how to share a variable across modules for all tests in py.test

Question

I have multiple tests run by py.test that are located in multiple classes in multiple files.

What is the simplest way to share a large dictionary - which I do not

Answer 1

I'm suprised no answer mentioned caching yet: since version 2.8, pytest has a powerful cache mechanism.

Usage example

@pytest.fixture(autouse=True)
def init_cache(request):
    data = request.config.cache.get('my_data', None)
    data = {'spam': 'eggs'}
    request.config.cache.set('my_data', data)

Access the data dict in tests via builtin request fixture:

def test_spam(request):
    data = request.config.cache.get('my_data')
    assert data['spam'] == 'eggs'

Sharing the data between test runs

The cool thing about request.cache is that it is persisted on disk, so it can be even shared between test runs. This comes handy when you running tests distributed (pytest-xdist) or have some long-running data generation which does not change once generated:

@pytest.fixture(autouse=True)
def generate_data(request):
    data = request.config.cache.get('my_data', None)
    if data is None:
        data = long_running_generation_function()
        request.config.cache.set('my_data', data)

Now the tests won't need to recalculate the value on different test runs unless you clear the cache on disk explicitly. Take a look what's currently in the cache:

$ pytest --cache-show
...
my_data contains:
  {'spam': 'eggs'}

Rerun the tests with the --cache-clear flag to delete the cache and force the data to be recalculated. Or just remove the .pytest_cache directory in the project root dir.

Where to go from here

The related section in pytest docs: Cache: working with cross-testrun state.