[Python-Dev] PEP 567 v2

[Victor Stinner]

Le 2 janv. 2018 18:57, "Guido van Rossum" <guido at python.org> a écrit :

Oh, the "Specification" section in the PEP is too brief on several of these
subjects. It doesn't really specify what var.get() does if the value is not
set, nor does it even mention var.get(<default>) except in the code
examples for var.reset(). It's also subtle that ctx[var] returns the
default (if there is one). I suppose it will raise if there isn't one --
resulting in the somewhat surprising behavior where `var in ctx` may be
true but `ctx[var]` may raise. And what does it raise? (All these questions
are answered by the code, but they should be clearly stated in the PEP.)


A variable has or has no default value. Would it make sense to expose the
default value as a public read-only attribute (it would be equal to
_NO_DEFAULT or Token.MISSING if there is no default) and/or add a is_set()
method? is_set() returns true if the variable has a default value or if it
was set in the "current context".

Currently, a custom sentinel is needed to check if var.get(), ctx.get(var)
and ctx[var] would raise an exception or not. Example:

my_sentinel = object()
is_set = (var.get(default=my_sentinel) is not my_sentinel)
# no exception if is_set is true

ContextVar.get() is non obvious because the variable has an optinal
default, get() has an optional default parameter, and the variable can be
set or not in the current context.

Victor
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