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)