Using len() and sum()

The len() and sum() functions provide two simple reductions—a count of the elements and the sum of the elements in a sequence. These two functions are mathematically similar, but their Python implementation is quite different.

Mathematically, we can observe this cool parallelism. The len() function returns the sum of ones for each value in a collection, .

The sum() function returns the sum of x for each value in a collection, .

The sum() function works for any iterable. The len() function doesn't apply to iterables; it only applies to sequences. This little asymmetry in the implementation of these functions is a little awkward around the edges of statistical algorithms.

For empty sequences, both of these functions return a proper additive identity element of zero:

>>> sum(())
0

Of course, sum(()) returns an integer zero. When other numeric types are used, the integer zero will be coerced to the proper type for the available data.