You're right. On Fri, Feb 13, 2015 at 7:55 PM, Isaac Schwabacher <ischwabac...@wisc.edu> wrote:
> On 15-02-13, Neil Girdhar wrote: > > 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 C calls super and gets D. The scenario I'm concerned with is that A > knows how to mimic B's constructor and B knows how to mimic D's, but A > doesn't know about D. So D asks A if it knows how to mimic D's constructor, > and it says no. Via super, B gets a shot, and it does know, so it > translates the arguments to D's constructor into arguments to B's > constructor, and again asks A if it knows how to handle them. Then A says > yes, translates the args, and constructs an A. If C ever gets consulted, it > responds "I don't know a thing" and calls super. > > > 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. > > This scheme (as I've written it) depends strongly on all the classes in > the MRO having __make_me__ methods with this very precisely defined > structure: test base against yourself, then any superclasses you care to > mimic, then call super. Any antisocial superclass ruins everyone's party. > > > Best, > > Neil > > > > > > On Fri, Feb 13, 2015 at 7:00 PM, Isaac Schwabacher < > alexander.belopol...@gmail.com <ischwabac...@wisc.edu')" target="1"> > 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 < > http://stackoverflow.com/questions/5490824/should-constructors-comply-with-the-liskov-substitution-principle(javascript:main.compose('new', > 't=alexander.belopol...@gmail.com>(java_script:main.compose()> wrote: > > > > > > > > > > > > > > On Fri, Feb 13, 2015 at 4:44 PM, Neil Girdhar < > mistersh...@gmail.com <mistersh...@gmail.com>(java_script:main.compose()> > wrote: > > > > > > > > > > > Interesting: > > 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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