[Python-checkins] peps: Chris A: Mention Mark Shannon's counterproposal of changing next(). One small

[guido.van.rossum]

https://hg.python.org/peps/rev/9695a84dc696
changeset:   5622:9695a84dc696
user:        Guido van Rossum <guido at python.org>
date:        Wed Nov 26 09:34:12 2014 -0800
summary:
  Chris A: Mention Mark Shannon's counterproposal of changing next(). One small copy-edit.

files:
  pep-0479.txt |  22 ++++++++++++++++++++--
  1 files changed, 20 insertions(+), 2 deletions(-)


diff --git a/pep-0479.txt b/pep-0479.txt
--- a/pep-0479.txt
+++ b/pep-0479.txt
@@ -143,8 +143,8 @@
 This is done by enclosing those places in the generator body where a
 ``StopIteration`` is expected (e.g. bare ``next()`` calls or in some
 cases helper functions that are expected to raise ``StopIteration``)
-in a ``try/except`` construct that returns when ``StopIteration``
-returns.  The ``try/except`` construct should appear directly in the
+in a ``try/except`` construct that returns when ``StopIteration`` is
+raised.  The ``try/except`` construct should appear directly in the
 generator function; doing this in a helper function that is not itself
 a generator does not work.  If ``raise StopIteration`` occurs directly
 in a generator, simply replace it with ``return``.
@@ -364,6 +364,21 @@
 bubble out would still be potentially wrong, depending on the use made
 of the distinction between the two exception types.
 
+Converting the exception inside next()
+--------------------------------------
+
+Mark Shannon suggested [11]_ that the problem could be solved in
+``next()`` rather than at the boundary of generator functions.  By
+having ``next()`` catch ``StopIteration`` and raise instead
+``ValueError``, all unexpected ``StopIteration`` bubbling would be
+prevented; however, the backward-incompatibility concerns are far
+more serious than for the current proposal, as every ``next()`` call
+now needs to be rewritten to guard against ``ValueError`` instead of
+``StopIteration`` - not to mention that there is no way to write one
+block of code which reliably works on multiple versions of Python.
+(Using a dedicated exception type, perhaps subclassing ``ValueError``,
+would help this; however, all code would still need to be rewritten.)
+
 
 Criticism
 =========
@@ -426,6 +441,9 @@
 .. [10] Tracker issue with Proof-of-Concept patch
    (http://bugs.python.org/issue22906)
 
+.. [11] Post from Mark Shannon with alternate proposal
+   (https://mail.python.org/pipermail/python-dev/2014-November/137129.html)
+
 Copyright
 =========
 

-- 
Repository URL: https://hg.python.org/peps