8

Based on my limited Python knowledge Metaclass is another class. In the following example I am just building a metaclass. Here I overloaded the __new__ method. My expectation is at the end of the class declaration when type.__new__() is called it will be delegated to MyMeta.__new__(). But that never happens.

print('Building MetaOne') class MyMeta(type): def __new__(meta, classname, supers, classdict): print('In MyMeta:', classname) def myfunc(self): print ('In myfunc')

Here from MyMeta's __new__() I don't see the print.

But on the contrary if I now use MyMeta class as a meta class to another class like:

print('Building Another class') class Super(metaclass = MyMeta): def myfunc(self): print ('In Super myfunc')

At the end of this class I see MyMeta's __new__ is getting invoked.

Can some one please help me understand why this happens?

Improve this question
4
  • 1
    No, __new__ is not called when you created the meta class; . Super is an instance of the metatype, a class object, so then __new__ is called to create it. Note that your __new__ method returns None, rather than a new class object. You may want to add return super().__new__(meta, classname, supers, classdict) to the end of it. Commented Dec 26, 2017 at 23:39
  • You can use __new__ on a class definition as well, and it would only be called when you call the class to create new instances. Commented Dec 26, 2017 at 23:39
  • Thanks Martin. Is this because when I create class object MyMeta (or any other class object), only the _new()_ function of it's metaclass or metaclass of the base classes get called ? Commented Dec 26, 2017 at 23:49
  • exactly, which is type(). You can't create meta metaclasses. Commented Dec 27, 2017 at 0:17
Load 3 more related questions Show fewer related questions

0

Reset to default

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.