diff -r 1a972140ab62 Lib/os.py
--- a/Lib/os.py	Thu Mar 12 09:12:48 2015 +0100
+++ b/Lib/os.py	Thu Mar 12 10:04:15 2015 +0100
@@ -354,7 +354,7 @@ def walk(top, topdown=True, onerror=None
 
     dirs = []
     nondirs = []
-    symlinks = set()
+    symlinks = {}
 
     # We may not have read permission for top, in which case we can't
     # get a list of the files the directory contains.  os.walk
@@ -379,13 +379,12 @@ def walk(top, topdown=True, onerror=None
 
             if is_dir and not followlinks:
                 try:
-                    if entry.is_symlink():
-                        symlinks.add(entry.name)
+                    symlinks[entry.name] = entry.is_symlink()
                 except OSError:
                     # If is_symlink() raises an OSError, consider that the
                     # entry is not a symbolik link, same behaviour than
                     # os.path.islink().
-                    pass
+                    symlinks[entry.name] = False
     except OSError as error:
         # scandir() or iterating into scandir() iterator raised an OSError
         if onerror is not None:
@@ -398,7 +397,15 @@ def walk(top, topdown=True, onerror=None
 
     # Recurse into sub-directories
     for name in dirs:
-        if followlinks or name not in symlinks:
+        if followlinks:
+            walk_into = True
+        else:
+            try:
+                walk_into = not symlinks[name]
+            except KeyError:
+                # directory added after dirs was created
+                walk_into = not path.islink(path.join(top, name))
+        if walk_into:
             new_path = path.join(top, name)
             yield from walk(new_path, topdown, onerror, followlinks)