4.4 Using len() and sum() on collections
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, X: ∑ x∈X1 = ∑ x∈Xx0.The
sum()function returns the sum of each value in a collection, X: ∑ x∈Xx = ∑ x∈Xx1.
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.
As noted above, for empty sequences, both of these functions return a proper additive identity value of zero:
>>> sum(())
0
>>> len(()) ...
