I personally don't think this is a big enough issue to warrant any changes, but I think Serhiy's solution would be the ideal best with one additional parameter: the caller's type. Something like def __make_me__(self, cls, *args, **kwargs) and the idea is that any time you want to construct a type, instead of self.__class__(assumed arguments…) where you are not sure that the derived class' constructor knows the right argument types, you do def SomeCls: def some_method(self, ...): return self.__make_me__(SomeCls, assumed arguments…) Now the derived class knows who is asking for a copy. In the case of defaultdict, for example, he can implement __make_me__ as follows: def __make_me__(self, cls, *args, **kwargs): if cls is dict: return default_dict(self.default_factory, *args, **kwargs) return default_dict(*args, **kwargs) essentially the caller is identifying himself so that the receiver knows how to interpret the arguments. Best, Neil On Fri, Feb 13, 2015 at 5:55 PM, 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-t...
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)