1 files changed, 19 insertions, 3 deletions
diff --git a/Lib/ntpath.py b/Lib/ntpath.py
index 549c35e..1f355ec 100644
--- a/Lib/ntpath.py
+++ b/Lib/ntpath.py
@@ -440,9 +440,25 @@ def normpath(path):
     """Normalize path, eliminating double slashes, etc."""
     path = path.replace("/", "\\")
     prefix, path = splitdrive(path)
-    while path[:1] == "\\":
-        prefix = prefix + "\\"
-        path = path[1:]
+    # We need to be careful here. If the prefix is empty, and the path starts
+    # with a backslash, it could either be an absolute path on the current
+    # drive (\dir1\dir2\file) or a UNC filename (\\server\mount\dir1\file). It
+    # is therefore imperative NOT to collapse multiple backslashes blindly in
+    # that case.
+    # The code below preserves multiple backslashes when there is no drive
+    # letter. This means that the invalid filename \\\a\b is preserved
+    # unchanged, where a\\\b is normalised to a\b. It's not clear that there
+    # is any better behaviour for such edge cases.
+    if prefix == '':
+        # No drive letter - preserve initial backslashes
+        while path[:1] == "\\":
+            prefix = prefix + "\\"
+            path = path[1:]
+    else:
+        # We have a drive letter - collapse initial backslashes
+        if path.startswith("\\"):
+            prefix = prefix + "\\"
+            path = path.lstrip("\\")
     comps = path.split("\\")
     i = 0
     while i < len(comps):