On 14 Feb 2015 08:57, "Alexander Belopolsky" <alexander.belopolsky@gmail.com> wrote:
>
>
> On Fri, Feb 13, 2015 at 4:44 PM, Neil Girdhar <mistersheik@gmail.com> wrote:
>>
>> Interesting: http://stackoverflow.com/questions/5490824/should-constructors-comply-with-the-liskov-substitution-principle
>
>
> Let me humbly conjecture that the people who wrote the top answers have background in less capable languages than Python.
>
> Not every language allows you to call self.__class__().  In the languages that don't you can get away with incompatible constructor signatures.
>
> However, let me try to focus the discussion on a specific issue before we go deep into OOP theory.
>
> With python's standard datetime.date we have:
>
> >>> from datetime import *
> >>> class Date(date):
> ...     pass
> ...
> >>> Date.today()
> Date(2015, 2, 13)
> >>> Date.fromordinal(1)
> Date(1, 1, 1)
>
> Both .today() and .fromordinal(1) will break in a subclass that redefines __new__ as follows:
>
> >>> class Date2(date):
> ...     def __new__(cls, ymd):
> ...         return date.__new__(cls, *ymd)
> ...
> >>> Date2.today()
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: __new__() takes 2 positional arguments but 4 were given
> >>> Date2.fromordinal(1)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: __new__() takes 2 positional arguments but 4 were given
>
> Why is this acceptable, but we have to sacrifice the convenience of having Date + timedelta
> return Date to make it work  with Date2:
>
> >>> Date2((1,1,1)) + timedelta(1)
> datetime.date(1, 1, 2)

Coupling alternative constructors to the default constructor signature is pretty normal - it just means that if you override the signature of the default constructor, you may need to update the alternative ones accordingly.

Cheers,
Nick.



>
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