[Python-Dev] PEP 550 v4

Greg Ewing greg.ewing at canterbury.ac.nz
Thu Sep 7 03:39:01 EDT 2017

Yury Selivanov wrote:
> 1. So essentially this means that we will have one "local context" per
> context manager storing one value.

I can't see that being a major problem. Context vars will
(I hope!) be very rare things, and needing to change a
bunch of them in one function ought to be rarer still.

But if you do, it would be easy to provide a context
manager whose sole effect is to introduce a new context:

    with new_local_context():

> 2. If somebody makes a mistake and calls "push_local_context" without
> a corresponding "pop_local_context"

You wouldn't normally call them directly, they would be
encapsulated in carefully-written context managers. If you
do use them, you're taking responsibility for using
them correctly.

If it would make you feel happier, they could be named
_push_local_context and _pop_local_context to emphasise
that they're not intended for everyday use.

> 3. Users will need to know way more to correctly use the mechanism.

Most users will simply be using already-provided context
managers, which they're *already used to doing*. So they
won't have to know anything more than they already do.

See my last decimal example, which required *no change*
to existing correct user code.

> So far, both you and Koos can't give us a realistic example which
> illustrates why we should suffer the implications of (1), (2), and
> (3).

And you haven't given a realistic example that convinces me
your proposed with-statement-elimination feature would be of
significant benefit.


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