[Python-checkins] bpo-26329: update os.path.normpath documentation (GH-20138) (#27095)

[ambv]

https://github.com/python/cpython/commit/47abf240365ddd54a91c6ac167900d4bf6806c4f
commit: 47abf240365ddd54a91c6ac167900d4bf6806c4f
branch: 3.9
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: ambv <lukasz at langa.pl>
date: 2021-07-12T17:22:33+02:00
summary:

bpo-26329: update os.path.normpath documentation (GH-20138) (#27095)

(cherry picked from commit 66c5853406bbcccecf35372795078c0641a5f385)

Co-authored-by: Furkan Onder <furkanonder at protonmail.com>

files:
M Doc/library/os.path.rst
M Lib/posixpath.py

diff --git a/Doc/library/os.path.rst b/Doc/library/os.path.rst
index d2fe4943524533..3be513e9099c78 100644
--- a/Doc/library/os.path.rst
+++ b/Doc/library/os.path.rst
@@ -340,6 +340,14 @@ the :mod:`glob` module.)
    that contains symbolic links.  On Windows, it converts forward slashes to
    backward slashes. To normalize case, use :func:`normcase`.
 
+  .. note::
+      On POSIX systems, in accordance with `IEEE Std 1003.1 2013 Edition; 4.13
+      Pathname Resolution <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13>`_,
+      if a pathname begins with exactly two slashes, the first component
+      following the leading characters may be interpreted in an implementation-defined
+      manner, although more than two leading characters shall be treated as a
+      single character.
+
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
 
diff --git a/Lib/posixpath.py b/Lib/posixpath.py
index ecb4e5a8f7072c..af2814bdb05804 100644
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -349,6 +349,7 @@ def normpath(path):
     initial_slashes = path.startswith(sep)
     # POSIX allows one or two initial slashes, but treats three or more
     # as single slash.
+    # (see http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13)
     if (initial_slashes and
         path.startswith(sep*2) and not path.startswith(sep*3)):
         initial_slashes = 2