class & metaclass.py

########
# auto #
########

class M(type):
    def __new__(mcls, *a, **kw):
        print("in metaclass M - __new__:", locals())
        return super().__new__(mcls, *a, **kw)

    def __init__(cls, name, base, dicts):
        print("in metaclass M - __init__:", locals())

    def __call__(cls, *a, **kw):
        print("in metaclass M - __call__:", locals())
        return super(M, cls).__call__(*a, **kw)

class C(metaclass=M):
    print('===== C code start =====')

    def __new__(cls, *a, **kw):
        print("in class C - __new__:", locals())
        return super(C, cls).__new__(cls)

    def __init__(self, *a, **kw):
        print("in class C - __init__:", locals())

    print("~~~~~ C code finish ~~~~~~~")

C(1,2,3)
"""
===== C code start =====
~~~~~ C code finish ~~~~~~~
in metaclass M - __new__: {'mcls': <class '__main__.M'>, 'kw': {}, 'a': ('C', (), {'__qualname__': 'C', '__module__': '__main__', '__init__': <function C.__init__ at 0xffee7078>, '__new__': <function C.__new__ at 0xffecadf8>}), '__class__': <class '__main__.M'>}
in metaclass M - __init__: {'name': 'C', 'dicts': {'__qualname__': 'C', '__module__': '__main__', '__init__': <function C.__init__ at 0xffee7078>, '__new__': <function C.__new__ at 0xffecadf8>}, 'base': (), 'cls': <class '__main__.C'>}
in metaclass M - __call__: {'kw': {}, '__class__': <class '__main__.M'>, 'a': (1, 2, 3), 'cls': <class '__main__.C'>}
in class C - __new__: {'kw': {}, '__class__': <class '__main__.C'>, 'a': (1, 2, 3), 'cls': <class '__main__.C'>}
in class C - __init__: {'kw': {}, 'a': (1, 2, 3), 'self': <__main__.C object at 0xffeda8d0>}
"""

##########
# manual #
##########

class M(type):
    def __new__(mcls, *a, **kw):
        print("in metaclass M - __new__:", locals())
        return super().__new__(mcls, *a, **kw)

    def __init__(cls, name, base, dicts):
        print("in metaclass M - __init__:", locals())

    def __call__(cls, *a, **kw):
        print("in metaclass M - __call__:", locals())
        return super(M, cls).__call__(*a, **kw)

print('===== C code start =====')

class C:
    def __new__(cls, *a, **kw):
        print("in class C - __new__:", locals())
        return super(C, cls).__new__(cls)

    def __init__(self, *a, **kw):
        print("in class C - __init__:", locals())

print("~~~~~ C code finish ~~~~~~~")

C = M(C.__name__, C.__bases__, {'__module__': C.__module__, '__new__': C.__new__,
                                '__init__': C.__init__, '__qualname__': C.__qualname__})
# which is same as:
#   >>> name, base, dict_ = (C.__name__, C.__bases__,
#   >>>                      {'__module__': C.__module__, '__init__': C.__init__,
#   >>>                       '__new__': C.__new__, '__qualname__': C.__qualname__})
#   >>> C = M.__new__(M, name, base, dict_)
#   >>> M.__init__(C, name, base, dict_)

# C(1,2,3)
C.__call__(1,2,3)
# which will do at the end:
#   >>> c = C.__new__(C, 1, 2, 3)
#   >>> C.__init__(c, 1, 2, 3)