Language reference is ambiguous regarding next() method lookup

[ncoghlan]

BPO	5739
Nosy	@birkenfeld, @terryjreedy, @ncoghlan

^{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 = None
created_at = <Date 2009-04-12.00:58:26.396>
labels = ['type-bug', 'docs']
title = 'Language reference is ambiguous regarding next() method lookup'
updated_at = <Date 2019-03-15.22:55:14.374>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2019-03-15.22:55:14.374>
actor = 'BreamoreBoy'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2009-04-12.00:58:26.396>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 5739
keywords = []
message_count = 5.0
messages = ['85883', '86104', '86113', '236179', '236187']
nosy_count = 3.0
nosy_names = ['georg.brandl', 'terry.reedy', 'ncoghlan']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue5739'
versions = []

[ncoghlan]

The language reference is currently silent as to whether or not an
iterator's next() method is looked up on every pass around a for loop,
or whether it is OK for an implementation to look the method up once at
the start of the loop, cache the result and call it again each time
around the loop without doing the lookup again.

The language reference should require implementations to follow
CPython's behaviour in this respect: the method should be looked up
again on each pass around the loop.

As per this email on python-ideas:
http://mail.python.org/pipermail/python-ideas/2009-April/004083.html

[terryjreedy]

I was surprised when this came up with another issue.
Why does CPython behave that way? Accident or justified decision?

[ncoghlan]

Guido didn't actually say whether or not this was originally just an
implementation accident or a deliberate design choice - he just
indicated that this was a case where caching the bound method should be
disallowed because it could change the semantics of existing programs.

[BreamoreBoy]

Thrown up by selecting "Random Issue", if nothing else it looks to me as if __next__ is missing from and so should be documented in https://docs.python.org/3.1/reference/datamodel.html#special-method-names

[terryjreedy]

Still true in the current doc
https://docs.python.org/3/reference/datamodel.html#special-method-names
object.__iter__ in listed in 3.3.6. Emulating container types
A listing for __next__ could follow that.

Both iterator special methods are documented in
https://docs.python.org/3/library/stdtypes.html#iterator-types