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[Python-Dev] Return type of datetime subclasses added to timedelta


OK, I approved the PR. Can some other core dev ensure that it gets merged?
No backports though!

On Mon, Feb 4, 2019 at 8:46 AM Paul Ganssle <paul at ganssle.io> wrote:

> There's already a PR, actually, #10902:
> https://github.com/python/cpython/pull/10902
>
> Victor reviewed and approved it, I think before I started this thread, so
> now it's just waiting on merge.
> On 2/4/19 11:38 AM, Guido van Rossum wrote:
>
> I recommend that you submit a PR so we can get it into 3.8 alpha 2.
>
> On Mon, Feb 4, 2019 at 5:50 AM Paul Ganssle <paul at ganssle.io> wrote:
>
>> Hey all,
>>
>> This thread about the return type of datetime operations seems to have
>> stopped without any explicit decision - I think I responded to everyone who
>> had objections, but I think only Guido has given a +1 to whether or not we
>> should go ahead.
>>
>> Have we got agreement to go ahead with this change? Are we still
>> targeting Python 3.8 here?
>>
>> For those who don't want to dig through your old e-mails, here's the
>> archive link for this thread:
>> https://mail.python.org/pipermail/python-dev/2019-January/155984.html
>>
>> If you want to start commenting on the actual implementation, it's
>> available here (though it's pretty simple):
>> https://github.com/python/cpython/pull/10902
>>
>> Best,
>>
>> Paul
>>
>>
>> On 1/6/19 7:17 PM, Guido van Rossum wrote:
>>
>> OK, I concede your point (and indeed I only tested this on 3.6). If we
>> could break the backward compatibility for now() we presumably can break it
>> for this purpose.
>>
>> On Sun, Jan 6, 2019 at 11:02 AM Paul Ganssle <paul at ganssle.io> wrote:
>>
>>> I did address this in the original post - the assumption that the
>>> subclass constructor will have the same arguments as the base constructor
>>> is baked into many alternate constructors of datetime. I acknowledge that
>>> this is a breaking change, but it is a small one - anyone creating such a
>>> subclass that *cannot* handled the class being created this way would
>>> be broken in myriad ways.
>>>
>>> We have also in recent years changed several alternate constructors
>>> (including `replace`) to retain the original subclass, which by your same
>>> standard would be a breaking change. I believe there have been no
>>> complaints. In fact, between Python 3.6 and 3.7, the very example you
>>> showed broke:
>>>
>>> Python 3.6.6:
>>>
>>> >>> class D(datetime.datetime):
>>> ...     def __new__(cls):
>>> ...         return cls.now()
>>> ...
>>> >>> D()
>>> D(2019, 1, 6, 13, 49, 38, 842033)
>>>
>>> Python 3.7.2:
>>>
>>> >>> class D(datetime.datetime):
>>> ...     def __new__(cls):
>>> ...         return cls.now()
>>> ...
>>> >>> D()
>>> Traceback (most recent call last):
>>>   File "<stdin>", line 1, in <module>
>>>   File "<stdin>", line 3, in __new__
>>> TypeError: __new__() takes 1 positional argument but 9 were given
>>>
>>>
>>> We haven't seen any bug reports about this sort of thing; what we *have*
>>> been getting is bug reports that subclassing datetime doesn't retain the
>>> subclass in various ways (because people *are* using datetime
>>> subclasses). This is likely to cause very little in the way of problems,
>>> but it will improve convenience for people making datetime subclasses and
>>> almost certainly performance for people using them (e.g. pendulum and
>>> arrow, which now need to take a slow pure python route in many situations
>>> to work around this problem).
>>>
>>> If we're *really* concerned with this backward compatibility breaking,
>>> we could do the equivalent of:
>>>
>>> try:
>>>     return new_behavior(...)
>>> except TypeError:
>>>     warnings.warn("The semantics of timedelta addition have "
>>>                   "changed in a way that raises an error in "
>>>                   "this subclass. Please implement __add__ "
>>>                   "if you need the old behavior.", DeprecationWarning)
>>>
>>> Then after a suitable notice period drop the warning and turn it to a
>>> hard error.
>>>
>>> Best,
>>>
>>> Paul
>>> On 1/6/19 1:43 PM, Guido van Rossum wrote:
>>>
>>> I don't think datetime and builtins like int necessarily need to be
>>> aligned. But I do see a problem -- the __new__ and __init__ methods defined
>>> in the subclass (if any) should allow for being called with the same
>>> signature as the base datetime class. Currently you can have a subclass of
>>> datetime whose __new__ has no arguments (or, more realistically, interprets
>>> its arguments differently). Instances of such a class can still be added to
>>> a timedelta. The proposal would cause this to break (since such an addition
>>> has to create a new instance, which calls __new__ and __init__). Since this
>>> is a backwards incompatibility, I don't see how it can be done -- and I
>>> also don't see many use cases, so I think it's not worth pursuing further.
>>>
>>> Note that the same problem already happens with the .fromordinal() class
>>> method, though it doesn't happen with .fromdatetime() or .now():
>>>
>>> >>> class D(datetime.datetime):
>>> ...   def __new__(cls): return cls.now()
>>> ...
>>> >>> D()
>>> D(2019, 1, 6, 10, 33, 37, 161606)
>>> >>> D.fromordinal(100)
>>> Traceback (most recent call last):
>>>   File "<stdin>", line 1, in <module>
>>> TypeError: __new__() takes 1 positional argument but 4 were given
>>> >>> D.fromtimestamp(123456789)
>>> D(1973, 11, 29, 13, 33, 9)
>>> >>>
>>>
>>> On Sun, Jan 6, 2019 at 9:05 AM Paul Ganssle <paul at ganssle.io> wrote:
>>>
>>>> I can think of many reasons why datetime is different from builtins,
>>>> though to be honest I'm not sure that consistency for its own sake is
>>>> really a strong argument for keeping a counter-intuitive behavior - and to
>>>> be honest I'm open to the idea that *all* arithmetic types *should*
>>>> have some form of this change.
>>>>
>>>> That said, I would say that the biggest difference between datetime and
>>>> builtins (other than the fact that datetime is *not* a builtin, and as
>>>> such doesn't necessarily need to be categorized in this group), is that
>>>> unlike almost all other arithmetic types, *datetime* has a special,
>>>> dedicated type for describing differences in datetimes. Using your example
>>>> of a float subclass, consider that without the behavior of "addition of
>>>> floats returns floats", it would be hard to predict what would happen in
>>>> this situation:
>>>>
>>>> >>> F(1.2) + 3.4
>>>>
>>>> Would that always return a float, even though F(1.2) + F(3.4) returns
>>>> an F? Would that return an F because F is the left-hand operand? Would it
>>>> return a float because float is the right-hand operand? Would you walk the
>>>> MROs and find the lowest type in common between the operands and return
>>>> that? It's not entirely clear which subtype predominates. With datetime,
>>>> you have:
>>>>
>>>> datetime - datetime -> timedelta
>>>> datetime ? timedelta -> datetime
>>>> timedelta ? timedelta -> timedelta
>>>>
>>>> There's no operation between two datetime objects that would return a
>>>> datetime object, so it's always clear: operations between datetime
>>>> subclasses return timedelta, operations between a datetime object and a
>>>> timedelta return the subclass of the datetime that it was added to or
>>>> subtracted from.
>>>>
>>>> Of course, the real way to resolve whether datetime should be different
>>>> from int/float/string/etc is to look at why this choice was actually made
>>>> for those types in the first place, and decide whether datetime is like
>>>> them *in this respect*. The heterogeneous operations problem may be a
>>>> reasonable justification for leaving the other builtins alone but changing
>>>> datetime, but if someone knows of other fundamental reasons why the
>>>> decision to have arithmetic operations always create the base class was
>>>> chosen, please let me know.
>>>>
>>>> Best,
>>>> Paul
>>>> On 1/5/19 3:55 AM, Alexander Belopolsky wrote:
>>>>
>>>>
>>>>
>>>> On Wed, Jan 2, 2019 at 10:18 PM Paul Ganssle <paul at ganssle.io> wrote:
>>>>
>>>>> .. the original objection was that this implementation assumes that
>>>>> the datetime subclass has a constructor with the same (or a sufficiently
>>>>> similar) signature as datetime.
>>>>>
>>>> While this was used as a possible rationale for the way standard types
>>>> behave, the main objection to changing datetime classes is that it will
>>>> make them behave differently from builtins.  For example:
>>>>
>>>> >>> class F(float):
>>>> ...     pass
>>>> ...
>>>> >>> type(F.fromhex('AA'))
>>>> <class '__main__.F'>
>>>> >>> type(F(1) + F(2))
>>>> <class 'float'>
>>>>
>>>> This may be a legitimate gripe, but unfortunately that ship has sailed
>>>>> long ago. All of datetime's alternate constructors make this assumption.
>>>>> Any subclass that does not meet this requirement must have worked around it
>>>>> long ago (or they don't care about alternate constructors).
>>>>>
>>>>
>>>> This is right, but the same argument is equally applicable to int,
>>>> float, etc. subclasses.  If you want to limit your change to datetime types
>>>> you should explain what makes these types special.
>>>>
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>>>>
>>>
>>>
>>> --
>>> --Guido van Rossum (python.org/~guido)
>>>
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>>
>>
>> --
>> --Guido van Rossum (python.org/~guido)
>>
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>
>
> --
> --Guido van Rossum (python.org/~guido)
>
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-- 
--Guido van Rossum (python.org/~guido)
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