缺陷：__mian__不能使用相对导入
PEP 328 Relative Imports and __name__ 中说明：
Relative imports use a module's __name__ attribute to determine that module's position in the package hierarchy. If the module's name does not contain any package information (e.g. it is set to '__main__') then relative imports are resolved as if the module were a top level module, regardless of where the module is actually located on the file system.
即：相对导入在 package 分层中，使用模块的__name__ 属性得到模块的位置。但是如果模块的名字没有包含任何 package 信息（例如，__main__模块），不管是否该模块实际位于文件系统，相对导入会被当作顶层模块导入（a top level module）。
修复
根据 6.4.2. Intra-package References 的建议：
Note that relative imports are based on the name of the current module. Since the name of the main module is always "__main__", modules intended for use as the main module of a Python application must always use absolute imports.
__mian__ 使用 绝对导入 方式，而其他模块仍使用 相对导入
if __name__ == '__main__':
sys.path.append(os.path.dirname(sys.path[0]))
后话
导入的方式
参考 stackoverflow vaultah 的回答，比较详细，这里不再赘述。
为什么选择相对导入的优势
PEP 328 Rationale for Relative Imports 中说明：
Several use cases were presented, the most important of which is being able to rearrange the structure of large packages without having to edit sub-packages.In addition, a module inside a package can't easily import itself without relative imports.
即：相对导入可以对大型的 python packages 重构时，不用重新编辑 sub-packages;另外，相对导入可以比较轻松地导入自身。