List comprehension for running total

Question

I want to get a running total from a list of numbers.

For demo purposes, I start with a sequential list of numbers using range

a = range         


        

Answer 1

Starting Python 3.8, and the introduction of assignment expressions (PEP 572) (:= operator), we can use and increment a variable within a list comprehension:

# items = range(7)
total = 0
[(x, total := total + x) for x in items]
# [(0, 0), (1, 1), (2, 3), (3, 6), (4, 10), (5, 15), (6, 21)]

This:

• Initializes a variable total to 0 which symbolizes the running sum
• For each item, this both:
  • increments total by the current looped item (total := total + x) via an assignment expression
  • and at the same time returns the new value of total as part of the produced mapped tuple