To create a function that flattens a nested list in Python, we can use recursion to handle the depth-first order of the elements. Below is the implementation of the flatten_nested_list function, along with some test cases to ensure its correctness.

def flatten_nested_list(nested_list):
flattened_list = []

for element in nested_list:
if isinstance(element, list):
# Recursively extend the flattened list with the flattened version of the current element
flattened_list.extend(flatten_nested_list(element))
else:
# Append the element directly if it’s not a list
flattened_list.append(element)

return flattened_list

# Test Cases
assert flatten_nested_list([1, 2, [3, 4], [5, [6, 7]]]) == [1, 2, 3, 4, 5, 6, 7]
assert flatten_nested_list([[], [1], [[2, 3]], [4, [5, []]]]) == [1, 2, 3, 4, 5]
assert flatten_nested_list([[[[1, 2], 3], 4], 5, [], [6, [7, [8, [9]]]]]) == [1, 2, 3, 4, 5, 6, 7, 8, 9]

print(“All test cases passed!”)

Explanation of the Code:

– Function Definition: The function flatten_nested_list is defined to take one parameter nested_list, which can be a list containing integers and/or other lists.
– Flattening Process: We initialize an empty list flattened_list to collect the flattened elements.
– Iteration: We iterate through each element in nested_list.- If an element is a list (checked using isinstance), we call the function recursively on that element and extend flattened_list with the result.
– If the element is not a list (i.e., it’s an integer), we simply append it to flattened_list.

– Return Statement: Finally, we return flattened_list which now contains all elements in a single flat structure.

Test Cases:

The test cases provided check various scenarios:

1. A mix of integers and nested lists.
2. Lists with empty sub-lists.
3. Deep nesting to ensure that the function handles all levels correctly.

When you run this code, you should see “All test cases passed!” if everything is working correctly.