[Python-ideas] Fwd: Why do equality tests between OrderedDict keys/values views behave not as expected?

[Wes Turner]

* DOC: OrderDict.values() comparisons in Python 3
  * Src:
https://hg.python.org/cpython/file/f2a0a4a45292/Doc/library/collections.rst#l793

What should it say?

```rst

.. `<
https://hg.python.org/cpython/file/f2a0a4a45292/Doc/library/collections.rst#l793
>`__

collections.OrderedDict.values().__eq__

* "is suprising"
* Python 3 has `dict views`_

* :class:`OrderedDict` matches the dict interface in Python 2.7 and
  Python 3.
  *
https://docs.python.org/2/library/collections.html#collections.OrderedDict
  *
https://docs.python.org/3/library/collections.html#collections.OrderedDict

* Python 2 dict interface:
  * dict.viewkeys(), dict.viewvalues(), dict.viewitems()
  * dict.keys(), dict.values(), dict.items()
  * https://docs.python.org/2/library/stdtypes.html#dict
  * https://docs.python.org/2/library/stdtypes.html#dict.values
  * https://docs.python.org/2/library/stdtypes.html#dictionary-view-objects
* Python 3 dict interface (:ref:`dictionary view objects`:
  * dict.keys(), dict.values(), dict.items()
  * list(dict.keys()), list(dict.values()), list(dict.items())
  * https://docs.python.org/3/library/stdtypes.html#dict
  * https://docs.python.org/3/library/stdtypes.html#dict.values
  * https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects


* In order to compare OrderedDict.values() **by value**
  you must either:

  * Cast values() to a sequence (e.g. a list) before comparison
  * Subclass :class:`OrderedDict` and wrap `values()`


.. code:: python

    from collections import OrderedDict
    a = 'a'
    x = 1
    y = x
    ab  =  [( a,  x), ('b', 2)]
    ba  =  [('b', 2),  (a,  y)]
    ab_odict      = OrderedDict(ab)
    ab_odict_     = OrderedDict(ab)
    ab_odict_copy = OrderedDict(ab.copy())
    ba_odict      = OrderedDict(ba)
    ab_dict = dict(ab)
    ba_dict = dict(ba)

    # In Python 3,
    # OrderedDict.values.__eq__ does not compare by value:
    assert (     ab_odict.values()  ==      ab_odict_.values()) is False
    assert (list(ab_odict.values()) == list(ab_odict_.values()) is True

    # In Python 2.7 and 3,
    # OrderedDict.__eq__ compares ordered sequences
    assert (ab_odict == ab_odict_)     is True
    assert (ab_odict == ab_odict_copy) is True
    assert (ab_odict == ba_odict) is False
    assert (ab_dict  == ba_dict)  is True


    # - [ ] How to explain the x, y part?
    #   - in terms of references, __eq__, id(obj), __hash__

```


On Wed, Jan 20, 2016 at 12:39 PM, Sven R. Kunze <srkunze at mail.de> wrote:

> Documentation is a very good idea.
>
> Maybe, even raise an error when comparing values.
>
> Best,
> Sven
>
>
> On 20.01.2016 12:13, Alexandre Figura wrote:
>
> If we put technical considerations aside, maybe we should just ask to
> ourselves what behavior do we expect when doing equality tests between
> ordered dictionaries. As a reminder:
>
> >>> xy = OrderedDict([('x', None), ('y', None)])
> >>> yx = OrderedDict([('y', None), ('x', None)])
> >>> xy == yx
> False
> >>> xy.items() == yx.items()
> True
> >>> xy.keys() == yx.keys()
> True
> >>> xy.values() == yx.values()
> False
>
> So, it appears that:
> 1. equality tests between odict_values use objects identity and not
> equality,
> 2. equality tests between odict_keys do not respect order.
>
> If it is not technically possible to change the current implementation,
> maybe all we can do is just add a warning about current behavior in the
> documentation?
>
> On Mon, Jan 11, 2016 at 4:17 AM, Guido van Rossum < <guido at python.org>
> guido at python.org> wrote:
>
>> Seems like we dropped the ball... Is there any action item here?
>>
>> --
>> --Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
>>
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>
>
>
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