With PEP 468, the lru cache not longer needs to sort keyword args

[rhettinger]

BPO	29203
Nosy	@rhettinger, @serhiy-storchaka
Files	lru_468.diff lru_cache_pep468.patch

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

Show more details

GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2017-01-09.07:21:45.489>
created_at = <Date 2017-01-08.10:01:29.924>
labels = ['3.7', 'type-bug', 'library']
title = 'With PEP 468, the lru cache not longer needs to sort keyword args'
updated_at = <Date 2017-01-09.07:27:42.854>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2017-01-09.07:27:42.854>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2017-01-09.07:21:45.489>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2017-01-08.10:01:29.924>
creator = 'rhettinger'
dependencies = []
files = ['46210', '46213']
hgrepos = []
issue_num = 29203
keywords = ['patch']
message_count = 6.0
messages = ['284971', '284972', '284973', '284984', '285013', '285014']
nosy_count = 3.0
nosy_names = ['rhettinger', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29203'
versions = ['Python 3.6', 'Python 3.7']

[rhettinger]

Since the ordering of keyword arguments is now guaranteed, the LRU cache doesn't need to sort any longer.

The will give a small change in behavior that I don't care about. A call f(a=1, b=2) would now be cached separately from f(b=2, a=1). That won't arise often and isn't really different than the status quo where f(1, b=2) or f(1, 2) are already cached separately. Overall it is a net win by saving the sorting step on every call.

[serhiy-storchaka]

I think this is a bug and it should be fixed in 3.6. Currenly lru_cache breaks PEP-468 (Preserving Keyword Argument Order).

>>> from functools import lru_cache
>>> @lru_cache()
... def f(**kwargs):
...     return list(kwargs.items())
... 
>>> f(a=1, b=2)
[('a', 1), ('b', 2)]
>>> f(b=2, a=1)
[('a', 1), ('b', 2)]

C implementation should be changed too.

[rhettinger]

I think this is a bug and it should be fixed in 3.6.

I concur. Do you care to whip-up a patch (it is late here).

[serhiy-storchaka]

Here is a patch that fixes also C implementation and has tests.

[python-dev]

New changeset 48c750c26710 by Raymond Hettinger in branch '3.6':
Issue bpo-29203: functools.lru_cache() now respects PEP-468
https://hg.python.org/cpython/rev/48c750c26710

[python-dev]

New changeset cc47d385512b by Raymond Hettinger in branch 'default':
Complete the merge for issue bpo-29203
https://hg.python.org/cpython/rev/cc47d385512b