Patch 1339796: add a relpath() function to os.path.

1 files changed, 27 insertions, 1 deletions

diff --git a/Lib/ntpath.py b/Lib/ntpath.py
index 60c1fd0..99d7a4a 100644
--- a/Lib/ntpath.py
+++ b/Lib/ntpath.py
@@ -16,7 +16,7 @@ __all__ = ["normcase","isabs","join","splitdrive","split","splitext",
            "getatime","getctime", "islink","exists","lexists","isdir","isfile",
            "ismount","walk","expanduser","expandvars","normpath","abspath",
            "splitunc","curdir","pardir","sep","pathsep","defpath","altsep",
-           "extsep","devnull","realpath","supports_unicode_filenames"]
+           "extsep","devnull","realpath","supports_unicode_filenames","relpath"]
 
 # strings representing various path-related bits and pieces
 curdir = '.'
@@ -465,3 +465,29 @@ realpath = abspath
 # Win9x family and earlier have no Unicode filename support.
 supports_unicode_filenames = (hasattr(sys, "getwindowsversion") and
                               sys.getwindowsversion()[3] >= 2)
+
+def relpath(path, start=curdir):
+    """Return a relative version of a path"""
+
+    if not path:
+        raise ValueError("no path specified")
+    start_list = abspath(start).split(sep)
+    path_list = abspath(path).split(sep)
+    if start_list[0].lower() != path_list[0].lower():
+        unc_path, rest = splitunc(path)
+        unc_start, rest = splitunc(start)
+        if bool(unc_path) ^ bool(unc_start):
+            raise ValueError("Cannot mix UNC and non-UNC paths (%s and %s)"
+                                                                % (path, start))
+        else:
+            raise ValueError("path is on drive %s, start on drive %s"
+                                                % (path_list[0], start_list[0]))
+    # Work out how much of the filepath is shared by start and path.
+    for i in range(min(len(start_list), len(path_list))):
+        if start_list[i].lower() != path_list[i].lower():
+            break
+    else:
+        i += 1
+
+    rel_list = [pardir] * (len(start_list)-i) + path_list[i:]
+    return join(*rel_list)