On 15-02-13, Neil Girdhar wrote: > 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
Such a method necessarily involves explicit switching on classes... ew. Also, to make this work, a class needs to have a relationship with its superclass's superclasses. So in order for DefaultDict's subclasses not to need to know about dict, it would need to look like this: class DefaultDict(dict): ....@classmethod # instance method doesn't make sense here ....def __make_me__(cls, base, *args, **kwargs): # make something like base(*args, **kwargs) ........# when we get here, nothing in cls.__mro__ above DefaultDict knows how to construct an equivalent to base(*args, **kwargs) using its own constructor ........if base is DefaultDict: ............return DefaultDict(*args, **kwargs) # if DefaultDict is the best we can do, do it ........elif base is dict: ............return cls.__make_me__(DefaultDict, None, *args, **kwargs) # subclasses that know about DefaultDict but not dict will intercept this ........else: ............super(DefaultDict, cls).__make_me__(base, *args, **kwargs) # we don't know how to make an equivalent to base.__new__(*args, **kwargs), so keep looking I don't even think this is guaranteed to construct an object of class cls corresponding to a base(*args, **kwargs) even if it were possible, since multiple inheritance can screw things up. You might need to have an explicit list of "these are the superclasses whose constructors I can imitate", and have the interpreter find an optimal path for you. > On Fri, Feb 13, 2015 at 5:55 PM, Alexander Belopolsky > <alexander.belopol...@gmail.com(javascript:main.compose()> wrote: > > > > > On Fri, Feb 13, 2015 at 4:44 PM, Neil Girdhar > > <mistersh...@gmail.com(javascript:main.compose()> 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) > > > > > > > > > > > > > > > > > > > > _______________________________________________ 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/archive%40mail-archive.com
