On 11/20/2014 2:36 PM, Guido van Rossum wrote:
There's still a lively discussion on python-ideas; Steven D'Aprano has
dug up quite a bit of evidence that StopIteration is used quite a bit in
ways that will break under the new behavior, and there also seems to be
quite a bit of third-party information that recommends StopIteration
over return to terminate a generator early.
However I don't see much evidence that the current behavior is *better*
than the proposal -- I see the current behavior as a definite wart, and
I have definitely seen people struggle to debug silent early loop
termination due to an "escaped" StopIteration.
That said, I think for most people the change won't matter, some people
will have to apply one of a few simple fixes, and a rare few will have
to rewrite their code in a non-trivial way (sometimes this will affect
"clever" libraries).
I wonder if the PEP needs a better transition plan, e.g.
- right now, start an education campaign
For StackOverflow, someone with a high-enough rep (you, Guido?) should
add a tag for StopIteration with text like "The Python `StopIteration`
exception." (Copied from the current AttributeError tag, typeerror and
NameError also exist.). If and when the PEP is accepted, I would be
willing to add comments and the new tag to existing and future
questions/answers.
- with Python 3.5, introduce "from __future__ import generator_return",
and silent deprecation warnings
- with Python 3.6, start issuing non-silent deprecation warnings
I agree that two deprecation periods are needed.
- with Python 3.7, make the new behavior the default (subject to some
kind of review)
It would also be useful if we could extend the PEP with some examples of
the various categories of fixes that can be applied easily, e.g. a few
examples of "raise StopIteration" directly in a generator that can be
replaced with "return" (or omitted, if it's at the end); a few examples
of situations where "yield from" can supply an elegant fix (and an
alternative for code that needs to be backward compatible with Python
3.2 or 2.7); and finally (to be honest) an example of code that will
require being made more complicated.
Agree that explicit fixes are needed. Deprecation message could refer
to section of PEP.
Oh, and it would also be nice if the PEP included some suggested words
that 3rd party educators can use to explain the relationship between
StopIteration and generators in a healthier way (preferably a way that
also applies to older versions).
Chris, are you up to drafting these additions?
--
Terry Jan Reedy
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