In metaclass, when to use __new__ vs. __init__?

[Matthew Wilson]

I have been experimenting with metaclasses lately. It seems possible to
define a metaclass by either subclassing type and then either redefining
__init__ or __new__.

Here's the signature for __init__:

def __init__(cls, name, bases, d):

and here's __new__:

def __new__(meta, classname, bases, d):

Every metaclass I have found monkeys with d, which is available in both
methods. So when is it better to use one vs the other?

Thanks for the help.

Matt

 

[bruno.desthuilliers]

I have been experimenting with metaclasses lately. It seems possible to
define a metaclass by either subclassing type and then either redefining
__init__ or __new__.

Here's the signature for __init__:

def __init__(cls, name, bases, d):

and here's __new__:

def __new__(meta, classname, bases, d):

Every metaclass I have found monkeys with d, which is available in both
methods. So when is it better to use one vs the other?

Well... The __new__ method is responsible for creating and returning a
new instance (so in this case, a new class), which is then passed to
the __init__ method. So which one you want to use depends on what you
want to do. If you only want to add some attributes/methods, register
either the class or some of it's methods somewhere etc, then __init__
is fine. If you have to transform / replace / whatever some of the
(not yet) attributes, fiddle with the inheritance tree, cache the
class object or such things, you'd better do it in __new__.