I have a Python function that contains a nested function. I want to write unit tests for the nested function using PyTest, but I don’t want to modify the way the nested function is defined or its behavior. I also don’t want to call or test the outer function itself, just the nested function.

Here is a simplified example of my code:

# example.py

def outer_function(a, b):
    """
    Example outer function that performs some operations.
    """
    def inner_function(x, y):
        """
        Nested function that I want to test.
        """
        return x + y
    
    # Some operations using inner_function
    result = inner_function(a, b)
    return result

I want to create a PyTest test function that only tests inner_function using parameterization, ensuring it covers various scenarios such as:

1. Adding two positive numbers
2. Adding positive and negative numbers
3. Adding two negative numbers
4. Adding zero to a number

How can I achieve this in a way that the tests are independent of outer_function?

Example Test Function:

# test_example.py

import pytest

@pytest.mark.parametrize("x, y, expected", [
    (1, 2, 3),         # Adding two positive numbers
    (1, -1, 0),        # Adding positive and negative numbers
    (-1, -2, -3),      # Adding two negative numbers
    (0, 5, 5),         # Adding zero to a number
    (3, 0, 3)          # Adding a number to zero
])
def test_inner_function(x, y, expected):
    """
    Describe how to test the inner_function here.
    """
    # Code to test inner_function goes here.

Any guidance or examples on how to properly structure and implement these tests would be greatly appreciated!