"Can prefect execute a flow in a single thread? See comments for more details and context, I’m trying to get Prefect to run Playwright, but it is single-threaded only." Hi Can Playwright be combined with Prefect?

[tool.poetry.dependencies]
python = "^3.10"
prefect = "^2.6.1"
playwright = "^1.27.1"

import prefect
from playwright.sync_api import Page, sync_playwright

@prefect.task
def go_to_a_web(page: Page):
    page.goto("<https://formulae.brew.sh/cask/slack>")


@prefect.flow
def main_flow():
    with sync_playwright() as pw:
        browser = pw.chromium.launch(headless=False)
        page = browser.new_page()
        go_to_a_web(page)


main_flow()

I got

greenlet.error: cannot switch to a different thread

any help would be much appreciated

Can you post the full error traceback? I don’t see anything inherently wrong with your code. Have you tried running the code without the

@prefect.flow

and

@prefect.task

decorators? Do you receive the same error?

Thank You for replying. No error without decorators. Full traceback is too long to post

16:27:52.985 | ERROR   | Flow run 'macho-porcupine' - Finished in state Failed('Flow run encountered an exception. greenlet.error: cannot switch to a different thread\n')
Traceback (most recent call last):
  File "/Users/me/code/python/test-greenlet/main.py", line 16, in <module>
    main_flow()
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/flows.py", line 429, in __call__
    return enter_flow_run_engine_from_flow_call(
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 150, in enter_flow_run_engine_from_flow_call
    return anyio.run(begin_run)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/_core/_eventloop.py", line 70, in run
    return asynclib.run(func, *args, **backend_options)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 292, in run
    return native_run(wrapper(), debug=debug)
  File "/Users/me/.pyenv/versions/3.10.1/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/Users/me/.pyenv/versions/3.10.1/lib/python3.10/asyncio/base_events.py", line 641, in run_until_complete
    return future.result()
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 287, in wrapper
    return await func(*args)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/client/utilities.py", line 47, in with_injected_client
    return await fn(*args, **kwargs)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 229, in create_then_begin_flow_run
    return await state.result(fetch=True)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/states.py", line 74, in _get_state_result
    raise await get_state_exception(state)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 578, in orchestrate_flow_run
    result = await run_sync(flow_call)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/utilities/asyncutils.py", line 68, in run_sync_in_worker_thread
    return await anyio.to_thread.run_sync(call, cancellable=True)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/to_thread.py", line 31, in run_sync
    return await get_asynclib().run_sync_in_worker_thread(
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 937, in run_sync_in_worker_thread
    return await future
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 867, in run
    result = context.run(func, *args)
  File "/Users/me/code/python/test-greenlet/main.py", line 14, in main_flow
    go_to_a_web(page)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/tasks.py", line 330, in __call__
    return enter_task_run_engine(
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 701, in enter_task_run_engine
    return run_async_from_worker_thread(begin_run)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/utilities/asyncutils.py", line 148, in run_async_from_worker_thread
    return anyio.from_thread.run(call)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/from_thread.py", line 49, in run
    return asynclib.run_async_from_thread(func, *args)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 970, in run_async_from_thread
    return f.result()
  File "/Users/me/.pyenv/versions/3.10.1/lib/python3.10/concurrent/futures/_base.py", line 445, in result
    return self.__get_result()
  File "/Users/me/.pyenv/versions/3.10.1/lib/python3.10/concurrent/futures/_base.py", line 390, in __get_result
    raise self._exception
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 842, in get_task_call_return_value
    return await future._result()
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/futures.py", line 237, in _result
    return await final_state.result(raise_on_failure=raise_on_failure, fetch=True)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/states.py", line 74, in _get_state_result
    raise await get_state_exception(state)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/engine.py", line 1190, in orchestrate_task_run
    result = await run_sync(task.fn, *args, **kwargs)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/prefect/utilities/asyncutils.py", line 68, in run_sync_in_worker_thread
    return await anyio.to_thread.run_sync(call, cancellable=True)
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/to_thread.py", line 31, in run_sync
    return await get_asynclib().run_sync_in_worker_thread(
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 937, in run_sync_in_worker_thread
    return await future
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 867, in run
    result = context.run(func, *args)
  File "/Users/me/code/python/test-greenlet/main.py", line 7, in go_to_a_web
    page.goto("<https://formulae.brew.sh/cask/slack>")
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/playwright/sync_api/_generated.py", line 8114, in goto
    self._sync(
  File "/Users/me/Library/Caches/pypoetry/virtualenvs/test-greenlet-R45Id2Qo-py3.10/lib/python3.10/site-packages/playwright/_impl/_sync_base.py", line 102, in _sync
    self._dispatcher_fiber.switch()
greenlet.error: cannot switch to a different thread

Definitely some good information. I wish I was of more assistance. I’m learning Prefect as I go and I have always found digging into the community helps me understand it better and faster. I’m not sure of a way to get Prefect to not execute in a thread. Maybe ask something more along the lines of “Can prefect execute a flow in a single thread? See comments for more details and context, I’m trying to get Prefect to run Playwright, but it is single-threaded only.” … then in the thread you can link back to this post.

Thank you so much. I just learned how to ask a good question.

If you write an async task e.g.

async def foo

it’ll run in the main thread.