The sum(), len(), max(), and min() functions are—in a way— all specializations of a more general algorithm expressed by the reduce() function. The reduce() function is a higher-order function that folds a function into each pair of items in an iterable.

A sequence object is given as follows:

d = [2, 4, 4, 4, 5, 5, 7, 9]

The function, reduce(lambda x,y: x+y, d), will fold in + operators to the list as follows:

2+4+4+4+5+5+7+9

Including () can show the effective grouping as follows:

((((((2+4)+4)+4)+5)+5)+7)+9

Python's standard interpretation of expressions involves a left-to-right evaluation of operators. The fold left isn't a big change in meaning.

We can also provide an initial value as follows:

reduce(lambda x,y: x+y**2, iterable, 0)

If we don't, the initial value from the sequence is used as the initialization. Providing an initial value is essential when there's a map() function as well as a reduce() function. Following is how the right answer is computed...