[Python-Dev] PEP 550 v4 (original) (raw)

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():
   cvar1.set(something)
   cvar2.set(otherthing)
   ...

2. If somebody makes a mistake and calls "pushlocalcontext" without a corresponding "poplocalcontext"

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.

-- Greg

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