python - Create child of str (or int or float or tuple) that accepts kwargs -

i need class behaves string takes additional kwargs. therefor subclass str:

class child(str):      def __init__(self, x, **kwargs):         # code ...         pass   inst = child('a', y=2) print(inst) 

this raises:

traceback (most recent call last):   file "/home/user1/project/exp1.py", line 8, in <module>     inst = child('a', y=2) typeerror: 'y' invalid keyword argument function 

which rather strange, since code below works without error:

class child(object):      def __init__(self, x, **kwargs):         # code ...         pass   inst = child('a', y=2) 

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questions:

• why different behavior when trying subclass str, int, float, tuple etc compared other classes object, list, dict etc?
• how can create class behaves string has additional kwargs?

you need override __new__ in case, not __init__:

>>> class child(str): ...    def __new__(cls, s, **kwargs): ...       inst = str.__new__(cls, s) ...       inst.__dict__.update(kwargs) ...       return inst ... >>> c = child("foo") >>> c.upper() 'foo' >>> c = child("foo", y="banana") >>> c.upper() 'foo' >>> c.y 'banana' >>> 

see here answer why overriding __init__ doesn't work when subclassing immutable types str, int, , float:

__new__() intended allow subclasses of immutable types (like int, str, or tuple) customize instance creation. commonly overridden in custom metaclasses in order customize class creation.