Creating inmutable types in Python using new()

My question is pretty simple, I have:

class upperstr(str):
    def __new__(cls, arg):
        return str.__new__(cls, str(arg).upper())

Why, if my __new__() method is directly using an instance of an inmutable type (str), instances of my new type (upperstr) are mutable?

>>> s = str("text")
>>> "__dict__" in dir(s)
False
>>> s = upperstr("text")
>>> "__dict__" in dir(s)
True

In what stage does the interpreter sets the __dict__ attribute to upperstr intances if I'm only overriding the __new__() method?

Thanks!

Solution

All user-defined classes in Python have a __dict__() attribute by default, even if you don't overwrite anything at all:

>>> x = object()
>>> x.__dict__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'object' object has no attribute '__dict__'
>>> class MyObject(object):
...     pass 
... 
>>> x = MyObject()
>>> x.__dict__
{}

If you don't want a new-style class to have a __dict__, use __slots__ (documentation, related SO thread):

>>> class MyObject(object):
...     __slots__ = []
... 
>>> x = MyObject()
>>> x.__dict__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'MyObject' object has no attribute '__dict__'

Creating inmutable types in Python using __new__()

Creating inmutable types in Python using new()