def Join(List, sep=None):

result = ''  # Initialize an empty string to store the result

# Iterate through the list of strings
for i in range(len(List)):

    # Append the current string to the result 
    result += List[i]

    # If there is a separator, append it as well. 
    if sep and i < len(List)-1: 
        result += sep

return result