Question

I read What is a metaclass in Python?

and I tried to replicate the upper metaclass from the example and found that this doesn’t work in all cases:

def upper(cls_name, cls_parents, cls_attr):                                     
    """ Make all class attributes uppper case """                               
    attrs = ((name, value) for name, value in cls_attr.items()                  
            if not name.startswith('__'))                                       
    upper_atts = dict((name.upper(), value) for name, value in attrs)           
    return type(cls_name, cls_parents, upper_atts)                              

__metaclass__ = upper #Module level
class Foo:                                                                      
    bar = 1                                                                     
f =  Foo()
print(f.BAR) #works in python2.6

The above fails (with an attribute error) in python3 which I think is natural because all classes in python3 already have object as their parent and metaclass resolution goes into the object class.

The question:

How do I make a module level metaclass in python3?

Answer 1

The module level metaclass isn’t really “module level”, it has to do with how class initialization worked. The class creation would look for the variable "__metaclass__" when creating the class, and if it wasn’t in the local environment it would look in the global. Hence, if you had a “module level” __metaclass__ that would be used for every class afterwards, unless they had explicit metaclasses.

In Python 3, you instead specify the metaclass with a metaclass= in the class definition. Hence there is no module level metaclasses.

So what do you do? Easy: You specify it explicitly for each class.

It’s really not much extra work, and you can even do it with a nice regexp search and replace if you really have hundreds of classes and don’t want to do it manually.