From e5f62ec5cd0dde14777754c6ea05034b738f3a1f Mon Sep 17 00:00:00 2001
From: barneygale <barney.gale@gmail.com>
Date: Sat, 6 Jul 2024 14:36:18 +0100
Subject: [PATCH] GH-119186: Slightly speed up `os.walk(topdown=True)`

When `os.walk()` traverses into subdirectories in top-down mode, call
`os.path.join()` once to add a trailing slash, and use string concatenation
thereafter to generate child paths.
---
 Lib/os.py                                      | 18 ++++++++++--------
 ...4-07-06-14-32-30.gh-issue-119186.E5B1HQ.rst |  2 ++
 2 files changed, 12 insertions(+), 8 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2024-07-06-14-32-30.gh-issue-119186.E5B1HQ.rst

diff --git a/Lib/os.py b/Lib/os.py
index 4b48afb040e565..75dd65f88a7077 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -432,14 +432,16 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
             # Yield before sub-directory traversal if going top down
             yield top, dirs, nondirs
             # Traverse into sub-directories
-            for dirname in reversed(dirs):
-                new_path = join(top, dirname)
-                # bpo-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):
-                    stack.append(new_path)
+            if dirs:
+                prefix = join(top, top[:0])  # Add trailing slash
+                for dirname in reversed(dirs):
+                    new_path = prefix + dirname
+                    # bpo-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):
+                        stack.append(new_path)
         else:
             # Yield after sub-directory traversal if going bottom up
             stack.append((top, dirs, nondirs))
diff --git a/Misc/NEWS.d/next/Library/2024-07-06-14-32-30.gh-issue-119186.E5B1HQ.rst b/Misc/NEWS.d/next/Library/2024-07-06-14-32-30.gh-issue-119186.E5B1HQ.rst
new file mode 100644
index 00000000000000..fbb4f58b59358f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-07-06-14-32-30.gh-issue-119186.E5B1HQ.rst
@@ -0,0 +1,2 @@
+Slightly speed up :func:`os.walk` by calling :func:`os.path.join` less
+often.