The definitions of the arithmetic mean have an appealingly trivial definition based on sum() and len(), which is as follows:
def mean(items):
return sum(items)/len(items)
While elegant, this doesn't actually work for iterables. This definition only works for collections that support the len() function. This is easy to discover when trying to write proper type annotations. The definition of mean(items: Iterable)->float won't work because Iterable types don't support len().
Indeed, we have a hard time performing a simple computation of mean or standard deviation based on iterables. In Python, we must either materialize a sequence object or resort to somewhat more complex operations.
The definition needs ...
