Document FileSystemStorage.get_created_time() limitation on Unix

[Jonathan Verner]

The method get_created_time uses os.path.getctime which only returns the creation time on Windows. On Unix it returns the change time , i.e. the last time the inode attributes were modified (see ​http://stackoverflow.com/questions/237079/how-to-get-file-creation-modification-date-times-in-python). The documentation says that

For storage systems unable to return the creation time this will raise NotImplementedError

so the correct behaviour would be, IHMO, to raise a NotImplementedError on Unix platforms. I am not sure whether the method is used anywhere---searching on github only came up with uses in tests, docs and the place where it is defined. Also, as noted by a commenter on the above stackoverflow question, it doesn't seem to be a useful value anyway:

Frankly, file creation time is usually fairly useless. When you open an existing file for write with mode "w", it's not replacing it, it just opens the existing file and truncates it. Even though the file contents are completely unrelated to whatever it had on creation, you'd still be told the file was "created" well before the current version. Conversely, editors that use atomic replace on save (original file is replaced by new work-in-progress temp file) would show a more recent creation date, even if you just deleted one character. Use the modification time, don't grub for creation time.

So it might make sense to remove the method altogether.

[Adam Johnson]

Perhaps ctime is good enough though, making the presumption that the inode attributes only change at creation time?

[Jonathan Verner]

Replying to Adam Chainz:

Perhaps ctime is good enough though, making the presumption that the inode attributes only change at creation time?

Unfortunately, that is not the case: ctime changes at least as often as mtime does---updating the mtime changes inode attributes.

[Tim Graham]

Accepting as a documentation clarification for now. I'm not sure if changing the method's implementation is worth it, but I think it would require a deprecation path. If you want to pursue that, please raise the idea on the DevelopersMailingList to get a consensus.

[Ingo Klöcker]

I'll look into this.

[Ingo Klöcker]

​PR

[Ingo Klöcker]

I've improved the patch.

[Tim Graham <timograham@…>]

In f3ac398d:

Fixed #27644 -- Doc'd FileSystemStorage.get_created_time().

[Tim Graham <timograham@…>]

In 4678957b:

[1.11.x] Fixed #27644 -- Doc'd FileSystemStorage.get_created_time().

Backport of f3ac398dd595c43f551502c742b0b3cbe8440e50 from master