Example:

   class MyBytes(bytes):
      def __init__(self, *args, **kw):
         bytes.__init__(self, *args, **kw)
   a = bytes(b"hello")   # ok
   b = MyBytes(b"hello") # error

=> DeprecationWarning: object.__init__() takes no parameters

The example works fine in Python 2.6 but fails in Python 3.0.

With immutable types, you must use __new__ instead. Passing the
arguments to __init__ is too late since the object is immutable and it
has already been built in memory, thus you can't initialize it with
another value.

>>> class B(bytes):
...  def __new__(cls, *args, **kargs):
...   print(args)
...   return bytes.__new__(cls, *args, **kargs)
...
>>> b = B(b"foo")
(b'foo',)
>>> b
b'foo'
>>> type(b)
<class '__main__.B'>