Python中list[-1]的含义

Question

I have a piece of code here that is supposed to return the least common element in a list of elements, ordered by commonality:

def getSingle(arr):
    from collections import Counter
    c = Counter(arr)

    return c.most_common()[-1]  # return the least common one -> (key,amounts) tuple

arr1 = [5, 3, 4, 3, 5, 5, 3]

counter = getSingle(arr1)

print (counter[0])

My question is in the significance of the -1 in return c.most_common()[-1] . Changing this value to any other breaks the code as the least common element is no longer returned. So, what does the -1 mean in this context?

Answer 1

One of the neat features of Python lists is that you can index from the end of the list. You can do this by passing a negative number to [] . It essentially treats len(array) as the 0th index. So, if you wanted the last element in array , you would call array[-1] .

All your return c.most_common()[-1] statement does is call c.most_common and return the last value in the resulting list, which would give you the least common item in that list. Essentially, this line is equivalent to:

temp = c.most_common()
return temp[len(temp) - 1]