stackoverflow question

Ethan Furman ethan at
Fri Mar 9 23:10:18 CET 2012

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 

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.  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.

What I need is some way to have:

     class Test(metaclass=Meta):
         mutable = list()

     t1 = Test()
     t2 = Test()
     t1.mutable  # prints ['one']
     t2.mutable  # prints ['two']

Any ideas on how this can be accomplished?

Also, the metaclass doing the checking doesn't care what type of object 
the attribute is, only that it is there.

and the answer text:
There are a couple ways to do this:

   1. Have the metaclass check all the attributes, and if they are one 
of the mutables (`list`, `dict`, `set`, etc.) replace the attribute with 
a descriptor that will activate on first access and update the instance 
with a fresh copy of the mutable.

   2. Provide the descriptor from (1) as a decorator to be used when 
writing the class.

I prefer (2) is it gives complete control to the class author, and 
simplifies those cases where the class-level mutable attribute *should* 
be shared amongst all the instances.

Here's the decorator-descriptor:

     class ReplaceMutable:
         def __init__(self, func):
             self.func = func
         def __call__(self):
             return self
         def __get__(self, instance, owner):
             if instance is None:
                 return self
             result = self.func()
             setattr(instance, self.func.__name__, result)
             return result

and the test class:

     class Test:
         def mutable():
             return list()

     t1 = Test()
     t2 = Test()

How it works:

Just like `property`, `ReplaceMutable` is a descriptor object with the 
same name as the attribute it is replacing.  Unlike `property`, it does 
not define `__set__` nor `__delete__`, so when code tries to rebind the 
name (`mutable` in the test above) in the instance Python will allow it 
to do so.  This is the same idea behind caching descriptors.

`ReplaceMutable` is decorating a function (with the name of the desired 
attribute) that simply returns whatever the instance level attribute 
should be initialized with (an empty `list` in the example above).  So 
the first time the attribute is looked up on an instance it will not be 
found in the instance dictionary and Python will activate the 
descriptor; the descriptor then calls the function to retrieve the 
initial object/data/whatever, stores it in the instance, and then 
returns it.  The next time that attribute is accessed *on that instance* 
it will be in the instance dictionary, and that is what will be used.


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