I was trying to make a custom complex class that looked something like this and it failed. I replaced complex with int and it also failed, but I tried float and it worked.

---fail
class A(complex):
    def __init__(self, test):
        super().__init__()

A(test=5)

---fail
class A(int):
    def __init__(self, test):
        super().__init__()

A(test=5)

---work
class A(float):
    def __init__(self, test):
        super().__init__()

A(test=5)

I tried some more classes and str is weird, but dict and set work fine. very weird

This is because int, str, and complex are immutable. If I have 

    class MyInt(int):
        def __init__(self, stuff):
            pass

then when I call MyInt("19"), the string "19" is passed to the constructor int.__new__ before the overridden initializer MyInt.__init__. You can only override that by implementing a MyInt.__new__ to override the int constructor.

This is not a bug.

Here's an example:

class A(complex):
    def __init__(self, *args, test):
        self.test = test
    def __new__(cls, *args, test):
        return super().__new__(cls, *args)

>>> a = A(1, test="TEST")
>>> a
(1+0j)
>>> a.test
'TEST'

>>> b = A(1, 1, test="TEST2")
>>> b
(1+1j)
>>> b.test
'TEST2'

When reporting a failure please copy and paste the exception and message and when non-trivial, the traceback.   In this case:  "TypeError: 'test' is an invalid keyword argument for complex()".  The difference between int and complex versus float is that the former have keyword arguments, so they check for invalid keyword arguments, whereas float take no keyword arguments, so it does not check for validity and, for subclasses, passes them on.