QuickSort Function Question

def partition(l):
    length = len(l)
    pivot = l[length-1]
    left = []
    right = []

    for i in range(length-1):
        element = l[i]
        if element <= pivot:
            left+=[element]
        else: 
            right +=[element]
    return (left, pivot, right)

print partition([1, 5, 7, 3])

def quick_sort(list):
    #base case
    if len(list) <= 1:
        return list
    # recursive case
    else:
        (left, pivot, right) = partition(list) 
#whats this line above doing exactly? importing the inputs from the other function?
        sorted_left = quick_sort(left)
        sorted_right = quick_sort(right)
        return sorted_left + [pivot] + sorted_right

print quick_sort([1, 5, 7, 3])