需要帮助理解如何使用多级函数调用处理完成处理程序

def read_data(file_path):
    """读取文件数据"""
    with open(file_path, 'r') as file:
        data = file.readlines()
    return data

def clean_data(data):
    """清洗数据"""
    cleaned_data = [line.strip() for line in data if line.strip()]
    return cleaned_data

def process_data(cleaned_data):
    """处理数据"""
    processed_data = []
    for item in cleaned_data:
        processed_item = item.upper()
        processed_data.append(processed_item)
    return processed_data

def main():
    file_path = 'data.txt'
    data = read_data(file_path)
    cleaned_data = clean_data(data)
    processed_data = process_data(cleaned_data)
    print(processed_data)

if __name__ == "__main__":
    main()