os.path.commonprefix returns the common prefix of a list of paths taken character-by-character. This can
return invalid paths. For example, os.path.commonprefix(["/export/home/dave", "/etc/passwd"]) will return "/e", which likely has no meaning as a path, at least in the context of the input list.

Ideally, os.path.commonprefix would operate component-by-component, but people rely on the existing
character-by-character operation, so it has been so far impossible to change semantics. There are several
possible ways to solve this problem. One, change how commonprefix behaves. Two, add a flag to
commonprefix to allow it to operate component-by-component if desired. Three, add a new function to
os.path.

I personally prefer the first option. Aside from the semantic change though, it presents the problem of
where to put the old definition of commonprefix. It's clearly of some use or people wouldn't have co-
opted it for non-filesystem use. It could go in the string module, but that's been living a life in limbo
since the creation of string methods. People have been loathe to add new functionality there. The second
option seems to me like would just be a hack on top of already broken behavior and probably require the
currently slightly broken behavior as the default to boot, so I won't go there. Since option one is
perhaps not going to be available to me, I've implemented the third option as a new function,
commonpathprefix. See the attached patch. It includes test cases and documentation changes.

A new function sounds like a good solution to me. How about just calling
it "os.path.commonpath" though?

I agree having a path component based prefix function in os.path is
highly desirable, particularly since the addition of relpath in 2.6:

base_dir = os.path.commonpath(paths)
rel_paths = [os.path.relpath(p, base_dir) for p in paths]

The documentation should explain what a "common path prefix" is. It
can't be the path to a common parent directory, since the new function
doesn't allow mixing absolute and relative directories. As Phillip Eby
points out, it also doesn't account for case-insensitivity that some
file systems or operating systems implement, nor does it take into
account short file names on Windows.

I think we need to recognize the inherent limitations of what we can expect
to do. It is perfectly reasonable for a user on Windows to import posixpath
and call posixpath.commonpathprefix. The function won't have access to the
actual filesystems being manipulated. Same for Unix folks importing ntpath
and manipulating Windows paths. While we can make it handle
case-insensitivity, I'm no sure we can do much, if anything, about shortened
filenames.

Also, as long as we are considering case sensitivity, what about HFS on Mac
OS X?

Skip

1. The discussion on python-dev shows that the current documentation of
   os.path.commonprefix is incorrect - it technically works element by
   element rather than character by character (since it will handle
   sequences other than strings, such as lists of path components)

2. Splitting on os.sep is not the correct way to break a string into
   path components. Instead, os.path.split needs to be applied repeatedly
   until "head" is a single character (a single occurrence of os.sep or
   os.altsep for an absolute path) or empty (for a relative path).
   (Alternatively, but with additional effects on the result, the
   separators can be normalised first with os.path.normpath or
   os.path.normcase)

For Windows, os.path.splitunc and os.path.splitdrive should also be
invoked first, and if either returns a non-empty string, that should
become the first path component (with the remaining components filled in
as above)

3. Calling any or all of
   abspath/expanduser/expandvars/normcase/normpath/realpath is the
   responsibility of the library user as far as os.path.commonprefix is
   concerned. Should that behaviour be retained for an os.path.commonpath
   function, or should some of them (such as os.path.abspath) be called
   automatically?

The regex based approach to the component splitting when os.altsep is
defined obviously works as well. Duplicating the values of sep and
altsep in the default regex that way grates a little though...

http://code.activestate.com/recipes/577016-path-entire-split-commonprefix/

There is more developed patch in bpo-10395.

Feel free to close this ticket. I long ago gave up on it.

bpo-10395 added “os.path.commonpath” in 3.5.