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Thu Dec 28 04:51:52 EST 2017 · https://mail.python.org/pipermail/python-dev/2017-December/151471.html

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?

2. ``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 :-)

Victor
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[Python-Dev] PEP 567 v2