8000 bpo-30969: Fix docs about the comparison in absence of __contains__ by ztane · Pull Request #2761 · python/cpython · GitHub
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bpo-30969: Fix docs about the comparison in absence of __contains__ #2761

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Merged
merged 4 commits into from
May 30, 2019
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bpo-30969: Fix docs about the comparison in absence of __contains__ #2761

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6d411c6
Fix the eq check that is used in absence of `__contains__`
ztane Jul 19, 2017
29abde0
lowercase the true, as it need not be `True`.
ztane Jul 19, 2017
511406d
Add the missing space here.
ztane Jul 19, 2017
b4f285f
Fix trailing whitespace
csabella May 30, 2019
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Fix the eq check that is used in absence of __contains__ 
Both the `__iter__` and the index-iteration method test the `is` first. While the document says that `x is x` means that `x == x` should be true, it is not true for example in the case of `nan`. 

```>>> x = float('nan') 
>>> x == x
False
>>> x is x
True
>>> class Foo:
...     def __iter__(self):
...         return iter([x])
... 
>>> x in Foo()
True
>>> any(x == i for i in Foo())
False
>>> class Bar:
...     def __getitem__(self):
...         return [x].__getitem__(self)
... 
>>> class Bar:
...     def __getitem__(self, i):
...         return [x].__getitem__(i)
... 
>>> x in Bar
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: argument of type 'type' is not iterable
>>> x in Bar()
True
>>> any(x == i for i in Bar())
False```
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@ztane
ztane authored Jul 19, 2017
commit 6d411c6ce4286b21948e1d7df182f510cd9857b2
11 changes: 6 additions & 5 deletions Doc/reference/expressions.rst
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Expand Up @@ -1443,14 +1443,15 @@ y`` returns ``True`` if ``y.__contains__(x)`` returns a true value, and
``False`` otherwise.

For user-defined classes which do not define :meth:`__contains__` but do define
:meth:`__iter__`, ``x in y`` is ``True`` if some value ``z`` with ``x == z`` is
produced while iterating over ``y``. If an exception is raised during the
iteration, it is as if :keyword:`in` raised that exception.
:meth:`__iter__`, ``x in y`` is ``True`` if some value ``z``, for which the
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@tirkarthi tirkarthi Jan 8, 2019 •
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@ztane trailing space in the three lines are causing CI failure. Removing them will fix the issue. Thanks for the patch.

Doc git:(pr_2761) python3 tools/rstlint.py -i tools -i ./venv -i README.rst
[1] reference/expressions.rst:1446: trailing whitespace
[1] reference/expressions.rst:1447: trailing whitespace
[1] reference/expressions.rst:1448: trailing whitespace
3 problems with severity 1 found.

expression``x is z or x == z`` is True, is produced while iterating over ``y``.
If an exception is raised during the iteration, it is as if :keyword:`in` raised
that exception.

Lastly, the old-style iteration protocol is tried: if a class defines
:meth:`__getitem__`, ``x in y`` is ``True`` if and only if there is a non-negative
integer index *i* such that ``x == y[i]``, and all lower integer indices do not
raise :exc:`IndexError` exception. (If any other exception is raised, it is as
integer index *i* such that ``x is y[i] or x == y[i]``, and no lower integer index
raises the :exc:`IndexError` exception. (If any other exception is raised, it is as
if :keyword:`in` raised that exception).

.. index::
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0