On 10 October 2017 at 22:51, Koos Zevenhoven <k7hoven@gmail.com> wrote:

​I see no reason why these two should be equivalent.

There is no "should" about it: it's a brute fact that the two forms *are* currently equivalent for lazy iterators (including generators), and both different from the form that uses eager evaluation of the values before the context change.

Where should enters into the picture is by way of PEP 550 saying that they should *remain* equivalent because we don't have an adequately compelling justification for changing the runtime semantics.

That is, given the following code:

    itr = make_iter()

    with decimal.localcontext() as ctx:

        ctc.prex = 30

        for i in itr:
          pass

Right now, today, in 3.6. the calculations in the iterator will use the modified decimal context, *not* the context that applied when the iterator was created. If you want to ensure that isn't the case, you have to force eager evaluation before the context change.

What PEP 550 is proposing is that, by default, *nothing changes*: the lazy iteration in the above will continue to use the updated decimal context by default.