[issue23986] Inaccuracy about "in" keyword for list and tuple

[Raymond Hettinger]

Changes by Raymond Hettinger :


--
resolution:  -> fixed
status: open -> closed

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[issue23986] Inaccuracy about "in" keyword for list and tuple

[Roundup Robot]

Roundup Robot added the comment:

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.
https://hg.python.org/cpython/rev/aba647a34ed4

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nosy: +python-dev

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[issue23986] Inaccuracy about "in" keyword for list and tuple

[Ezio Melotti]

Changes by Ezio Melotti :


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stage:  -> patch review

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[issue23986] Inaccuracy about "in" keyword for list and tuple

[Raymond Hettinger]

Raymond Hettinger added the comment:

I'll add James' suggested wording, but with the reversed-order suggested by 
David Murray.

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assignee: docs@python -> rhettinger
nosy: +rhettinger

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[issue23986] Inaccuracy about "in" keyword for list and tuple

[R. David Murray]

R. David Murray added the comment:

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).

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nosy: +r.david.murray

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[issue23986] Inaccuracy about "in" keyword for list and tuple

[James Edwards]

James Edwards added the comment:

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.

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keywords: +patch
nosy: +jedwards
Added file: http://bugs.python.org/file39087/issue23986.diff

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[issue23986] Inaccuracy about "in" keyword for list and tuple

[wim glenn]

New submission from wim glenn:

The comparisons section of the python 2 docs says:

--- 

https://docs.python.org/2/reference/expressions.html#comparisons

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)

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assignee: docs@python
components: Documentation
messages: 241317
nosy: docs@python, wim.glenn
priority: normal
severity: normal
status: open
title: Inaccuracy about "in" keyword for list and tuple
type: enhancement
versions: Python 2.7

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