[Python-ideas] PEP draft: context variables

[Yury Selivanov]

On Tue, Oct 10, 2017 at 10:22 AM, Koos Zevenhoven <k7hoven at gmail.com> wrote:
> On Tue, Oct 10, 2017 at 5:01 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>
>> On 10 October 2017 at 22:51, Koos Zevenhoven <k7hoven at gmail.com> wrote:
>>>
>>> I see no reason why these two should be equivalent.
>>
>>
>> There is no "should" about it: it's a brute fact that the two forms *are*
>> currently equivalent for lazy iterators (including generators), and both
>> different from the form that uses eager evaluation of the values before the
>> context change.
>>
>> Where should enters into the picture is by way of PEP 550 saying that they
>> should *remain* equivalent because we don't have an adequately compelling
>> justification for changing the runtime semantics.
>>
>> That is, given the following code:
>>
>>     itr = make_iter()
>>     with decimal.localcontext() as ctx:
>>         ctc.prex = 30
>>         for i in itr:
>>           pass
>>
>> Right now, today, in 3.6. the calculations in the iterator will use the
>> modified decimal context, *not* the context that applied when the iterator
>> was created. If you want to ensure that isn't the case, you have to force
>> eager evaluation before the context change.
>>
>> What PEP 550 is proposing is that, by default, *nothing changes*: the lazy
>> iteration in the above will continue to use the updated decimal context by
>> default.
>
>
> That's just an arbitrary example. There are many things that *would* change
> if decimal contexts simply switched from using thread-local storage to using
> PEP 550. It's not at all obvious which of the changes would be most likely
> to cause problems. If I were to choose, I would probably introduce a new
> context manager which works with PEP 555 semantics, because that's the only
> way to ensure full backwards compatibility, regardless of whether PEP 555 or
> PEP 550 is used. But I'm sure one could decide otherwise.

Please stop using "many things .. would", "most likely" etc.  We have
a very focused discussion here.  If you know of any particular issue,
please demonstrate it with a realistic example.  Otherwise, we only
increase the number of emails and make things harder to track for
everybody.

If decimal switches to use PEP 550, there will be no "many things that
*would* change".  The only thing that will change is this:

  def g():
    with decimal_context(...):
       yield

  next(g())   # this will no longer leak decimal context to the outer world

I consider the above a bug fix, because nobody in their right mind
relies on partial iteration of generator expecting that some of it's
internal code would affect your code indirectly.  The only such case
is contextlib.contextmanager, and PEP 550 provides mechanisms to make
generators "leaky" explicitly.

Yury