python os.path.dirname yields wrong results with windows network path

Question

I've got this code snippet

print "tmp = ", tmp
print "dirname = ", os.path.dirname(tmp)
print "tmp sane = ", (os.path.dirname(tmp.replace("\\", "/"))).replace("/", "\\")

that produces this result

tmp =  \\aaaa.aaa\aaaaaaaa\aaaaaaaaa\aaaa\aaaa\aaaaa\aaaaaaa\aaaaaaa\aaaa.aaaaaaaa_aaaaaa\aa\aaaaa\aaaaaa\aaa\aaaaaa\aa\aaaaa\aaaaaaa_aaaaaaaaa_aaa\aaaa_aa.aaa
dirname =
tmp sane =  \\aaaa.aaa\aaaaaaaa\aaaaaaaaa\aaaa\aaaa\aaaaa\aaaaaaa\aaaaaaa\aaaa.aaaaaaaa_aaaaaa\aa\aaaaa\aaaaaa\aaa\aaaaaa\aa\aaaaa\aaaaaaa_aaaaaaaaa_aaa

Any ideas why "tmp sane" works and the simple dirname doesn't? I couldn't find anything related to the windows network names/backslashes.

Answer 1

You are most likely not running this on Windows. The behaviour is entirely consistent with using the os.path module on anything but an actual Windows environment.

os.path adjusts behaviour to match the current platform. On Windows, both forward and backward slashes are supported, but on Linux and Mac only forward slashes are recognized. That's to match the actual convention used on the platform you are running your code on.

If you want to split dirnames from Windows paths on a POSIX OS like Mac OS X or Linux, you'll need to import the ntpath module; that module is what is really used on Windows but it is available still on other platforms:

import ntpath
result = ntpath.dirname(path)

See the top of the os.path module documentation:

Note: Since different operating systems have different path name conventions, there are several versions of this module in the standard library. The os.path module is always the path module suitable for the operating system Python is running on, and therefore usable for local paths. However, you can also import and use the individual modules if you want to manipulate a path that is always in one of the different formats. They all have the same interface:

• posixpath for UNIX-style paths
• ntpath for Windows paths
• macpath for old-style MacOS paths
• os2emxpath for OS/2 EMX paths

Cygwin is considered a POSIX environment, not Windows, in this context.