For https://docs.python.org/3.7/library/contextvars.html#contextvars.contextvars... and newer, an "is" should be "if". Token.old_value says "It points to Token.MISSING is the variable..." instead of "It points to Token.MISSING if the variable..." (Per https://docs.python.org/3/bugs.html#documentation-bugs, I am "short on time") Cheers.