How do I take a function from a user as a test variable throughout the testing with pytest?

I am new to pytest and I have been struggling to implement this part of my own library: Taking a function from a user and save that function as a variable so that that could be used and accessed throughout the testing. I am trying to pass a function that I defined in playground.py(pretending I am a random user) to the sortoy.py. I heard using global_variable for this is not recommended. Is there any recommended way of doing it?

My whole Library structure looks like below:

my_library
-my_library
    -__init__.py
    -sortoy.py
-LICENSE
-playground.py --> This is where I am testing my code
-README.md
-setup.py

This is the code for playground.py

import my_library.sortoy

# The defined array
def bubbleSort(arr): 
    n = len(arr)
    for i in range(n): 
        for j in range(0, n-i-1): 
            if arr[j] > arr[j+1] : 
                arr[j], arr[j+1] = arr[j+1], arr[j] 
    return arr

my_library.sortoy.sort_function(bubbleSort) # Taking user defined function

This is the code for sortoy.py

import pytest

small_cases = [([1], [1]), ([2, 1], [1, 2]), ([2, 2], [2, 2])]

@pytest.fixture # I was trying to use this for taking user function (It does not work however).
def sort_function(f):
    return f

# The test I am trying to run
@pytest.mark.parametrize("test_case, sorted", small_cases)
def test_one_element(sort_function, test_case, sorted):
    assert sort_function(test_case) == sorted

# Script version of pytest so that I could run the test_case above once I have got the user-defined function.
pytest.main(["-v"])

For the information, the library I am trying to implement is to test the user-defined function for users when they pass in the sort_function they have created (like sorting). When my_library.sortoy.sort_function(bubblesort) is called, this library is supposed to execute the testings(test_one_element() in this case) automatically and show on console if it has failed or passed the tests.