[Python-Dev] PEP 550 v4

Yury Selivanov yselivanov.ml at gmail.com
Mon Aug 28 19:16:33 EDT 2017

On Mon, Aug 28, 2017 at 6:56 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Yury Selivanov wrote:
>> I saying that the following should not work:
>>     def nested_gen():
>>         set_some_context()
>>         yield
>>     def gen():
>>        # some_context is not set
>>        yield from nested_gen()
>>        # use some_context ???
> And I'm saying it *should* work, otherwise it breaks
> one of the fundamental principles on which yield-from
> is based, namely that 'yield from foo()' should behave
> as far as possible as a generator equivalent of a
> plain function call.

Consider the following generator:

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

We don't want gen's context to leak to the outer scope -- that's one
of the reasons why PEP 550 exists.  Even if we do this:

     g = gen()
     # the decimal.context won't leak out of gen

So a Python user would have a mental model: context set in generators
doesn't leak.

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

     def gen():

If we iterate gen() with next(), it still won't leak its context.  But
if "yield from" has semantics that you want -- "yield from" to be just
like function call -- then calling

     yield from gen()

will corrupt the context of the caller.

I simply want consistency.  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".


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