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

Guido van Rossum guido at python.org
Thu Jan 4 19:52:09 EST 2018

On Thu, Jan 4, 2018 at 4:56 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

Guido van Rossum wrote:

It was getcontext() in an earlier version of PEP 567. We changed it to copycontext() believing that that would clarify that you get a clone that is unaffected by subsequent ContextVar.set() operations (which affect the current context rather than the copy you just got).

In that case it seems clear to me that "the context" is conceptually a mutable mapping. The fact that it happens to be built out of immutable components is an implementation detail that user-level docs should not be talking about.

Well, it's not immutable (it shouldn't support hash()), but it doesn't follow the MutableMapping protocol -- it only follows the Mapping protocol. Note that the latter carefully doesn't call itself ImmutableMapping. Context is a mutable object that implements the Mapping protocol. The only way to mutate a Context is to use var.set() when that Context is the current context. (Modulo the caching bug discussed in the subthread with Nathaniel.)

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