[Python-Dev] PEP 567 v2

[Victor Stinner]

Currently, Context.get(var) returns None when "var in context" is false.
That's surprising and different than var.get(), especially when var has a
default value.

Code:
---
import contextvars

name = contextvars.ContextVar('name', default='victor')
context = contextvars.copy_context()
print(name in context)
print(context.get(name))
print(name.get())
---

Output:
---
False
None
victor
---

Context.get() must raise a lookup error by default if var is not in
context. It should return the default argument if it's set, it's just that
the default parameter must not have a default value (None).

I'l fine that Context.get(default=None) and var.get() behaves differently
(return None vs victor in my example) when var isn't set and var has a
default value.

Victor
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