Race-condition creating folder in Python

Instead of

if not os.path.exists(self.cache_location):    os.mkdir(self.cache_location)

you could do

try:    os.makedirs(self.cache_location)except OSError:    pass

As you would end up with the same functionality.

_{DISCLAIMER: I don't know how Pythonic this might be.}

Using SQLite3, might be a bit of overkill, but would add a lot of functionality and flexibility to your use case.

If you have to do a lot of "selecting", concurrent inserting and filtering, it's a great idea to use SQLite3, as it wont add too much complexity over simple files (it could be argued that it removes complexity).

Rereading your question (and comments) I can better understand your problem.

What is the possibility that a file could create the same race condition?

If it is small enough, then I'd do something like:

if not os.path.isfile(self.cache_location):    try:        os.makedirs(self.cache_location)    except OSError:        pass

Also, reading your code, I'd change

else:    # Our target dir is already a file, or different error,    # relay the error!    raise OSError(e)

to

else:    # Our target dir is already a file, or different error,    # relay the error!    raise

as it's really what you want, Python to reraise the exact same exception _{(just nitpicking)}.

One more thing, may be this could be of use for you (Unix-like only).

The code I ended up with was:

import osimport errnofolder_location = "/tmp/example_dir"try:    os.mkdir(folder_location)except OSError as e:    if e.errno == errno.EEXIST and os.path.isdir(folder_location):        # File exists, and it's a directory,        # another process beat us to creating this dir, that's OK.        pass    else:        # Our target dir exists as a file, or different error,        # reraise the error!        raise

In Python 3.x, you can use os.makedirs(path, exist_ok=True), which will not raise any exception if such directory exists. It will raise FileExistsError: [Errno 17] if a file exists with the same name as the requested directory (path).

Verify it with:

import osparent = os.path.dirname(__file__)target = os.path.join(parent, 'target')os.makedirs(target, exist_ok=True)os.makedirs(target, exist_ok=True)os.rmdir(target)with open(target, 'w'):    passos.makedirs(target, exist_ok=True)