[Tutor] Challenge supporting custom deepcopy with inheritance

W W srilyk at gmail.com
Mon Jun 1 12:27:52 CEST 2009

On Mon, Jun 1, 2009 at 2:27 AM, Alan Gauld <alan.gauld at btinternet.com>wrote:

> "Kent Johnson" <kent37 at tds.net> wrote
>> > Yesterday, I posted a question to python-list involving custom
>> > deepcopies in an inheritance hierarchy. I haven't received any
>> ISTM that in general B.__deepcopy__() should call A.__deepcopy__() to do
>> the "A" part of the work. In your example, this won't work because
>> A.__deepcopy__() assumes that subclasses have a one-argument constructor.
>> So, I would say that B is not fulfilling the contract assumed by A.
> I've been trying to think of a clear way to reply to this but kept getting
> sucked into discussions of the Liskoff Substitution Principle and Law of
> Demeter and such. Kent's explanation is much clearer!
> But this example highlights a real problem (IMHO) with dynamic OOP
> languages like Python. You can write classes that examine themselves at
> runtime and manipulate attributes that were actually
> provided by subclasses (or even by the application writer!). This makes any
> attempt at things like deepcopy fraught with difficulty because the clear
> separation of concerns between parent and subclass has been broken.
> It is very important for good OO design that classes only operate on their
> own data. But when you can dynamically add attributes to instances after
> theit creation, as you can in Python, it's almost impossible to distinguish
> between attributes of the parent class and the subclass. It's one of the
> penalties of Python's dynamic nature and there is no easy solution  to the
> general case. In the specific case you need to read the code of both parent
> and subclass and the application(s) using them!
>  What if you give B a one-arg constructor by making the bTag argument
>> optional? Then I think B.__deepcopy__() can call A.__deepcopy__(), then do
>> the "B" part of the copy on the result.
> As a minimum subclasses should adhere to the parent interface.
> Unfortunately because Python only allows a single constructor that can be a
> limiting factor :-(
> ( Multiple constructors (or factory methods) is one feature I  would like
> to see added to Python! )

Wouldn't it be possible to create sort of a... bastardization? i.e.

def __init__(self, *args):
    if len(args) == 0:
        #do something
    if len(args) == 1:
       #do something else


Or would that create more problems than is worth it?

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