TypeError: 'str' object is not callable-Python

Define a function test_sort that takes a tuple containing a sort function reference and a function description as a parameter and executes that sort function with the data from the previous task. Track the comparisons for each set of data, calculate the average number of comparisons for the list of random lists.

The sort function reference just means that you can put a function definition into a variable just like any other value, and then execute that function variable. You can also pass a function definition as an argument to a another function and then execute the resulting parameter as a function.

this is the code

def test_sort(function_tuple, a_sorte, a_reverse, a_random):
    Number.comparisons = 0
    f = function_tuple[0]
   f(a_sorte)
    x = Number.comparisons

    Number.comparisons = 0
    f = function_tuple[0]
    f(a_reverse)
    y = Number.comparisons

    Number.comparisons = 0
    f = function_tuple[0]
    for i in range(len(a_random)):
        f(a_random[i])
    z = Number.comparisons
    print("{0}        {1}         {2}        {3}".format(
        function_tuple[1], x, y, z))
    return

the main:

import copy

from sorts_array import Sorts
import functions
SORTS = (
    ('Bubble Sort', Sorts.bubble_sort),
    ('Insertion Sort', Sorts.insertion_sort),
    ('Selection Sort', Sorts.selection_sort),
    ('Merge Sort', Sorts.merge_sort),
    ('Quick Sort', Sorts.quick_sort),
    ('Heap Sort', Sorts.heap_sort),
    ('Shell Sort', Sorts.shell_sort),
    ('Cocktail Sort', Sorts.cocktail_sort),
    ('Comb Sort', Sorts.comb_sort),
    ('Bin. Ins. Sort', Sorts.binary_insert_sort)
)

a_sorte = functions.create_sorted()

a_reverse = functions.create_reversed()

a_random = functions.create_randomly()


for i in range(0, 9):
    x = copy.deepcopy(a_sorte)
    y = copy.deepcopy(a_reverse)
    z = copy.deepcopy(a_random)
    functions.test_sort(SORTS[i], x, y, z)

The error I get:

Traceback (most recent call last):
 functions.test_sort(SORTS[i], x, y, z)
        f(a_sorte)
  TypeError: 'str' object is not callable

This what I did in the previous task as mentioned in the question above:

def create_sorted():

    value = []
    for i in range(0, SIZE):
        n = Number(i)
        value.append(copy.deepcopy(n))
    return value


def create_reversed():
    value = []
    for i in range(SIZE, -1, -1):
        n = Number(i)
        value.append(copy.deepcopy(n))
    return value


def create_randomly():
    value = []
    for i in range(N):
        n = Number(random.randint(0, RANGE))
        value.append(copy.deepcopy(n))
    return value