@twhughes @keewis In my case, I encounter this when I: - Have xarray + dask installed in my environment - Run any pytest which imports xarray (which in turn imports dask) - Uninstall dask, but pytest artefacts remain:

sh ❯ ls venv/lib/python3.9/site-packages/dask/__pycache__ utils_test.cpython-39-pytest-6.2.5.pyc

Here are the steps which would hopefully reproduce the behaviour:

```py

Create this small unit test named:

test_import_xarray.py

def test_xr(): import xarray ```

```sh python -m venv venv source venv/bin/activate pip install xarray pytest dask pytest test_import_xarray.py

pip uninstall dask ls venv/lib/python3.9/site-packages/dask/pycache # See output above python -c 'import dask' # No ImportError! python -c 'import xarray' ```

Exact versions used Python 3.9.9 along with: ```sh

requirements.txt

attrs==21.2.0 cloudpickle==2.0.0 dask==2021.11.2 fsspec==2021.11.1 iniconfig==1.1.1 locket==0.2.1 numpy==1.21.4 packaging==21.3 pandas==1.3.4 partd==1.2.0 pluggy==1.0.0 py==1.11.0 pyparsing==3.0.6 pytest==6.2.5 python-dateutil==2.8.2 pytz==2021.3 PyYAML==6.0 six==1.16.0 toml==0.10.2 toolz==0.11.2 xarray==0.20.1 ```