[Python-Dev] PEP 567 v2 (original) (raw)

Victor Stinner victor.stinner at gmail.com
Fri Jan 5 05:05:18 EST 2018

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.

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