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By default, threading.local raises an AttributeError (unless you subclass it.) Similar to that and to NameErrors, I think it's a good idea for ContextVars to raise a LookupError if a variable was not explicitly set.

Yury

On Tue, Jan 9, 2018 at 7:15 PM Victor Stinner <victor.stinner@gmail.com> wrote:

2018-01-09 12:41 GMT+01:00 Yury Selivanov <yselivanov.ml@gmail.com>:
\> But I'd be -1 on making all ContextVars have a None default
\> (effectively have a "ContextVar.get(default=None)" signature. This
\> would be a very loose semantics in my opinion.

Why do you think that it's a loose semantics? For me
ContextVar/Context are similar to Python namespaces and thread local
storage.

To "declare" a variable in a Python namespace, you have to set it:
"global x" doesn't create a variable, only "x = None".

It's not possible to define a thread local variable without specifying
a "default" value neither.

Victor