Mastering Object-Oriented Python - Second Edition

A statistical list

It makes good sense to incorporate mean and standard deviation features directly into a subclass of list. We can extend list like this:

class StatsList(list):    def __init__(self, iterable: Optional[Iterable[float]]) -> None:        super().__init__(cast(Iterable[Any], iterable))    @property    def mean(self) -> float:        return sum(self) / len(self)    @property    def stdev(self) -> float:        n = len(self)        return math.sqrt(            n * sum(x ** 2 for x in self) - sum(self) ** 2        ) / n

With this simple extension to the built-in list class, we can accumulate data and report statistics on the collection of data items.

Note the relative complexity involved in narrowing the type of the list class. The built-in list structure's type, List, is effectively List[Any] ...