In both Python 2 and 3, the following code
class A(object):
pass
A(1)
expectedly raises the following exception
TypeError: object() takes no parameters
However, the following code
class A(object):
def __new__(cls, *args, **kwargs):
return super(A, cls).__new__(cls)
A(1)
Does not raise anything. How come?
The answer lies in how object.__new__ [1] and object.__init__ [2] handle extra arguments.
Let's walk through a couple examples:
class A(object):
pass
>>> A(1)
TypeError: object() takes no parameters
object.__new__ is called. It gets the 1 argument. We didn't override __new__ so this is false [1] and it doesn't throw an error. We didn't override __init__ so this is true [2] and throws an error.class A(object):
def __init__(self, *args, **kwargs):
print("A.__init__({0}, {1}, {2})".format(self, args, kwargs))
>>> A(1)
A.__init__(<__main__.A object at 0xdeadbeef>, (1, ), {})
<__main__.A at 0xdeadbeef>
object.__new__ is called. It gets the 1 argument. We didn't override __new__ so this is false [3] and it doesn't throw an error. We did override __init__ so this is false [4] and doesn't throw an error.A.__init__ is called. Because it doesn't call super, object.__init__ is never called.class A(object):
def __new__(cls, *args, **kwargs):
print("A.__new__({0}, {1}, {2})".format(cls, args, kwargs))
return super(A, cls).__new__(cls)
>>> A(1)
A.__new__(<class '__main__.A'>, (1,), {})
<__main__.A at 0xdeadbeef>
A.__new__ is called. It gets the 1 argument. We call object.__new__ with no arguments.object.__init__ is called. It gets the 1 argument. We didn't override __init__ so this is false [5] and it doesn't throw an error. We did override __new__ so this is false [6] and it doesn't throw an error.