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

Greg Ewing greg.ewing at canterbury.ac.nz
Tue Aug 29 17:45:08 EDT 2017

Yury Selivanov wrote:

Consider the following generator:

def gen(): with decimal.context(...): yield We don't want gen's context to leak to the outer scope

That's understandable, but fixing that problem shouldn't come at the expense of breaking the ability to refactor generator code or async code without changing its semantics.

I'm not convinced that it has to, either. In this example, the with-statement is the thing that should be establishing a new nested context. Yielding and re-entering the generator should only be swapping around between existing contexts.

Not, let's consider a "broken" generator:

def gen(): decimal.context(...) yield

The following non-generator code is "broken" in exactly the same way:

def foo():
   decimal.context(...)
   do_some_decimal_calculations()
   # Context has now been changed for the caller

I simply want consistency.

So do I! We just have different ideas about what consistency means here.

It's easier for everybody to say that generators never leaked their context changes to the outer scope, rather than saying that "generators can sometimes leak their context".

No, generators should always leak their context changes to exactly the same extent that normal functions do. If you don't want to leak a context change, you should use a with statement.

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

-- Greg

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