缺陷：__mian__不能使用相對導入
PEP 328 Relative Imports and __name__ 中說明：
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：相對導入在 package 分層中，使用模塊的__name__ 屬性得到模塊的位置。但是如果模塊的名字沒有包含任何 package 信息（例如，__main__模塊），不管是否該模塊實際位於文件系統，相對導入會被當作頂層模塊導入（a top level module）。
修復
根據 6.4.2. Intra-package References 的建議：
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 絕對導入 方式，而其他模塊仍使用 相對導入
if __name__ == '__main__':
sys.path.append(os.path.dirname(sys.path[0]))
后話
導入的方式
參考 stackoverflow vaultah 的回答，比較詳細，這里不再贅述。
為什么選擇相對導入的優勢
PEP 328 Rationale for Relative Imports 中說明：
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：相對導入可以對大型的 python packages 重構時，不用重新編輯 sub-packages;另外，相對導入可以比較輕松地導入自身。