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

Victor Stinner victor.stinner at gmail.com
Thu Dec 28 04:51:52 EST 2017

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Hi,

I like the new version of the PEP using "read only mapping" and copy_context(). It's easier to understand.

I'm ok with seeing a context as a mapping, but I am confused about a context variable considered as a mapping item. I still see a context variable as a variable, so something which has a value or not. I just propose to rename the default parameter of the ContextVar constructor.

Le 28 déc. 2017 7:10 AM, "Yury Selivanov" <yselivanov.ml at gmail.com> a écrit :

ContextVar

The ContextVar class has the following constructor signature: ContextVar(name, *, default=_NO_DEFAULT). The name parameter is used only for introspection and debug purposes, and is exposed as a read-only ContextVar.name attribute. The default parameter is optional. Example::

# Declare a context variable 'var' with the default value 42.
var = ContextVar('var', default=42)

In term of API, "default" parameter name is strange. Why not simply calling it "value"?

var = ContextVar('var', default=42)

and:

var = ContextVar('var') var.set (42)

behaves the same, no?

The implementation explains where the "default" name comes from, but IMHO "value" is a better name.

(The _NO_DEFAULT is an internal sentinel object used to detect if the default value was provided.)

I would call it _NOT_SET.

• a read-only attribute Token.old_value set to the value the variable had before the set() call, or to Token.MISSING if the variable wasn't set before.

Hum, I also suggest to rename Token.MISSING to Token.NOT_SET. It would be more conistent with the last sentence.

C API

Would it be possible to make this API private?

1. int PyContextVar_Get(PyContextVar *, PyObject *default_value, PyObject **value): (...) value is always a borrowed reference.

I'm not sure that it's a good idea to add a new public C function which returns a borrowed reference. I would prefer to only use (regular) strong references in the public API.

I don't want to elaborate here. You may see:

http://vstinner.readthedocs.io/python_new_stable_api.html

Internally, I don't care, do whatever you want for best performances :-)

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