasks while preserving all their dependencies!

Call Signatures

Note that flow.replace preserves edges - this means the old and new tasks need to have the exact same call signature.

# Resuming failing Flows

Sometimes flows fail, and so it can be useful to interactively debug them from the command line. The Prefect API provides a number of different ways of achieving this:

# Manually mocking out long-running successful tasks

The following Flow fails at run-time due to a ZeroDivisionError in the failure task

from time import sleep

import prefect
from prefect import Flow, task
from prefect.engine import FlowRunner

def i_will_take_forever() -> int:
    return 42

def failure(a_number: int):
    return a_number / 0

with Flow("Success/Failure") as flow:

    a_great_number = i_will_take_forever()

Running this flow in an iPython command shell fails


Swapping out 1/0 with 1/1 in the task failure() using flow.replace would fix this error, however, rerunning the whole flow including the slow i_will_take_forever is unncessary.

Every time a Prefect flow is run, the state of the flow after it is run is returned. The flow run state result is a dictionary whose keys are Task objects and whose values are the states of those tasks after the run is complete.

from prefect.engine.state import Success

long_task = flow.get_tasks(name="i_will_take_forever")[0]
task_states =  {long_task : Success("Mocked success", result=42)}


We can access our long-running task using the convenient get_tasks method, and then set it's state to Success via a task_states dictionary which can be passed as an argument to flow.run. As a result the flow skips the slow task and resumes from failure.

# Locally check your Flow's Docker storage

Another reason a flow might unexpectedly break in production (or fail to run at all) is if its storage is broken (e.g., if you forget a Python dependency in defining your Docker storage for the flow). Luckily, checking your flow's storage locally is easy! Let's walk through an example:

from prefect import task, Flow
from prefect.storage import Docker

# import a non-prefect package used for scraping reddit
import praw

def whoami():
    reddit = praw.Reddit(client_id='SI8pN3DSbt0zor',
                         user_agent='testscript by /u/fakebot3',
    return reddit.user.me()

storage = Docker(base_image="python:3.7", registry_url="http://my.personal.registry")
flow = Flow("reddit-flow", storage=storage, tasks=[whoami])

If you were to deploy this Flow to Cloud, you wouldn't hear from it again. Why? Let's find out - first, build the Docker storage locally without pushing to a registry:

# note that this will require either a valid registry_url, or no registry_url
# push=False is important here; otherwise your local image will be deleted
built_storage = flow.storage.build(push=False)

Note the Image ID contained in the second to last line of Docker output:

Successfully built 0f3b0851148b # your ID will be different

Now we have a Docker container on our local machine which contains our flow. To access it, we first identify where the flow is stored:

# {"reddit-flow": "/root/.prefect/reddit-flow.prefect"}

and then connect to the container via:

# connect to an interactive python session running in the container
docker run -it 0f3b0851148b python

Finally, we can use cloudpickle to deserialize the file into a Flow object:

import cloudpickle

with open("/root/.prefect/reddit-flow.prefect", "rb") as f:
    flow = cloudpickle.load(f)

Which will result in a very explicit traceback!

Traceback (most recent call last):
    flow = cloudpickle.loads(decrypted_pickle)
  File "/usr/local/lib/python3.7/site-packages/cloudpickle/cloudpickle.py", line 944, in subimport
ModuleNotFoundError: No module named 'praw'

In this particular case, we forgot to include praw in our python_dependencies for the Docker storage; in general, this is one way to ensure your Flow makes it through the deployment process uncorrupted.

The More You Know

We actually found this process to be so useful, we've automated it for you! Prefect now performs a "health check" prior to pushing your Docker image, which essentially runs the above code and ensures your Flow is deserializable inside its container. However, the mechanics by which this occurs is still useful to know.