diff --git a/Doc/library/os.rst b/Doc/library/os.rst
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -2227,7 +2227,9 @@
    directory is generated before the triples for any of its subdirectories
    (directories are generated top-down).  If *topdown* is ``False``, the triple for a
    directory is generated after the triples for all of its subdirectories
-   (directories are generated bottom-up).
+   (directories are generated bottom-up). In either case the list of
+   subdirectories is retrieved before the tuples for the directory and
+   its subdirectories are generated.
 
    When *topdown* is ``True``, the caller can modify the *dirnames* list in-place
    (perhaps using :keyword:`del` or slice assignment), and :func:`walk` will only
diff --git a/Lib/os.py b/Lib/os.py
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -241,6 +241,8 @@
     the search, or to impose a specific order of visiting.  Modifying
     dirnames when topdown is false is ineffective, since the directories in
     dirnames have already been generated by the time dirnames itself is
+    generated. In either case the list of subdirectories is retrieved
+    before the tuples for the directory and its subdirectories are
     generated.
 
     By default errors from the os.listdir() call are ignored.  If