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Title: Inaccuracy about "in" keyword for list and tuple
Type: enhancement Stage: patch review
Components: Documentation Versions: Python 2.7
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: rhettinger Nosy List: docs@python, jedwards, python-dev, r.david.murray, rhettinger, wim.glenn
Priority: normal Keywords: patch

Created on 2015-04-17 07:22 by wim.glenn, last changed 2022-04-11 14:58 by admin. This issue is now closed.

File name Uploaded Description Edit
issue23986.diff jedwards, 2015-04-17 17:06 review
Messages (5)
msg241317 - (view) Author: wim glenn (wim.glenn) * Date: 2015-04-17 07:22
The comparisons section of the python 2 docs says:


For the list and tuple types, x in y is true if and only if there exists an index i such that x == y[i] is true.


But it's not strictly speaking correct.  Simplest counter-example:

x = float('nan')
y = [x]

The python 3 docs instead mention correct equivalent which is 

any(x is e or x == e for e in y)
msg241344 - (view) Author: James Edwards (jedwards) * Date: 2015-04-17 17:06
What about:

 For the list and tuple types, ``x in y`` is true if and only if there exists an
-index *i* such that ``x == y[i]`` is true.
+index *i* such that either ``x == y[i]`` or ``x is y[i]`` is true.

Seems to address your issue, and match the semantics of the Python3 any() approach.
msg241363 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2015-04-17 20:25
Those two should be in the reverse order, though.  Identity is checked before equality.  But why not backport the python3 wording?  (Note that there is an open issue for slightly improving that wording).
msg241598 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2015-04-20 04:05
I'll add James' suggested wording, but with the reversed-order suggested by David Murray.
msg264230 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2016-04-26 07:59
New changeset aba647a34ed4 by Raymond Hettinger in branch '2.7':
Issue #23986:  Note that the in-operator for lists and tuples check identity before equality.
Date User Action Args
2022-04-11 14:58:15adminsetgithub: 68174
2016-04-26 08:00:31rhettingersetstatus: open -> closed
resolution: fixed
2016-04-26 07:59:24python-devsetnosy: + python-dev
messages: + msg264230
2016-01-03 10:06:50ezio.melottisetstage: patch review
2015-04-20 04:05:23rhettingersetassignee: docs@python -> rhettinger

messages: + msg241598
nosy: + rhettinger
2015-04-17 20:25:51r.david.murraysetnosy: + r.david.murray
messages: + msg241363
2015-04-17 17:06:31jedwardssetfiles: + issue23986.diff

nosy: + jedwards
messages: + msg241344

keywords: + patch
2015-04-17 07:22:58wim.glenncreate