Issue #10395: Added os.path.commonpath(). Implemented in posixpath and ntpath.

Based on patch by Rafik Draoui.

Lib/posixpath.py

@@ -22,7 +22,8 @@ __all__ = ["normcase","isabs","join","splitdrive","split","splitext",
            "ismount", "expanduser","expandvars","normpath","abspath",
            "samefile","sameopenfile","samestat",
            "curdir","pardir","sep","pathsep","defpath","altsep","extsep",
-           "devnull","realpath","supports_unicode_filenames","relpath"]
+           "devnull","realpath","supports_unicode_filenames","relpath",
+           "commonpath"]
 
 # Strings representing various path-related bits and pieces.
 # These are primarily for export; internally, they are hardcoded.
@@ -455,3 +456,45 @@ def relpath(path, start=None):
     except (TypeError, AttributeError, BytesWarning, DeprecationWarning):
         genericpath._check_arg_types('relpath', path, start)
         raise
+
+
+# Return the longest common sub-path of the sequence of paths given as input.
+# The paths are not normalized before comparing them (this is the
+# responsibility of the caller). Any trailing separator is stripped from the
+# returned path.
+
+def commonpath(paths):
+    """Given a sequence of path names, returns the longest common sub-path."""
+
+    if not paths:
+        raise ValueError('commonpath() arg is an empty sequence')
+
+    if isinstance(paths[0], bytes):
+        sep = b'/'
+        curdir = b'.'
+    else:
+        sep = '/'
+        curdir = '.'
+
+    try:
+        split_paths = [path.split(sep) for path in paths]
+
+        try:
+            isabs, = set(p[:1] == sep for p in paths)
+        except ValueError:
+            raise ValueError("Can't mix absolute and relative paths") from None
+
+        split_paths = [[c for c in s if c and c != curdir] for s in split_paths]
+        s1 = min(split_paths)
+        s2 = max(split_paths)
+        common = s1
+        for i, c in enumerate(s1):
+            if c != s2[i]:
+                common = s1[:i]
+                break
+
+        prefix = sep if isabs else sep[:0]
+        return prefix + sep.join(common)
+    except (TypeError, AttributeError):
+        genericpath._check_arg_types('commonpath', *paths)
+        raise