Peter Otten wrote:
Sorry for the mess; second attempt:
A class is an instance of its metaclass.
class A:
pass
is roughly equivalent to
A = type("A", (), {}) # classname, base classes, class attributes
and
class B(A):
foo = 42
is roughly equivalent to
B = type(A)("B", (A,), {"foo": 42})
When you subclass from an instance of A instead of A itself this becomes
a = A()
B = type(a)("B", (a,), {"foo": 42})
which can be simplified to
B = A("B", (a,), {"foo": 42})
If this succeeds B is bound to an instance of A, but usually you'll see a
TypeError, either immediately as the OP,
>>> class A: pass
...
>>> class B(A()): pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object() takes no parameters
or later when you try to instantiate B:
>>> class A:
... def __init__(self, *args):
... print("__init__{}".format(args))
...
>>> class B(A()): pass
...
__init__()
__init__('B', (<__main__.A object at 0x7f3db8a1c048>,), {'__module__':
'__main__', '__qualname__': 'B'})
>>> isinstance(B, A)
>>>
>>>
True
>>> B()
>>>
>>>
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'A' object is not callable
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