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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 "", line 1, in
TypeError: \_\_new\_\_() takes 2 positional arguments but 4 were given
>>> Date2.fromordinal(1)
Traceback (most recent call last):
File "", line 1, in
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)