NameError: name 'METHOD_NAME' is not defined Python Unittest

I've got the following project structure:

The code for each of the files are as follows (nothing is included in init.py):

my_sum.py

from service import calculate_sum

def main():
    calculate_sum.create_sum()

if __name__ == "__main__":
    main()

calculate_sum.py:

def calculate(arg):
    total = 0
    for val in arg:
        total += val
    return total

def create_sum():
    print("hello")
    total = calculate([5, 5])
    print(total)

test_sum.py:

import unittest

from dir import *

class TestSum(unittest.TestCase):
    def test_list_int(self):
        """
        Test that it can sum a list of integers
        """
        data = [1, 2, 3]
        result = calculate(data)
        self.assertEqual(result, 6)

if __name__ == '__main__':
    unittest.main()

As you can see, this is a very simple program that calculates the sum of a list. Note: I am mimicking a larger project hence why I've got multiple (rather unnecessary) calls to methods.

When I run python -m unittest discover -s test/ from the MainDir directory, I get an error:

testsum.py", line 11, in test_list_int result = calculate(data) NameError: name 'calculate' is not defined

However, if I change the "calculate" method to "sum" (i.e. the below), my test runs fine. Note: I should note that I copied this example from a site online, and therefore, I think "sum" is defined somewhere, but I'm not really sure where and therefore don't know why the error is occurring.

Changing calculate_sum.py to the below passes the test:

def sum(arg):
    total = 0
    for val in arg:
        total += val
    return total

def create_sum():
    print("hello")
    total = sum([5, 5])
    print(total)

testsum.py:

import unittest

from dir import *

class TestSum(unittest.TestCase):
    def test_list_int(self):
        """
        Test that it can sum a list of integers
        """
        data = [1, 2, 3]
        result = sum(data)
        self.assertEqual(result, 6)

if __name__ == '__main__':
    unittest.main()

---------------------------------------------------------------------- Ran 1 test in 0.000s

OK