<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def listdir(directory):
 """Returns list of nested files and directories for local directory by path :param directory: absolute or relative path to local directory :return: list nested of file or directory names """

    file_names = list()
    for filename in os.listdir(directory):
        file_path = os.path.join(directory, filename)
        if os.path.isdir(file_path):
            filename = f'{filename}{os.path.sep}'
        file_names.append(filename)
    return file_names