Python `contextvars` don't persist across cells in notebook

Python 3.7 added a new contextvars package to “manage, store, and access context-local state”.

However, if I set a ContextVar in one cell and then try to get it in another, it isn’t persisted:

This may be an antipattern (not sure if there is a good use case to set a variable across many cells) and I worked around it by setting default, as recommended. However it is surprising and is different than how ipython works.

[mdickinson]

This produces surprising results when changing the decimal precision (or other aspects of the decimal context) in notebooks, since decimal now uses a PEP 567 context variable for the decimal context.

Related: https://github.com/ipython/ipykernel/issues/494, https://github.com/jupyter/notebook/issues/5260

[njsmith]

I believe tornado on py3 uses the asyncio loop by default, and if you’re on py 3.7 then asyncio will automatically use different contexts for different tasks. And probably each cell execution counts as a different task as far as tornado/aio is concerned.

Sounds like the kernel should be explicitly holding a Context object and reusing it for each cell execution, instead of letting tornado/aio set the Context implicitly.