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Fix object.__reversed__ doc #50284

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terryjreedy opened this issue May 16, 2009 · 2 comments
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Fix object.__reversed__ doc #50284

terryjreedy opened this issue May 16, 2009 · 2 comments
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@birkenfeld
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docs Documentation in the Doc dir

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@terryjreedy
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BPO 6034
Nosy @birkenfeld, @terryjreedy

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 = 'https://github.com/birkenfeld'
closed_at = <Date 2009-05-16.11:13:35.522>
created_at = <Date 2009-05-16.03:18:06.635>
labels = ['docs']
title = 'Fix object.__reversed__ doc'
updated_at = <Date 2009-05-16.11:13:35.520>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2009-05-16.11:13:35.520>
actor = 'georg.brandl'
assignee = 'georg.brandl'
closed = True
closed_date = <Date 2009-05-16.11:13:35.522>
closer = 'georg.brandl'
components = ['Documentation']
creation = <Date 2009-05-16.03:18:06.635>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 6034
keywords = []
message_count = 2.0
messages = ['87852', '87878']
nosy_count = 2.0
nosy_names = ['georg.brandl', 'terry.reedy']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue6034'
versions = ['Python 2.6', 'Python 3.0', 'Python 3.1', 'Python 2.7', 'Python 3.2']
@terryjreedy
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3.3.5. Emulating container types
object.__reversed__(self)
says in 3.0 and 3.1 and I assume in 2.x:
"Objects should normally only provide __reversed__() if they do not
support the sequence protocol and an efficient implementation of reverse
iteration is possible."

The builtin sequences violate this because because they do support the
sequence and have __ reversed__ methods anyway. And iterables that do
not support that protocol obviously *must* provide a method to be
reverse iterable.

I believe the point is that it is hard for Python code to beat the
C-coded version of the obvious

def __reversed__(self):
   for i in reversed(range(self.__len__)):
      yield self.__getitem__(i)

So I think the entry should say: "Objects that support the sequence
protocol should only provide __reversed__ if they can provide an
implementation that is more efficient than the one provided by
reversed()." possibly followed by "Objects that do not supposrt the
sequence protocol must provide __reversed__ to be reverse iterable."

@terryjreedy terryjreedy added the docs Documentation in the Doc dir label May 16, 2009
@birkenfeld
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Thanks, fixed in r72675.

@ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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