EXAMPLE! How to understand the python metaclasses. Less1.

metaclassexample.py

from functools import wraps


def debug(func):
    print('debug(', func.__qualname__, ')')
    name = func.__qualname__

    @wraps(func)
    def wrapper(*args, **kwargs):
        print('CALL:', name, args, kwargs)
        return func(*args, **kwargs)

    return wrapper


def debugclass(cls):
    print('debugclass(', cls.__qualname__, ')')

    for name, val in vars(cls).items():
        if callable(val):
            setattr(cls, name, debug(val))
    return cls


class debugmeta(type):
    def __new__(cls, clsname, bases, clsdict):
        print('CONSTRUCT NEW CLASS:', clsname, bases, clsdict)
        clsobj = super().__new__(
            cls, clsname, bases, clsdict
        )
        clsobj = debugclass(clsobj)
        return clsobj


print(1)


class Base(object, metaclass=debugmeta):
    def foo(self):
        pass


print(2)


class A(Base):
    @staticmethod
    def __new__(cls, *args, **kwargs):
        print('CONSTRUCT NEW INSTANCE:', cls, args, kwargs)
        return super().__new__(cls, *args)

    def bar(self):
        pass


print(3)

a = A()
a.foo()
a.bar()

print(4)


# output:
"""
1
CONSTRUCT NEW CLASS: Base (<class 'object'>,) {'__qualname__': 'Base', 'foo': <function Base.foo at 0x025264F8>, '__module__': '__main__'}
debugclass( Base )
debug( Base.foo )
2
CONSTRUCT NEW CLASS: A (<class '__main__.Base'>,) {'__qualname__': 'A', 'bar': <function A.bar at 0x0256FA08>, '__module__': '__main__', '__new__': <staticmethod object at 0x02535170>}
debugclass( A )
debug( A.bar )
3
CONSTRUCT NEW INSTANCE: <class '__main__.A'> () {}
CALL: Base.foo (<__main__.A object at 0x02535630>,) {}
CALL: A.bar (<__main__.A object at 0x02535630>,) {}
4
"""