Is there any reason to choose __new__ over __init__ when defining a metaclass?

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I've always set up metaclasses something like this:

class SomeMetaClass(type):
    def __new__(cls, name, bases, dict):
        #do stuff here

But I just came across a metaclass that was defined like this:

class SomeMetaClass(type):
    def __init__(self, name, bases, dict):
        #do stuff here

Is there any reason to prefer one over the other?

Update: Bear in mind that I'm asking about using __new__ and __init__ in a metaclass. I already understand the difference between them in another class. But in a metaclass, I can't use __new__ to implement caching because __new__ is only called upon class creation in a metaclass.

metaclass init new-operator constructor python
5 Answers

Several differences, in fact.

For one thing, the first argument in __new__ and __init__ are not the same, which is not made clear by everyone insisting on just using cls in both cases (despite the fact that the variable name doesn't hold any particular meaning). Someone pointed this out and it's core to understanding the difference:

  • __new__ gets the metaclass - MyType in my example (remember the application-level class is not created yet). This is where you can alter bases (which can cause MRO resolution errors if you're not careful). I'll call that variable mcls, to differentiate it from the usual cls referring to application level class.

  • __init__ gets the newly-created application-level class, Bar and Foo and, by that time, this class's namespace has been populated, see cls_attrib in example below. I'll stick to cls as per usual naming convention.

Sample code:

class Mixin:
    pass

class MyType(type):


    def __new__(mcls, name, bases, attrs, **kwargs):
        print("  MyType.__new__.mcls:%s" % (mcls))

        if not Mixin in bases:
            #could cause MRO resolution issues, but if you want to alter the bases
            #do it here
            bases += (Mixin,)

        #The call to super.__new__ can also modify behavior:
        #                                    classes Foo and Bar are instances of MyType
        return super(MyType, mcls).__new__(mcls, name, bases, attrs)

        #now we're back to the standard `type` 
        #doing this will neuter most of the metaclass behavior, __init__ wont
        #be called.                         
        #return super(MyType, mcls).__new__(type, name, bases, attrs)

    def __init__(cls, name, bases, attrs):
        print("  MyType.__init__.cls:%s." % (cls))
        
        #I can see attributes on Foo and Bar's namespaces
        print("    %s.cls_attrib:%s" % (cls.__name__, getattr(cls, "cls_attrib", None)))
        return super().__init__(name, bases, attrs)


print("\n Foo class creation:")
class Foo(metaclass=MyType):
    pass


print("\n bar class creation:")
class Bar(Foo):
    #MyType.__init__ will see this on Bar's namespace
    cls_attrib = "some class attribute"

output:

 Foo class creation:
  MyType.__new__.mcls:<class '__main__.test.<locals>.MyType'>
  MyType.__init__.cls:<class '__main__.test.<locals>.Foo'>.
    Foo.cls_attrib:None

 Bar class creation:
  MyType.__new__.mcls:<class '__main__.test.<locals>.MyType'>
  MyType.__init__.cls:<class '__main__.test.<locals>.Bar'>.
    Bar.cls_attrib:some class attribute
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