[Python-Dev] PEP 550 v4

Greg Ewing greg.ewing at canterbury.ac.nz
Wed Sep 6 20:00:39 EDT 2017

Nathaniel Smith wrote:
> Literally the first motivating example at the beginning of the PEP
> ('def fractions ...') involves only generators, not coroutines, and
> only works correctly if generators get special handling. (In fact, I'd
> be curious to see how Greg's {push,pop}_local_storage could handle
> this case.)

I've given a decimal-based example, but it was a bit
scattered. Here's a summary and application to the
fractions example.

I'm going to assume that the decimal module has been
modified to keep the current context in a context var,
and that getcontext() and setcontext() access that
context var.

THe decimal.localcontext context manager is also
redefined as:

    class localcontext():

       def __enter__(self):
          ctx = getcontext().copy()
          return ctx

       def __exit__(self):

Now we can write the fractions generator as:

    def fractions(precision, x, y):
       with decimal.localcontext() as ctx:
         ctx.prec = precision
         yield Decimal(x) / Decimal(y)
         yield Decimal(x) / Decimal(y ** 2)

You may notice that this is exactly the same as
what you would write today for the same task...


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