How to emulate sum() using a list comprehension?

Question

Is it possible to emulate something like sum() using list comprehension ?

For example - I need to calculate the product of all elements in a list :

l         


        

Answer 1

>>> reduce(int.__mul__,[1,2,3])
6

C:\Users\Henry>python -m timeit -s "" "reduce(int.__mul__,range(10000))" 
1000 loops, best of 3: 910 usec per loop

C:\Users\Henry>python -m timeit -s "from operator import mul" "reduce(mul,range(10000))"
1000 loops, best of 3: 399 usec per loop

C:\Users\Henry>

Answer 2

It is possible to achieve by using lambda with list comprehension Since we can't assign a value in list comprehension we go with lambda

Solution:

>>> (lambda number_list, sum=0:[sum for number in number_list for sum in [sum + number]][-1])([1, 2, 3, 4, 5])
>>> 15

Answer 3

I might be a bit late for this discussion, but I would like to mention that list comprehentions are turing complete, and thus this can be done with a list comprehention!

This however is messy, so I have used the following trick, which makes a cummulative array, and returns the last element

def sum(l):
    return [c[-1] for c in [[0]] for e in l if c.append(c[-1] + e) is None][-1]

Answer 4

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

total = 0
[total := total + x for x in [1, 2, 3, 4, 5]]
# 15

This:

• Initializes a variable total to 0
• For each item, total is incremented by the current looped item (total := total + x) via an assignment expression

Answer 5

Something like this:

>>> a = [1,2,3]
>>> reduce(lambda x, y: x*y, a)
6