Make a unique filesystem path, without creating the file

Thomas 'PointedEars' Lahn PointedEars at web.de
Sun Feb 14 17:48:25 EST 2016


Ben Finney wrote:

> How should a program generate a unique filesystem path and *not* create
> the filesystem entry?

The Python documentation suggests that it should not.
 
> The ‘tempfile.mktemp’ function is strongly deprecated, and rightly so
> <URL:https://docs.python.org/3/library/tempfile.html#tempfile.mktemp>
> because it leaves the program vulnerable to insecure file creation.
> 
> In some code (e.g. unit tests) I am calling ‘tempfile.mktemp’ to
> generate a unique path for a filesystem entry that I *do not want* to
> exist on the real filesystem. In this case the filesystem security
> concerns are irrelevant because there is no file.

I do not think that you have properly understood the problems with 
tmpfile.mktemp().
 
> […]
> It is also prone to that API function disappearing at some point in the
> future, because it is explicitly and strongly deprecated.
> 
> So I agree with the deprecation, but the library doesn't appear to
> provide a replacement.

| mktemp() usage can be replaced easily with NamedTemporaryFile(), passing 
| it the delete=False parameter: [example]

> What standard library function should I be using to generate
> ‘tempfile.mktemp’-like unique paths, and *not* ever create a real file
> by that path?

I do not think it is possible to avoid the creation of a real file using the 
PSL; in fact, that a file is created appears to be precisely what fixes the 
problems with tempfile.mktemp() because then it cannot happen that someone 
else creates a file with the same name at the same time:

| tempfile.NamedTemporaryFile(mode='w+b', buffering=None, encoding=None, 
| newline=None, suffix=None, prefix=None, dir=None, delete=True)
| 
| This function operates exactly as TemporaryFile() does, except that the 
| file is guaranteed to have a visible name in the file system (on Unix, the 
| directory entry is not unlinked). […] If delete is true (the default), the 
| file is deleted as soon as it is closed. […]

It is of course possible to generate a filename that is not currently used, 
but I am not aware of a PSL feature that does this, and if there were such a 
feature there would be the same problems with it as with mktemp().

-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.


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