os.link() vs. os.rename() vs. os.replace() for writing atomic write files. What is the best approach?

Question

Hi am trying to write an atomic write function like so...

with tempfile.NamedTemporaryFile(mode= "w", dir= target_directory) as f: 
     #perform file writing operation  
     os.replace(f.name, target_file_name) 

I am struggling to figure out what would be the best action to do in line 3. Should I use os.replace(), os.rename() or should I create a hard link between tempfile and target file using os.link()?

Does os.link() use more memmory? What are the benefits of each and are all of them atomic?

Answer 1

os.rename / os.replace are both implemented using this function

The only difference is os.replace uses is_replace=1 which has no effect on posix but sets MOVEFILE_REPLACE_EXISTING flag on windows:

If a file named lpNewFileName exists, the function replaces its contents with the contents of the lpExistingFileName file, provided that security requirements regarding access control lists (ACLs) are met. For more information, see the Remarks section of this topic.

If lpNewFileName or lpExistingFileName name a directory and lpExistingFileName exists, an error is reported.

os.link isn't really suitable for this function unless you can guarantee that the target file does not exist (as os.link will error):

$ touch a b
$ link a b
link: cannot create link 'b' to 'a': File exists
$ python3 -c 'import os; os.link("a", "b")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
FileExistsError: [Errno 17] File exists: 'a' -> 'b'