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