[Python-Dev] subclassing builtin data structures

[Isaac Schwabacher]

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):
.... at 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.belopolsky at gmail.com(javascript:main.compose()> wrote:
> 
> > 
> > On Fri, Feb 13, 2015 at 4:44 PM, Neil Girdhar <mistersheik at 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)
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >