stackoverflow question

[Ethan Furman]

Terry Reedy wrote:

Thanks for the review, Terry!

> On 3/9/2012 5:10 PM, Ethan Furman wrote:
>>
>> http://stackoverflow.com/q/9638921/208880
>>
>> If anyone here is willing to take a look at it and let me know if I did
>> not write it well, I would appreciate the feedback
>>
>> Here's the question text:
>> ------------------------
>> I'm writing a metaclass to do some cool stuff, and part of its
>> processing is to check that certain attributes exist when the class is
>> created. Some of these are mutable, and would normally be set in
>> `__init__`, but since `__init__` isn't run until the instance is created
>> the metaclass won't know that the attribute *will* be created, and
>> raises an error.
 >
> a. Create an instance and see if it has the attribute. Then delete it.

If the class takes any arguments, I won't be able to create instances of it.

> b. Put such tests in unit tests.

Always a good idea -- but this is the responsibility of the class author 
(who may not be me ;).

> c. Check the code object to see if the attribute will be created.

I have no idea how to do this -- pointers?


>> I could do something like:
>>
>> class Test(meta=Meta):
>> mutable = None
>> def __init__(self):
>>     self.mutable = list()
>>
>> But that isn't very elegant, and also violates DRY.
> 
> It works. It is a standard idiom for default values that are 
> conditionally masked with an instance value.
>>
>> What I need is some way to have:
>>
>> class Test(metaclass=Meta):
>> mutable = list()
>>
>> t1 = Test()
>> t2 = Test()
>> t1.mutable.append('one')
>> t2.mutable.append('two')
>> t1.mutable # prints ['one']
>> t2.mutable # prints ['two']
>>
>> Any ideas on how this can be accomplished?
> 
> Rewrite the __init__ code object ;-).

Ouch.  Can that be done in a cross-implementation way?  Any pointers?

Still, I'm not sure I'd want to go this route, anyway, as it makes it 
more difficult to get an actual class-wide mutable (rare though they are).

Thanks again for your feedback, I really appreciate it.

~Ethan~