Example of metaclass

metaclass_example.py

class ModelBase(type):
    a_variable_from_metaclass = 3
    _instances = dict() # keep track of the instances created per class

    @classmethod
    def __prepare__(metacls, name, bases, **kwargs):  # called first
        print(f"\nInside MetaClass __prepare__, name: {name}, bases: {bases}, metacls: {metacls}, kwargs: {kwargs}")
        namespace = super().__prepare__(name, bases, **kwargs)
        print(f"Returned namespace: {namespace}")
        return namespace

    def __new__(cls, name, bases, namespaces, **kwargs):  # called second, and constructs the new class
        print(
            f"\nInside MetaClass __new__(), name: {name}, bases: {bases}, cls: {cls}, kwargs: {kwargs}, namespaces: {namespaces}")

        namespaces["some_variable"] = 5  # setting a variable
        for attribute_name, attribute_value in namespaces.items():
            # iterate through the provided namespace, and change all the integer values to instances of ModelField
            if isinstance(attribute_value, int):
                namespaces[attribute_name] = ModelField(attribute_value)
        new_class = super().__new__(cls, name, bases, namespaces, **kwargs)
        print(f"Output of __new__: {new_class}")
        return new_class

    def __init__(cls, name, bases, namespace, **kwargs):  # called third, initializes the newly created class with values
        # By the time __init__ is called, the new class exists
        print("\nInside __init__ of MetaClass")
        print(f"cls: {cls}, name: {name}, bases: {bases}, namespace: {namespace}, kwargs: {kwargs}")
        ModelBase.__setattr__(cls, "some_variable", 4)
        cls.variable_added_during_init = "boom"
        namespace["some_variable"] = 4  # attempt to change the attribute some_variable's value to 4
        super().__init__(name, bases, namespace)

    def __call__(cls, *args, **kwargs):  # called when new instances are created, delegates initialization to __init__
        print(f"Inside __call__ of MetaClass.")
        print(f"cls: {cls}, args: {args}, kwargs: {kwargs}")
        if cls not in cls._instances: # https://stackoverflow.com/questions/6760685/creating-a-singleton-in-python
            cls._instances[cls] = super().__call__(*args, **kwargs)
        print(f"Output of __call__: {cls._instances[cls]}")
        return cls._instances[cls]

Hi @ychennay. Thank you for creating this. Isn't line 34-36 unnecessary?

Hi @ychennay. Thank you for creating this. Isn't line 34-36 unnecessary?

Yes you are right - silly typo. I've removed it.