async pytest-bdd step definitions

[mathew-jithin]

Just shooting this question with reference to pytest-dev/pytest-bdd#223

Is there any way the same can be handled with pytest-asyncio? Hope someone will please help if you have worked on something similar.

[seifertm]

My understanding is that there are two questions in the room:

1. Does pytest-asyncio work with pytest-bdd when using async test steps?
2. Can pytest-asyncio be used to provide support for async step definitions in pytest-bdd?

pytest-asyncio basically works like this: If an asynchronous test case is marked with the keyword asyncio, the plugin surrounds that test case with a synchronous wrapper. The wrapper executes the async function in an event loop. When you use the @when decorator, the decorated function will be synchronous:
https://github.com/pytest-dev/pytest-bdd/blob/02e667f239618e24dab6df787be43b670bcc9df8/pytest_bdd/steps.py#L105

Therefore, the decorated function cannot be executed in an event loop. In other words: No, I don't think pytest-bdd and pytest-asyncio can work together without modifications.

Here is the code that wraps the async function:

pytest-asyncio/pytest_asyncio/plugin.py

Lines 166 to 184 in 1bb7f30

	def wrap_in_sync(func, _loop):
	"""Return a sync wrapper around an async function executing it in the
	current event loop."""
	@functools.wraps(func)
	def inner(**kwargs):
	coro = func(**kwargs)
	if coro is not None:
	task = asyncio.ensure_future(coro, loop=_loop)
	try:
	_loop.run_until_complete(task)
	except BaseException:
	# run_until_complete doesn't get the result from exceptions
	# that are not subclasses of `Exception`. Consume all
	# exceptions to prevent asyncio's warning from logging.
	if task.done() and not task.cancelled():
	task.exception()
	raise
	return inner

But it should be possible to make both plugins work together. For example, there is already a custom integration for Hypothesis:

pytest-asyncio/pytest_asyncio/plugin.py

Lines 153 to 157 in 1bb7f30

	if getattr(pyfuncitem.obj, 'is_hypothesis_test', False):
	pyfuncitem.obj.hypothesis.inner_test = wrap_in_sync(
	pyfuncitem.obj.hypothesis.inner_test,
	_loop=pyfuncitem.funcargs['event_loop']
	)

This solution relies on markers provided by Hypothesis. We could create a similar mechanism with the following steps:

1. pytest-bdd needs to provide metadata for BDD steps decorated with when, given, …, so that pytest-asyncio can recognize them as such
2. pytest-asyncio recognizes the fact that a @when decorator is present and wraps the underlying async function, instead of the decorator.

I personally don't like the approach, though, as we would implement pytest-bdd support as another special case. It would be much nicer to find a more general way for pytest-asyncio to play with other plugins. But I'm also not in the position to decide :)

[mathew-jithin]

Thanks @seifertm. Really appreciate the response.

Saw this PR which still remains in review in pytest-bdd. Maybe if a similar handling is done in their side the issue can be addressed better.

[seifertm]

At the time of writing pytest-asyncio is compatible with flaky and hypothesis. I think that compatibility with pytest-bdd is a reasonable feature request.
@Tinche Does it make sense to label this issue accordingly? I think labels would help keeping track of triaged issues. I'm thinking of the labels feature and interop, but I'm open to other solutions.

[Tinche]

@seifertm Added the 'feature' label!

[KelvinSan]

Has this been done or is it still open ?

[seifertm]

@KelvinSan There has been no progress on this from the pytest-asyncio side.

In the meantime, the pytest-bdd issue linked by the OP received some attention. One comment presents a "comaptibility decorator" for using pytest-bdd steps with pytest-asyncio.