list.index and rest of list methods disagree if a value is in the list if it's mutated during the call

[ssbr]

BPO	23204
Nosy	@rhettinger, @ssbr, @serhiy-storchaka

^{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 2015-01-09.07:59:28.058>
created_at = <Date 2015-01-09.07:25:48.726>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = "list.index and rest of list methods disagree if a value is in the list if it's mutated during the call"
updated_at = <Date 2015-01-09.08:07:29.056>
user = 'https://github.com/ssbr'

bugs.python.org fields:

activity = <Date 2015-01-09.08:07:29.056>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2015-01-09.07:59:28.058>
closer = 'rhettinger'
components = ['Interpreter Core']
creation = <Date 2015-01-09.07:25:48.726>
creator = 'Devin Jeanpierre'
dependencies = []
files = []
hgrepos = []
issue_num = 23204
keywords = []
message_count = 3.0
messages = ['233721', '233723', '233725']
nosy_count = 3.0
nosy_names = ['rhettinger', 'Devin Jeanpierre', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23204'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6']

[ssbr]

>>> class AppendOnUnequal(object):
...     def __init__(self, append_to):
...         self.append_to = append_to
...     def __eq__(self, other):
...         if self is other:
...             return True
...         self.append_to.append(self)
...         return False
... 
>>> L = [1]; AppendOnUnequal(L) in L
True
>>> L = [1]; L.count(AppendOnUnequal(L))
1
>>> L = [1]; L.remove(AppendOnUnequal(L))
>>> L = [1]; L.index(AppendOnUnequal(L))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: <__main__.AppendOnUnequal object at 0x7f2562d071d0> is not in list

.index() is the only odd one out here. Looks like a bug to me.

[rhettinger]

This is a non-guaranteed behavior. It is allowed to be different from other list methods. The behavior is also very old, stable, and has not been a problem in practice. No good would come from changing it.

[serhiy-storchaka]

This is because list.index() has start and stop parameters, and L.index(x) is equivalent to L.index(x, 0, len(L)). In list.count() and list.remove() the limit is dynamic during iteration, but in list.index() it is specified by arguments before iterating. It is possible to make the limit static in list.count() and list.remove() or to make it floating if stop is absent or negative in list.index(), but this will complicate and slow down the code without good reason. I suggest to close this issue as "wont fix".