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

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:

??? 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.



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
> <mailto: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
>>     <mailto: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 <http://python.org/~guido>)
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