Python打开一个文件并正在读取它的数据，为什么python仍然可以读取它的数据，即使我删除了这个文件？

Question

My python program is to read data from a txt file and insert data into my postgre database. The program has already started and keeps running. Then I accidently deleted the file on the disk, but the program still keeps running and inserting data into database.

Is that because when python open a file, it loads the file into memory, so that even I delete the file on the disk, it doesn't affect the running program? But my file is more than 3GB, does python really load my file into memory? I'm worrying about whether or not my data in database is correct.

Here is my code that opens the file:

f = open("/home/minjian/Documents/tweets2009-07.txt")

My operating system is:

Linux minjian-OptiPlex-9020 3.16.0-46-generic #62~14.04.1-Ubuntu SMP Tue Aug 11 16:27:16 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Answer 1

符合POSIX标准的操作系统将实际文件数据保留在磁盘上，直到所有文件句柄都关闭，即使没有任何指向数据的链接也是如此。

Answer 2

The path to a file is not the file, and removing that path does not "delete the file on the disk". The path by which you access the file is merely a link to the file. When python opens the file via that link, it gets access to the file and subsequent removal of the link (eg, rm foo ) does not change python's access to the file. The file system will not delete the file until all references are removed, including those held by running processes.

The file is not loaded into memory. It's still just sitting there on the disk.

Answer 3

Most filesystems do not delete the file contents, but only the reference to the file. The content gets deleted when a new file requests the space of the old one or the free space is explicitly overwritten by a tool.

Let me explain this with NTFS. A File is first an entry in the MFT (Master File Table). Its like an index saying there is a file called "xyz" and can be found at address 0x87-0x95. When you delete the file, the index entry is deleted, but the contents at the adresses 0x87-0x95 is not. Your programm knows theses addresses from when it opened the file, so it can still read them, even the index entry is not longer there.

Answer 4

Python won't load the file into memory unless you tell it to.

What is going on is When your python program opens the file it creates a link to it and this linking is key in linux/unix to file handling.

When you list a directory and see a file, you are seeing the link that directory has to the file. When you delete the file, you are actually removing the link. The operating system then notes that the file has no more links so deletes it. In this case when you delete the file, the python program still has a link so the file is still on disk but you can't see it in its directory because that link is gone. When your python program closes the file handle or exits, under the hood, python is asking the OS to remove the link it has to the file and the OS notes this is the last link and the file is gone.

You can use the ln command to create links so a file appears in two directories. If you do a ls -l, the number on the left after the permissions is the number of hard links.