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

Yury Selivanov yselivanov.ml at gmail.com
Tue Aug 29 18:01:40 EDT 2017

On Tue, Aug 29, 2017 at 5:45 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote: [..]

What you seem to be suggesting is that generators shouldn't leak context changes even when you don't use a with-statement.

Yes, generators shouldn't leak context changes regardless of what and how changes the context inside them:

var = new_context_var()

def gen(): old_val = var.get() try: var.set('blah') yield yield yield finally: var.set(old_val)

with the above code, when you do "next(gen())" it would leak the state without PEP 550. "finally" block (or "with" block wouldn't help you here) and corrupt the state of the caller.

That's the problem the PEP fixes. The EC interaction with generators is explained here with a great detail: https://www.python.org/dev/peps/pep-0550/#id4

We explain the motivation behind desiring a working context-local solution for generators in the Rationale section: https://www.python.org/dev/peps/pep-0550/#rationale

Basically half of the PEP is about isolating context in generators.

If you're going to to that, you'd better make sure that the same thing applies to regular functions, otherwise you've introduced an inconsistency.

Regular functions cannot pause/resume their execution, so they can't leak an inconsistent context change due to out of order or partial execution.

PEP 550 positions itself as a replacement for TLS, and clearly defines its semantics for regular functions in a single thread, regular functions in multithreaded code, generators, and asynchronous code (async/await). Everything is specified in the High-level Specification section. I wouldn't call slightly differently defined semantics for generators/coroutines/functions an "inconsistency" -- they just have a different EC semantics given how different they are from each other.

Drawing a parallel between 'yield from' and function calls is possible, but we shouldn't forget that you can 'yield from' a half-iterated generator.

Yury

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