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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_folder_list(folder='.'):
 """ Get list of sub-folders contained in input folder :param folder: input folder to list sub-folders. Default is ``'.'`` :type folder: str :return: list of sub-folders :rtype: list(str) """

    dir_list = get_content_list(folder)
    return [f for f in dir_list if not os.path.isfile(os.path.join(folder, f))]