Mask a string by replacing a specific range of characters with asterisks (*)

mask_string.py

def mask_string(
  string: str,
  end: int = 4
) -> str:

    """
    Mask a string by replacing all but the last number of characters with asterisks(*).
    
    Parameters:
        string (str): The string to be masked.
        end (int): The number of characters to keep visible.
        
    Returns:
        str: The masked string.
        
    Example:
        original_string = "This is a test string."
        masked_string = mask_string(original_string)
        print(masked_string)
        # '******************ing.'
    """
    
    # Check if the value end is positive integer
    if not isinstance(end, int) or end <= 0:
        raise ValueError("end must be a positive integer")
    
    # Mask all but the last characters
    mask = '*' * (len(string) - end)
    
    # Replace all but the last characters with the mask
    return mask + string[-end:]