python f.readlines() 会耗完所有内存 https://stackoverflow.com/questions/17246260/python-readlines-usage-and-efficient-practice-for-reading https://stackoverflow.com/questions/28538388/python-open-function-memory-usage https://www.quora.com/When-we-open-a-file-using-Python-will-the-whole-file-be-moved-to-the-RAM-or-will-only-the-file-descriptor-with-the-portion-of-the-file-be-moved-to-the-RAM