[Python-ideas] PEP draft: context variables

[Pavol Lisy]

On 9/4/17, Koos Zevenhoven <k7hoven at gmail.com> wrote:

> Core concept
> ''''''''''''
>
> A context-local variable is represented by a single instance of
> ``contextvars.Var``, say ``cvar``. Any code that has access to the ``cvar``
> object can ask for its value with respect to the current context. In the
> high-level API, this value is given by the ``cvar.value`` property::
>
>     cvar = contextvars.Var(default="the default value",
>                            description="example context variable")
>
>     assert cvar.value == "the default value"  # default still applies
>
>     # In code examples, all ``assert`` statements should
>     # succeed according to the proposed semantics.
>
>
> No assignments to ``cvar`` have been applied for this context, so
> ``cvar.value`` gives the default value. Assigning new values to contextvars
> is done in a highly scope-aware manner::
>
>     with cvar.assign(new_value):
>         assert cvar.value is new_value
>         # Any code here, or down the call chain from here, sees:
>         #     cvar.value is new_value
>         # unless another value has been assigned in a
>         # nested context
>         assert cvar.value is new_value
>     # the assignment of ``cvar`` to ``new_value`` is no longer visible
>     assert cvar.value == "the default value"

I feel that of "is" and "==" in assert statements in this PEP has to
be used (or described) more precisely.

What if new_value above is 123456789?

maybe using something like could be better? ->

   def equals(a, b):
      return a is b or a == b

Doesn't PEP need to think about something like "context level overflow" ?

Or members like: cvar.level ?