On 14 Feb 2015 08:57, "Alexander Belopolsky" <alexander.belopol...@gmail.com> wrote: > > > On Fri, Feb 13, 2015 at 4:44 PM, Neil Girdhar <mistersh...@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. > > > > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: https://mail.python.org/mailman/options/python-dev/ncoghlan%40gmail.com >
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