On Fri, Feb 13, 2015 at 4:44 PM, Neil Girdhar <[email protected]> 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)
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