Incorrect str inheritance

mystr.py

class MyString(str):
    def __init__(self, data):
        self.data = data

    def append(self, other):
        self.data += other

    def __str__(self):
        return self.data


a = MyString("hello")
print(a)
print(a + " world")
a.append(" world")
print(a)
print(a + " again")