Flatten a list using Python with test cases against the flatten function

flatten.py

def flatten(input_list):
    flattened_output = []
    for item in input_list:
        if isinstance(item, int):
            flattened_output.append(item)
        else:
            flattened_output += flatten(item)
    return flattened_output


# In all tests below, the assumption is, the lists contain only integers and no other types of data.
# Tests are written to run using py.test

def test_flatten_empty_list():
    """
    assert empty list remains as it is
    """
    input_list = []
    assert flatten(input_list) == input_list


def test_flatten_no_nested_list():
    """
    assert non empty list of integers with no nested lists inside it
    """
    input_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100, 1000, 100000, 1223456]
    assert flatten(input_list) == input_list


def test_flatten_nested_list_of_integers():
    """
    assert non empty list of integers and a few nested lists
    """
    input_list = [1, 2, [3, 4], 5, 6, 7, 8, 9, 10, 100, 1000, [100000, 1223456]]
    expected_value = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100, 1000, 100000, 1223456]
    assert flatten(input_list) == expected_value