Unlike a regular method, you would never need to call super since you should know everyone that could be calling you. Typically, when you call super, you have something like this:
A < B, C B < D so you end up with mro: A, B, C, D And then when A calls super and B calls super it gets C which it doesn't know about. But in the case of make_me, it's someone like C who is calling make_me. If it gets a method in B, then that's a straight-up bug. make_me needs to be reimplemented in A as well, and A would never delegate up since other classes in the mro chain (like B) might not know about C. Best, Neil On Fri, Feb 13, 2015 at 7:00 PM, Isaac Schwabacher <ischwabac...@wisc.edu> wrote: > 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) > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
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