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

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 nestedgen(): setsomecontext() yield def gen(): # somecontext is not set yield from nestedgen() # use somecontext ??? 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(...):
        yield

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()
 next(g)
 # 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():
      decimal.context(...)
      yield

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".

Yury

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