# Python Code Example: Flattening a Nested List

Flattening a nested list involves converting a list of lists into a single list containing all the elements from the nested lists. This can be useful in various data processing tasks where a simplified, single-level list is required. There are different ways to achieve this in Python, including using recursion or list comprehensions. Here, we’ll demonstrate a recursive approach to flatten a nested list.

### Code Example

``````def flatten(nested_list):
# Initialize an empty list to store the flattened elements
flattened_list = []

# Recursive function to process each element in the nested list
def flatten_element(element):
# If the element is a list, recursively flatten it
if isinstance(element, list):
for item in element:
flatten_element(item)
else:
# If the element is not a list, append it to the flattened list
flattened_list.append(element)

# Start flattening from the top level of the nested list
flatten_element(nested_list)

return flattened_list

# Example usage
nested_list = [1, [2, 3], [4, [5, 6]], 7]
flattened_list = flatten(nested_list)
print(f"Flattened list: {flattened_list}")``````

### Code Explanation

#### Defining the `flatten` Function

The `flatten` function is defined to take a nested list as an argument. It initializes an empty list `flattened_list` to store the flattened elements.

``````def flatten(nested_list):
flattened_list = []``````

#### Recursive Helper Function

A nested helper function `flatten_element` is defined within `flatten` to process each element in the nested list. This function uses recursion to handle elements that are lists themselves.

``````def flatten_element(element):
if isinstance(element, list):
for item in element:
flatten_element(item)
else:
flattened_list.append(element)``````
• Checking if Element is a List: The function checks if the current element is a list using `isinstance(element, list)`. If it is, the function iterates over each item in the list and recursively calls `flatten_element` on it.
• Appending Non-list Elements: If the element is not a list, it is appended to the `flattened_list`.

#### Flattening the Top Level

The `flatten_element` function is initially called with the top-level `nested_list`. This starts the recursion process to flatten all nested lists.

``````flatten_element(nested_list)
return flattened_list``````

#### Example Usage

An example nested list is defined and passed to the `flatten` function. The resulting flattened list is printed.

``````nested_list = [1, [2, 3], [4, [5, 6]], 7]
flattened_list = flatten(nested_list)
print(f"Flattened list: {flattened_list}")``````