On 17 October 2017 at 15:02, Nick Coghlan <ncogh...@gmail.com> wrote:
> On 17 October 2017 at 14:31, Guido van Rossum <gu...@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 | ncogh...@gmail.com | Brisbane, Australia
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