diff -r 211e29335e72 Lib/os.py
--- a/Lib/os.py	Fri Mar 13 11:01:30 2015 +0100
+++ b/Lib/os.py	Fri Mar 13 11:34:41 2015 +0100
@@ -354,7 +354,7 @@ def walk(top, topdown=True, onerror=None
 
     dirs = []
     nondirs = []
-    symlinks = set()
+    catch_oserror = True
 
     # 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
@@ -377,16 +377,30 @@ def walk(top, topdown=True, onerror=None
             else:
                 nondirs.append(entry.name)
 
-            if is_dir and not followlinks:
-                try:
-                    if entry.is_symlink():
-                        symlinks.add(entry.name)
-                except OSError:
-                    # If is_symlink() raises an OSError, consider that the
-                    # entry is not a symbolik link, same behaviour than
-                    # os.path.islink().
-                    pass
+            if not topdown and is_dir:
+                # Bottom-up: recurse into sub-directory, but exclude symlinks
+                # to directories if followlinks is False
+                if followlinks:
+                    walk_into = True
+                else:
+                    try:
+                        is_symlink = 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().
+                        is_symlink = False
+                    walk_into = not is_symlink
+
+                if walk_into:
+                    # Don't catch OSError in recursive calls to walk()
+                    catch_oserror = False
+                    yield from walk(entry.path, topdown, onerror, followlinks)
+                    catch_oserror = True
     except OSError as error:
+        if not catch_oserror:
+            # OSError raised by the recursive call to walk()
+            raise
         # scandir() or iterating into scandir() iterator raised an OSError
         if onerror is not None:
             onerror(error)
@@ -396,14 +410,18 @@ def walk(top, topdown=True, onerror=None
     if topdown:
         yield top, dirs, nondirs
 
-    # Recurse into sub-directories
-    for name in dirs:
-        if followlinks or name not in symlinks:
-            new_path = path.join(top, name)
-            yield from walk(new_path, topdown, onerror, followlinks)
-
-    # Yield after recursion if going bottom up
-    if not topdown:
+        # Recurse into sub-directories
+        islink, join = path.islink, path.join
+        for name in dirs:
+            new_path = join(top, name)
+            # Issue #23605: os.path.islink() is used instead of caching
+            # entry.is_symlink() result during the loop on os.scandir() because
+            # the caller can replace the directory entry during the "yield"
+            # above.
+            if followlinks or not islink(new_path):
+                yield from walk(new_path, topdown, onerror, followlinks)
+    else:
+        # Yield after recursion if going bottom up
         yield top, dirs, nondirs
 
 __all__.append("walk")