[Python-Dev] Timeout for PEP 550 / Execution Context discussion

[Nick Coghlan]

On 17 October 2017 at 15:02, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 17 October 2017 at 14:31, Guido van Rossum <guido at python.org> wrote:
>
>> No, that version just defers to magic in ContextVar.get/set, whereas what
>> I'd like to see is that the latter are just implemented in terms of
>> manipulating the mapping directly. The only operations for which speed
>> matters would be __getitem__ and __setitem__; most other methods just defer
>> to those. __delitem__ must also be a primitive, as must __iter__ and
>> __len__ -- but those don't need to be as speedy (however __delitem__ must
>> really work!).
>>
>
> To have the mapping API at the base of the design, we'd want to go back to
> using the ContextKey version of the API as the core primitive (to ensure we
> don't get name conflicts between different modules and packages), and then
> have ContextVar be a convenience wrapper that always accesses the currently
> active context:
>
>     class ContextKey:
>         ...
>     class ExecutionContext:
>         ...
>
>     class ContextVar:
>         def __init__(self, name):
>             self._key = ContextKey(name)
>
>         def get(self):
>             return get_execution_context()[self._key]
>
>         def set(self, value):
>             get_execution_context()[self._key] = value
>
>         def delete(self, value):
>             del get_execution_context()[self._key]
>

Tangent: if we do go this way, it actually maps pretty nicely to the idea
of a "threading.ThreadVar" API that wraps threading.local():

    class ThreadVar:
        def __init__(self, name):
            self._name = name
            self._storage = threading.local()

        def get(self):
            return self._storage.value

        def set(self, value):
            self._storage.value = value

        def delete(self):
            del self._storage.value

(Note: real implementations of either idea would need to pay more attention
to producing clear exception messages and instance representations)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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