[Python-ideas] PEP draft: context variables

M.-A. Lemburg mal at egenix.com
Sun Oct 15 13:17:57 EDT 2017

On 15.10.2017 07:13, Nick Coghlan wrote:
> On 15 October 2017 at 14:53, M.-A. Lemburg <mal at egenix.com> wrote:
>> The behavior of also deferring the context to time of
>> execution should be the non-standard form to not break
>> this intuition, otherwise debugging will be a pain and
>> writing fully working code would be really hard in the
>> face of changing contexts (e.g. say decimal rounding
>> changes in different parts of the code).
> No, it really wouldn't, since "the execution context is the context that's
> active when the code is executed" is relatively easy to understand based
> entirely on the way functions, methods, and other forms of delayed
> execution work (including iterators).
> "The execution context is the context that's active when the code is
> executed, *unless* the code is in a generator, in which case, it's the
> context that was active when the generator-iterator was instantiated" is
> harder to follow.

I think you're mixing two concepts here: the context defines a
way code is supposed to be interpreted at runtime. This doesn't
have anything to do with when the code is actually run.

Just think what happens if you write code using a specific
context (let's say rounding to two decimal places), which then
get executed deferred within another context (say rounding
to three decimal places) for part of the generator run and
yet another context (say rounding to whole integers) for the
remainder of the generator.

I can't think of a good use case where such behavior would
be intuitive, expected or even reasonable ;-)

The context should be inherited by the generator when instantiated
and not change after that, so that the context defining the
generator takes precedent over any later context in which the
generator is later run.

Note that the above is not the same as raising an exception and
catching it somewhere else (as Nathaniel brought up). The context
actually changes semantics of code, whereas exceptions only flag
a special state and let other code decide what to do with it
(defined where the exception handling is happening, not where the
raise is caused).

Just for clarification: I haven't followed the thread, just saw
your posting and found the argument you put forward a bit
hard to follow. I may well be missing some context or evaluating
the argument in a different one as the one where it was defined ;-)

Marc-Andre Lemburg

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