>>> v = object()
>>> id(v)
4473343200

>>> v = "hello"
>>> x = v
>>> id(x) == id(v)
True
>>> x += "o"
>>> id(x) == id(v)
False

>>> v = "hello"
>>> type(v)
<class 'str'>
>>> v = 42
>>> type(v)
<class 'int'>

>>> class C: 
...     pass 

>>> id(C) 
140559438115784 

>>> dir(C) 
[ ... ] 

>>> type(C) 
<class 'type'>

>>> type(C) 
<class 'type'>

>>> class X: 
...      a = 1 

>>> X = type('X', (object,), dict(a=1))

>>> X
<class '__main__.X'>

class MyClass: 
    z = 1
    
    def f(self,):
        return x

from collections import OrederedDict

class MyMeta(type):
    @classmethod
    def __prepare__(mcs, name, bases, **kwargs):
        return OrderedDict()

class MyClass(metaclass=MyMeta):
    z = 1

    def f(self,):
        return x

print({k: v for k, v in MyClass.__dict__.items() if not k.startswith("__")})

# {'z': 1, 'f': <function MyClass.f at 0x10efc8bf8>}

class MyMeta(type):
    def __new__(mcs, *args, **kwargs):
        print(f"new class: {args}")
        r = super().__new__(mcs, *args, **kwargs)

        return r

class MyClass(metaclass=MyMeta):
    z = 1

    def f(self, x):
        return x

# new class: ('MyClass', (), {'__module__': '__main__', '__qualname__': 'MyClass', 'z': 1, 'f': <function MyClass.f at 0x101cc89d8>})
# same as type('MyClass', (), {...})

class MyMeta(type):
    def __call__(cls, *args, **kwargs):
        x = super().__call__(*args, **kwargs)
        print(f"A new object created: {x}")
        return x

class MyClass(metaclass=MyMeta):
    z = 1

    def f(self, x):
        return x

m = MyClass()
print(f"created object is {m}")

# A new object created: <__main__.MyClass object at 0x1073d54e0>
# created object is <__main__.MyClass object at 0x1073d54e0>

class DisallowMultipleInheritance(type):

    def __new__(mcs, *args, **kwargs):
        if len(args[1]) > 1:
            raise Exception("...")

        new_cls = super().__new__(mcs, *args, **kwargs)
        
        return new_cls

class Foo(metaclass=DisallowMultipleInheritance): 
    pass
 
class Bar: 
    pass
 
class Zee(Foo, Bar): 
    pass

Traceback (most recent call last):
  File "/.../disallow_multiple_inheritance.py", line 17, in <module>
    class Zee(Foo, Bar):
  File "/.../disallow_multiple_inheritance.py", line 4, in __new__
    raise Exception(f"Can't be subclassed with multiple inheritance: {args[1]}")

Exception: Can't be subclassed with multiple inheritance: (<Foo>, <Bar>)

class DisallowInheritance(type):
    def __new__(mcs, *args, **kwargs):
        cls = [c for c in args[1] if isinstance(c, mcs)]

        if cls:
            raise Exception(f"can't subclass {cls}")

        r = super().__new__(mcs, *args, **kwargs)

        return r

class UserForm(ModelForm):
    class Meta:
        model = User
        fields = ['name', 'email', 'birth_date']

class CheckMeta(type):
    def __new__(mcs, *args, **kwargs):
        name, bases, namespace = args

        if (not namespace.get("Meta", None)) and (bases != ()):
            raise Exception("Can not configure class. Meta is missing")

        r = super().__new__(mcs, *args, **kwargs)

        return r

class Singleton(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(...)

        return cls._instances[cls]

class C(metaclass=Singleton):
    pass

>>> a = C()

>>> b = C()

>>> print(a is b) 
 True

class Product:
    price = Price(name="price", unit="KRW") 

>>> pd = Product()

>>> pd.price = "NotANumber"
Please set a valid integer

>>> pd.price = 5000

>>> print(pd.price)
5,000 KRW

class Meta(type):
    def __new__(mcs, *args, **kwargs):
        name, bases, namespace = args

        self = super().__new__(mcs, name, bases, namespace)

        for k, v in self.__dict__.items():
            func = getattr(v, '__set_name__', None) 
            if func is not None:  # trigger hook if exists
                func(self, k)

        return self

class Desc:
    def __get__(self, instance, owner):
        return instance.__dict__[self.name]

    def __set__(self, instance, value):
        instance.__dict__[self.name] = value

    def __set_name__(self, owner, name):
        self.name = name

class C(metaclass=Meta):
    v = Desc()

>>> c = C()

>>> c.v = 3

>>> print(c.v == 3) 
 True

class TaskType(type):
    """Meta class for tasks.
    Automatically registers the task in the task registry, except
    if the `abstract` attribute is set.
    If no `name` attribute is provided, then no name is automatically
    set to the name of the module it was defined in, and the class name.
    """

    def __new__(cls, name, bases, attrs):
        new = super(TaskType, cls).__new__
        task_module = attrs.get("__module__") or "__main__"

        # - Abstract class: abstract attribute should not be inherited.
        if attrs.pop("abstract", None) or not attrs.get("autoregister", True):
            return new(cls, name, bases, attrs)

        # The 'app' attribute is now a property, with the real app located
        # in the '_app' attribute.  Previously this was a regular attribute,
        # so we should support classes defining it.
        _app1, _app2 = attrs.pop("_app", None), attrs.pop("app", None)
        app = attrs["_app"] = _app1 or _app2 or current_app

        # - Automatically generate missing/empty name.
        autoname = False
        if not attrs.get("name"):
            try:
                module_name = sys.modules[task_module].__name__
            except KeyError:  # pragma: no cover
                # Fix for manage.py shell_plus (Issue #366).
                module_name = task_module
            attrs["name"] = '.'.join(filter(None, [module_name, name]))
            autoname = True

        # - Create and register class.
        # Because of the way import happens (recursively)
        # we may or may not be the first time the task tries to register
        # with the framework.  There should only be one class for each task
        # name, so we always return the registered version.
        tasks = app._tasks

        # - If the task module is used as the __main__ script
        # - we need to rewrite the module part of the task name
        # - to match App.main.
        if MP_MAIN_FILE and sys.modules[task_module].__file__ == MP_MAIN_FILE:
            # - see comment about :envvar:`MP_MAIN_FILE` above.
            task_module = "__main__"
        if autoname and task_module == "__main__" and app.main:
            attrs["name"] = '.'.join([app.main, name])

        task_name = attrs["name"]
        if task_name not in tasks:
            tasks.register(new(cls, name, bases, attrs))
        instance = tasks[task_name]
        instance.bind(app)
        return instance.__class__

class TaskType(type):
    def __new__(cls, name, bases, attrs):
        # get __new__ form super
        # get app from attrs. if not exists, get from current_app
        # automatically generate missing/empty name.

        ...

        tasks = app._tasks

        ...

        task_name = attrs["name"]

        if task_name not in tasks:
            tasks.register(new(cls, name, bases, attrs))

        instance = tasks[task_name]
        instance.bind(app)

        return instance.__class__

class Form(BaseForm, metaclass=DeclarativeFieldsMetaclass):
    "A collection of Fields, plus their associated data."
    # This is a separate class from BaseForm in order to abstract the way
    # self.fields is specified. This class (Form) is the one that does the
    # fancy metaclass stuff purely for the semantic sugar -- it allows one
    # to define a form using declarative syntax.
    # BaseForm itself has no way of designating self.fields.

class DeclarativeFieldsMetaclass(MediaDefiningClass):
    """Collect Fields declared on the base classes."""
    def __new__(mcs, name, bases, attrs):
        # Collect fields from current class.
        current_fields = []
        for key, value in list(attrs.items()):
            if isinstance(value, Field):
                current_fields.append((key, value))
                attrs.pop(key)
        attrs['declared_fields'] = dict(current_fields)

        new_class = super(DeclarativeFieldsMetaclass, mcs).__new__(mcs, name, bases, attrs)

        # Walk through the MRO.
        declared_fields = {}
        for base in reversed(new_class.__mro__):
            # Collect fields from base class.
            if hasattr(base, 'declared_fields'):
                declared_fields.update(base.declared_fields)

            # Field shadowing.
            for attr, value in base.__dict__.items():
                if value is None and attr in declared_fields:
                    declared_fields.pop(attr)

        new_class.base_fields = declared_fields
        new_class.declared_fields = declared_fields

        return new_class

class DeclarativeFieldsMetaclass(MediaDefiningClass):
    """Collect Fields declared on the base classes."""
    def __new__(mcs, name, bases, attrs):
        # Collect fields from current class.
        current_fields = []
        for key, value in list(attrs.items()):
            if isinstance(value, Field):
                current_fields.append((key, value))
                attrs.pop(key)
        attrs['declared_fields'] = dict(current_fields)

        new_class = super(DeclarativeFieldsMetaclass, mcs).__new__(mcs, name, bases, attrs)

        # Walk through the MRO.
        declared_fields = {}
        
        ...

        new_class.base_fields = declared_fields
        new_class.declared_fields = declared_fields

        return new_class

class ModelFormMetaclass(DeclarativeFieldsMetaclass):
    def __new__(mcs, name, bases, attrs):
        base_formfield_callback = None
        for b in bases:
            if hasattr(b, 'Meta') and hasattr(b.Meta, 'formfield_callback'):
                base_formfield_callback = b.Meta.formfield_callback
                break

        formfield_callback = attrs.pop('formfield_callback', base_formfield_callback)

        new_class = super(ModelFormMetaclass, mcs).__new__(mcs, name, bases, attrs)

        if bases == (BaseModelForm,):
            return new_class

        opts = new_class._meta = ModelFormOptions(getattr(new_class, 'Meta', None))
        
        ...

class ModelFormMetaclass(DeclarativeFieldsMetaclass):
    def __new__(mcs, name, bases, attrs):
        ...
        
        # We check if a string was passed to `fields` or `exclude`,
        # which is likely to be a mistake where the user typed ('foo') instead
        # of ('foo',)
        for opt in ['fields', 'exclude', 'localized_fields']:
            value = getattr(opts, opt)
            if isinstance(value, str) and value != ALL_FIELDS:
                msg = ("%(model)s.Meta.%(opt)s cannot be a string. "
                       "Did you mean to type: ('%(value)s',)?" % {
                           'model': new_class.__name__,
                           'opt': opt,
                           'value': value,
                       })
                raise TypeError(msg)

        ...

class ModelFormMetaclass(DeclarativeFieldsMetaclass):
    def __new__(mcs, name, bases, attrs):
        ...

        if opts.model:
            # If a model is defined, extract form fields from it.

            # make sure opts.fields doesn't specify an invalid field

            # Override default model fields with any custom declared ones
            # (plus, include all the other declared fields).
            ...
            fields.update(new_class.declared_fields)
        else:
            fields = new_class.declared_fields

        new_class.base_fields = fields

        return new_class