[Python-Dev] subclassing builtin data structures

[Neil Girdhar]

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 <ischwabacher at 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):
> .... 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)
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
>
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