[Python-checkins] peps: The folks at the Twisted sprint made a good case :)

nick.coghlan python-checkins at python.org
Sat Apr 19 04:07:33 CEST 2014


http://hg.python.org/peps/rev/27829cec6787
changeset:   5463:27829cec6787
user:        Nick Coghlan <ncoghlan at gmail.com>
date:        Fri Apr 18 22:07:27 2014 -0400
summary:
  The folks at the Twisted sprint made a good case :)

files:
  pep-0469.txt |  126 +++++++++++++++++++++++++++++++++++++++
  1 files changed, 126 insertions(+), 0 deletions(-)


diff --git a/pep-0469.txt b/pep-0469.txt
new file mode 100644
--- /dev/null
+++ b/pep-0469.txt
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+PEP: 469
+Title: Simplified migration of iterator-based mapping code to Python 3
+Version: $Revision$
+Last-Modified: $Date$
+Author: Nick Coghlan <ncoghlan at gmail.com>
+Status: Draft
+Type: Standards Track
+Content-Type: text/x-rst
+Created: 2014-04-18
+Python-Version: 3.5
+Post-History: 2014-04-18
+
+
+Abstract
+========
+
+For Python 3, PEP 3106 changed the design of the ``dict`` builtin and the
+mapping API in general to replace the separate list based and iterator based
+APIs in Python 2 with a merged, memory efficient set and multiset view
+based API.
+
+This means that Python 3 code always requires an additional qualifier to
+reliably reproduce classic Python 2 mapping semantics:
+
+    * List based (e.g. ``d.keys()``): ``list(d.keys())``
+    * Iterator based (e.g. ``d.iterkeys()``): ``iter(d.keys())``
+
+Some Python 2 code that uses ``d.keys()`` may be migrated to Python 3
+(or the common subset of Python 2 and Python 3) without alteration, but
+*all* code using the iterator based API requires modification. Code that
+is migrating to the common subset of Python 2 and 3 and needs to retain the
+memory efficient implementation that avoids creating an unnecessary list
+object must switch away from using a method to instead using a helper
+function (such as those provided by the ``six`` module)
+
+To simplify the process of migrating Python 2 code that uses the existing
+iterator based APIs to Python 3, this PEP proposes the reintroduction
+of the Python 2 spelling of the iterator based semantics in Python 3.5, by
+restoring the following methods to the builtin ``dict`` API and the
+``collections.abc.Mapping`` ABC definition:
+
+    * ``iterkeys()``
+    * ``itervalues()``
+    * ``iteritems()``
+
+
+Proposal
+========
+
+Methods with the following exact semantics will be added to the builtin
+``dict`` type and ``collections.abc.Mapping`` ABC::
+
+    def iterkeys(self):
+        return iter(self.keys())
+
+    def itervalues(self):
+        return iter(self.values())
+
+    def iteritems(self):
+        return iter(self.items())
+
+These semantics ensure that the methods also work as expected for subclasses
+of these base types.
+
+
+Rationale
+=========
+
+Similar in spirit to PEP 414 (which restored explicit Unicode literal
+support in Python 3.3), this PEP is aimed primarily at helping users
+that currently feel punished for making use of a feature that needed to be
+requested explicitly in Python 2, but was effectively made the default
+behaviour in Python 3.
+
+Users of list-based iteration in Python 2 that aren't actually relying on
+those semantics get a free memory efficiency improvement when migrating to
+Python 3, and face no additional difficulties when migrating via the common
+subset of Python 2 and 3.
+
+By contrast, users that actually want the increased efficiency will have
+faced a three phase migration process by the time they have fully migrated
+to Python 3:
+
+* original migration to the iterator based APIs after they were added in
+  Python 2.2
+* migration to a separate function based API in order to run in the common
+  subset of Python 2 and 3
+* eventual migration back to unprefixed method APIs when eventually dropping
+  Python 2.7 support
+
+The view based APIs that were added to Python 2.7 don't actually help with
+the transition process, as they don't exist in Python 3 and hence aren't
+part of the common subset of Python 2 and Python 3, and also aren't supported
+by most Python 2 mappings (including the collection ABCs).
+
+This PEP proposes to just eliminate all that annoyance by making the iterator
+based APIs work again in Python 3.5+. As with the restoration of Unicode
+literals, it does add a bit of additional noise to the definition of Python
+3, but it does so while bringing a significant benefit in increasing the size
+of the common subset of Python 2 and Python 3 and so simplifying the process
+of migrating to Python 3 for affected Python 2 users.
+
+
+Acknowledgements
+================
+
+Thanks to the folks at the Twisted sprint table at PyCon for a very
+vigorous discussion of this idea (and several other topics), and especially
+to Hynek Schlawack for acting as a moderator when things got a little too
+heated :)
+
+
+Copyright
+=========
+
+This document has been placed in the public domain.
+
+
+..
+   Local Variables:
+   mode: indented-text
+   indent-tabs-mode: nil
+   sentence-end-double-space: t
+   fill-column: 70
+   coding: utf-8
+   End:

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


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