How to intercept class creation and add attribute using a metaclass?

Question

In the following code, I want metaclass NameMeta to add attribute gender to MyName class in case this class does not declare that attribute.

class NameMeta(type):
    def __new__(cls, name, bases, dic):
        if 'gender' not in dic:
            setattr(name, 'gender', 'Male')
        return super().__new__(cls, name, bases, dic)

class MyName(metaclass=NameMeta):
    def __init__(self, fname, lname):
        self.fname = fname
        self.lname = lname       
    def fullname(self):
        self.full_name = self.fname + self.lname
        return self.full_name 
inst = MyName('Joseph ', 'Vincent')
print(MyName.gender)

This is the output that I am getting:

<ipython-input-111-550ff3cfae41> in __new__(cls, name, bases, dic)
      2     def __new__(cls, name, bases, dic):
      3         if 'gender' not in dic:
----> 4             setattr(name, 'gender', 'Male')
      5         return super().__new__(cls, name, bases, dic)
      6 

AttributeError: 'str' object has no attribute 'gender'

I know this error makes sense since name is a string. My question is, how can I access MyName class as an object in the metaclass so that I can add the attribute?

Answer 1

You were close. Your problem is that you were trying to add your attribute to the name of the meta-class using name, which is a string. You need to assign the attribute to the class object you're creating. This can be done using dic:

class NameMeta(type):
    def __new__(cls, name, bases, dic):
        if 'gender' not in dic:
            dic['gender'] = 'Male'
        return super().__new__(cls, name, bases, dic)

With the above change your code outputs:

Male

Answer 2

You can just add it to the dic if it is not present, as it holds the class's attribute:

def __new__(mcs, name, bases, dict):
    if 'gender' not in dict:
        dict['gender'] = 'Male'
    # or just `dict.setdefault('gender', 'Male')`
    return super().__new__(mcs, name, bases, dic)

    # Or you can create the class and set it

    cls = super().__new__(mcs, name, bases, dic)
    if not hasattr(cls, 'gender'):
        cls.gender = 'Male'
    return cls

Or you could have a class attribute:

class NameMeta(type):
    gender = 'Male'
    # `gender = 'Male'` will be inherited by all classes
    # but not instances of those classes.