[Python] Immutable vs Mutable types

I'm confused on what an immutable type is. I know the float object is considered to be immutable, with this type of example from my book:

class RoundFloat(float):
def __new__(cls, val):
return float.__new__(cls, round(val, 2))


Is this considered to be immutable because of the class structure / hierarchy?, meaning float is at the top of the class and is its own method call. Similar to this type of example (even though my book says dict is mutable):

class SortedKeyDict(dict):
def __new__(cls, val):
return dict.__new__(cls, val.clear())


Whereas something mutable has methods inside the class, with this type of example:

class SortedKeyDict_a(dict):
def example(self):
return self.keys()


Also, for the last class(SortedKeyDict_a), if I pass this type of set to it:

d = (('zheng-cai', 67), ('hui-jun', 68),('xin-yi', 2))


without calling the example method, it returns a dictionary. The SortedKeyDict with __new__ flags it as an error. I tried passing integers to the RoundFloat class with __new__ and it flagged no errors.

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