On 3/9/2012 5:10 PM, Ethan Furman wrote:
Hey all!
I posted a question/answer on SO earlier, but there seems to be some
confusion around either the question or the answer (judging from the
comments).
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.
b. Put such tests in unit tests.
c. Check the code object to see if the attribute will be created.
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 ;-).
--
Terry Jan Reedy
--
http://mail.python.org/mailman/listinfo/python-list
