GH-119169: Slightly speed up os.walk(topdown=True) (GH-121431)

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

@@ -417,8 +417,10 @@ 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
+            if dirs:
+                prefix = join(top, top[:0])  # Add trailing slash
                 for dirname in reversed(dirs):
-                new_path = join(top, dirname)
+                    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"

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.