Rain Water Trapping program in Python

def rain_water_trap(arr):
    size = len(arr)
    left = size * [0]
    right = size * [0]
    left[0] = arr[0]
    max_so_far_left = arr[0]
    water = 0

    for index in range(size):
        if max_so_far_left < arr[index]:
            max_so_far_left = arr[index]
            left[index] = max_so_far_left
        else:
            left[index] = max_so_far_left
     
    max_so_far_right = arr[-1]

    for index in range(size-1, -1, -1):
        if max_so_far_right < arr[index]:
            max_so_far_right = arr[index]
            right[index] = max_so_far_right
        else:
            right[index] = max_so_far_right

    for index in range(size):
        water = water + min(left[index],right[index]) - arr[index]

    return water

list1 = [1, 0, 2, 0, 1, 0, 3, 1, 0, 2]
print(rain_water_trap(list1))