[Python-Dev] subclassing builtin data structures (original) (raw)

Neil Girdhar mistersheik at gmail.com
Sat Feb 14 01:05:17 CET 2015

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 makeme(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 somemethod(self, ...): > return self.makeme(SomeCls, assumed arguments…) > > > Now the derived class knows who is asking for a copy. In the case of defaultdict, for example, he can implement makeme as follows: > > > def makeme(self, cls, *args, **kwargs): > if cls is dict: return defaultdict(self.defaultfactory, *args, **kwargs) > return defaultdict(*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 makeme(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.makeme(DefaultDict, None, *args, **kwargs) # subclasses that know about DefaultDict but not dict will intercept this ........else: ............super(DefaultDict, cls).makeme(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 "", line 1, in > > TypeError: new() takes 2 positional arguments but 4 were given > > >>> Date2.fromordinal(1) > > Traceback (most recent call last): > > File "", line 1, in > > 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) > > > > > > > > > > > > > > > > > > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20150213/d5eaf48f/attachment.html>

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