I cannot make pytest-bdd work with pytest-asyncio

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05-10-2024

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Bridging the Gap: pytest-bdd and pytest-asyncio

Problem: Trying to use pytest-bdd features (like scenario outlines and data tables) in tests that require asynchronous operations using pytest-asyncio results in unexpected errors.

Simplified: You want to write BDD tests that interact with asynchronous code, but it doesn't work as smoothly as expected.

Scenario: You've written a test using pytest-bdd's @scenario decorator and are trying to integrate it with asynchronous code using pytest-asyncio's @pytest.mark.asyncio decorator. However, you encounter errors related to either the scenario steps not being executed correctly or the asynchronous operations not working as intended.

Original Code (Example):

import pytest
from pytest_bdd import scenarios, scenario, given, when, then

@pytest.mark.asyncio
@scenarios('features/my_feature.feature')
def test_my_feature(request):
    scenario = request.param
    request.param.name = scenario.name
    request.param.steps = scenario.steps
    request.param.steps[0].run()
    # ... rest of the steps

Analysis:

The issue arises from the conflicting nature of the two libraries. pytest-bdd expects the steps to be executed synchronously, while pytest-asyncio aims to manage asynchronous operations.

Here are some key reasons why you might be encountering problems:

1. Step Execution Order: pytest-bdd expects steps to be executed in order, whereas pytest-asyncio's asynchronous nature might lead to unexpected step execution order.
2. Step Function Signature: pytest-bdd might not recognize the signature of a coroutine function, which is essential for pytest-asyncio.
3. Fixture Scopes: Asynchronous fixtures need to be defined with specific scopes (session, module, class, function) to work correctly with pytest-asyncio and might clash with the default scopes of fixtures in pytest-bdd.

Solutions:

1. Embrace the @pytest.mark.asyncio decorator: Use @pytest.mark.asyncio on the step functions instead of the scenario itself to ensure that each step is executed asynchronously.

@given("...")
@pytest.mark.asyncio
async def given_step(context):
    # Asynchronous code

2. Utilize Asynchronous Fixtures: Define fixtures with the @pytest.mark.asyncio decorator to create resources that are compatible with asynchronous code.

@pytest.fixture
@pytest.mark.asyncio
async def async_resource():
    # Asynchronous resource setup
    return resource

3. Explicitly Run Async Steps: You can use asyncio.run to explicitly run asynchronous steps within the context of the scenario.

@scenario(...)
def test_my_feature(request):
    scenario = request.param
    request.param.name = scenario.name
    request.param.steps = scenario.steps
    asyncio.run(request.param.steps[0].run())
    # ... rest of the steps

Example:

import pytest
from pytest_bdd import scenarios, scenario, given, when, then
import asyncio

@pytest.mark.asyncio
@scenarios('features/my_feature.feature')
def test_my_feature(request):
    scenario = request.param
    request.param.name = scenario.name
    request.param.steps = scenario.steps

    async def run_step(step):
        await step.run()

    for step in request.param.steps:
        asyncio.run(run_step(step))

Additional Considerations:

• Scope: Carefully consider the scope of your fixtures to ensure they are available and managed correctly within the context of your scenario.
• Context: Ensure your BDD steps properly interact with the asynchronous context to avoid errors.
• Debugging: Use asyncio.debug to gain insights into the execution of your asynchronous code.

Conclusion:

Bridging the gap between pytest-bdd and pytest-asyncio requires a thoughtful approach. By understanding the challenges and implementing appropriate solutions, you can successfully utilize the power of BDD for testing your asynchronous applications.

References:

• pytest-bdd documentation
• pytest-asyncio documentation
• asyncio documentation