GH-119169: Speed up os.walk(topdown=False)

Lib/os.py

@@ -365,7 +365,9 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
 
         dirs = []
         nondirs = []
-        walk_dirs = []
+        if not topdown:
+            # Yield after sub-directory traversal if going bottom up
+            stack.append((top, dirs, nondirs))
 
         # We may not have read permission for top, in which case we can't
         # get a list of the files the directory contains.
@@ -375,41 +377,34 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
         try:
             with scandir(top) as entries:
                 for entry in entries:
+                    is_dir = False
                     try:
                         if followlinks is _walk_symlinks_as_files:
                             is_dir = entry.is_dir(follow_symlinks=False) and not entry.is_junction()
                         else:
                             is_dir = entry.is_dir()
+                        if is_dir and not topdown:
+                            # Exclude symlink if followlinks is False
+                            if followlinks or not entry.is_symlink():
+                                # Traverse into sub-directory
+                                stack.append(entry.path)
                     except OSError:
                         # If is_dir() raises an OSError, consider the entry not to
                         # be a directory, same behaviour as os.path.isdir().
-                        is_dir = False
+                        pass
 
                     if is_dir:
                         dirs.append(entry.name)
                     else:
                         nondirs.append(entry.name)
 
-                    if not topdown and is_dir:
-                        # Bottom-up: traverse 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 the
-                                # entry not to be a symbolic link, same behaviour
-                                # as os.path.islink().
-                                is_symlink = False
-                            walk_into = not is_symlink
-
-                        if walk_into:
-                            walk_dirs.append(entry.path)
         except OSError as error:
             if onerror is not None:
                 onerror(error)
+            if not topdown:
+                # Undo additions to stack.
+                while not isinstance(stack.pop(), tuple):
+                    pass
             continue
 
         if topdown:
@@ -424,12 +419,6 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
                 # above.
                 if followlinks or not islink(new_path):
                     stack.append(new_path)
-        else:
-            # Yield after sub-directory traversal if going bottom up
-            stack.append((top, dirs, nondirs))
-            # Traverse into sub-directories
-            for new_path in reversed(walk_dirs):
-                stack.append(new_path)
 
 __all__.append("walk")
 

Misc/NEWS.d/next/Library/2024-05-19-20-13-09.gh-issue-119169.CuE7q0.rst

@@ -0,0 +1 @@
+Slightly speed up :func:`os.walk` by simplifying exception handling.