Sum a list of numbers in Python

Given a list of numbers such as:

[1, 2, 3, 4, 5, ...]

How do I calculate their total sum:

1 + 2 + 3 + 4 + 5 + ...

How do I calculate their pairwise averages:

[(1+2)/2, (2+3)/2, (3+4)/2, (4+5)/2, ...]