[Python-ideas] PEP 550 v2

[Nick Coghlan]

On 16 August 2017 at 18:37, Nathaniel Smith <njs at pobox.com> wrote:
> On Tue, Aug 15, 2017 at 11:53 PM, Jelle Zijlstra
> <jelle.zijlstra at gmail.com> wrote:
>> Minor suggestion: Could we allow something like
>> `sys.set_new_context_item(description='mylib.context',
>> initial_value='spam')`? That would make it easier for type checkers to infer
>> the type of a ContextItem, and it would save a line of code in the common
>> case.
>
> This is a really handy feature in general, actually! In fact all of
> asyncio's thread-locals define initial values (using a trick involving
> subclassing threading.local), and I recently added this feature to
> trio.TaskLocal as well just because it's so convenient.
>
> However, something that you realize almost immediately when trying to
> use this is that in many cases, what you actually want is an initial
> value *factory*. Like, if you write new_context_item(initial_value=[])
> then you're going to have a bad time. So, should we support something
> like new_context_item(initializer=lambda: [])?
>
> The semantics are a little bit subtle. I guess it would be something
> like: if ci.get() goes to find the value and fails at all levels, then
> we call the factory function and assign its return value to the
> *deepest* LC, EC[0]. The idea being that we're pretending that the
> value was there all along in the outermost scope, you just didn't
> notice before now.

I actually wondered about this in the context of the PEP saying that
"context items are set to None by default", as it isn't clear what
that means for the behaviour of sys.new_execution_context().

The PEP states that the latter API creates an "empty" execution
context, but the notion of a fresh EC being truly empty conflicts with
the notion of all defined config items having a default value of None.

I think your idea resolves that nicely: if context_item.get() failed
to find a suitable context entry, it would do:

    base_context = ec.local_contexts[0]
    default_value = sys.run_with_local_context(base_context,
self.default_factory)
    sys.run_with_local_context(base_context, self.set, default_value)

The default setting for default_factory could then be to raise
RuntimeError complaining that the context item isn't set in the
current context.

Cheers,
Nick.

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