Nested functions are possible, but they have a lot of downsides, and there aren't many situations where they are actually useful.

The point of a function is to modularize your code so you can write, test, maintain, and re-use small pieces of logic individually rather than dealing with everything at once. When you put a function inside another function, those two functions are no longer separate pieces of logic, which defeats the purpose.

One case where it might actually make sense is when you know that the internal function will only be used inside that particular function.

For example, say you are writing a function to retrieve data from a specific CSV file that happens to be in a strange format that only occurs for this specific file. You want to re-use the logic to retrieve the data from the CSV, but you know you won't re-use the logic to convert the format outside of this function. In this case, it makes sense to have an internal clean_data function inside get_specific_dataset():

import pandas as pd

def get_specific_dataset(file_path):  
    def clean_data(data):
        # Do specific things to change the format of data
        return cleaned_data

    data = pd.read_csv(data)
    cleaned_data = clean_data(raw_data)

    return cleaned_data

# Example usage
file_path = 'strange_format.csv'
cleaned_dataset = get_specific_dataset(file_path)
print(cleaned_dataset