Backport dictviews to 2.7

[Yhg1s]

BPO	1967
Nosy	@Yhg1s, @avassalotti, @benjaminp, @ezio-melotti, @graingert
PRs	[2.7] bpo-1967: proxy dictviews in dictproxy #15143
Files	dictviews_backport.diff backport_dictviews.diff 2to3_fixer_dictviews.diff backport_dictviews-2.diff backport_dictviews-3.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2010-01-11.23:17:53.498>
created_at = <Date 2008-01-29.22:39:04.556>
labels = ['interpreter-core', 'type-feature']
title = 'Backport dictviews to 2.7'
updated_at = <Date 2019-08-06.13:24:16.339>
user = 'https://github.com/Yhg1s'

bugs.python.org fields:

activity = <Date 2019-08-06.13:24:16.339>
actor = 'python-dev'
assignee = 'none'
closed = True
closed_date = <Date 2010-01-11.23:17:53.498>
closer = 'alexandre.vassalotti'
components = ['Interpreter Core']
creation = <Date 2008-01-29.22:39:04.556>
creator = 'twouters'
dependencies = []
files = ['9336', '14651', '14652', '15705', '15706']
hgrepos = []
issue_num = 1967
keywords = ['patch', 'needs review']
message_count = 11.0
messages = ['61834', '61881', '70359', '91285', '91286', '96258', '96276', '97066', '97070', '97613', '349107']
nosy_count = 5.0
nosy_names = ['twouters', 'alexandre.vassalotti', 'benjamin.peterson', 'ezio.melotti', 'graingert']
pr_nums = ['15143']
priority = 'critical'
resolution = 'accepted'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue1967'
versions = ['Python 2.7']

[Yhg1s]

Patch to backport dictviews to trunk. Consists of some trickery:

• new 'viewkeys', 'viewvalues' and 'viewitems' methods of dicts,
  returning exactly what 'keys', 'values' and 'items' return in 3.0: three
  new types defined in dictobject.c
• a future import (dictviews) that changes which opcodes are generated
  for (some) attribute access
• special opcodes for getting and setting 'keys', 'values' and 'items'
  attributes from an object. These opcodes do nothing special unless a
  future import is in effect in the calling code block *and* the type they
  are called on is a dict subclass, in which case they translate 'keys',
  'values' and 'items' to 'viewkeys', 'viewvalues' and 'viewitems'.
• similar specialcasing in getattr() and setattr()

[Yhg1s]

After talking to Guido, got rid of the future import magic in favour of
just providing 'viewkeys', 'viewitems' and 'viewvalues' methods of
dicts. This makes efficient 2.6-and-3.0 dict-using code possibly by
making 2to3 translate the view-methods directly to keys/values/items in
3.0, and not wrapping everything in list().

[benjaminp]

I'm going to defer this to 2.7.

[avassalotti]

I have refreshed Thomas's patch to reflect recent updates to dictviews.
I also added the documentation for dictviews.

[avassalotti]

Here is a patch for 2to3 to support the translation of code using dictviews.

[avassalotti]

Can someone review the patches? I would like to commit this later this week.

[ezio-melotti]

The patch (backport_dictviews.diff) doesn't seem to work for me. I
applied it on trunk after a svn up, did "./configure && make" and when I
tried d.view{items|keys|values} Python segfaulted:

Python 2.7a1+ (trunk:76759M, Dec 12 2009, 00:28:52)
[GCC 4.4.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> d = dict(a=3, b=5)
>>> d.viewitems()
Segmentation fault

I'm tested it on Ubuntu 9.10.

The docstrings of the three methods are also wrong:

+PyDoc_STRVAR(viewkeys__doc__,
+ "D.keys() -> a set-like object providing a view on D's keys");

This should be "D.viewkeys() -> ...", same with the other two.

[avassalotti]

It looks like that crash is caused by some bug in the implementation of
PyUnicode_FromFormat(). I rewrote my patch to avoid it and added some
additional unit tests.

[avassalotti]

The tests were missing in my last patch. So, here's a new one.

[avassalotti]

Committed in r77428. Thanks for your comments!

[graingert]

It seems that 'viewkeys', 'viewvalues' and 'viewitems' methods were not added to types.DictProxyType, was that intentional?