The proposed patch a little cleans and optimizes some bits of _bootstrap.py.

It all LGTM. Nice cleanup! Totally didn't remember about to_bytes and from_bytes.

If you are feeling adventurous you can look at possibly porting the changes you made to _path_join() and _path_split() to os.py since I mostly copied them.

_path_join() and _path_split() do not look as join() and split() from ntpath or posixpath. They rather look as very simplified and limited versions of join() and split(). Perhaps they are enough for _bootstrap.py, but real os.path functions are more complicated.

New changeset 2528e4aea338 by Serhiy Storchaka in branch 'default':
Issue #17220: Little cleanup of _bootstrap.py.
http://hg.python.org/cpython/rev/2528e4aea338

2528e4aea338 seems to have broken building on Windows: http://buildbot.python.org/all/builders/AMD64%20Windows7%20SP1%203.x/builds/1517

I dug into http://buildbot.python.org/all/builders/AMD64%20Windows7%20SP1%203.x/builds/1517/steps/compile/logs/stdio and found this traceback:

EXEC : Fatal Python error : Py_Initialize: unable to load the file system codec [C:\buildbot.python.org\3.x.kloth-win64\build\PCbuild\ssl.vcxproj]
  Traceback (most recent call last):
    File "<frozen importlib._bootstrap>", line 1570, in _find_and_load
    File "<frozen importlib._bootstrap>", line 1537, in _find_and_load_unlocked
    File "<frozen importlib._bootstrap>", line 572, in _check_name_wrapper
    File "<frozen importlib._bootstrap>", line 1035, in load_module
    File "<frozen importlib._bootstrap>", line 1016, in load_module
    File "<frozen importlib._bootstrap>", line 536, in module_for_loader_wrapper
    File "<frozen importlib._bootstrap>", line 859, in is_package
    File "<frozen importlib._bootstrap>", line 77, in _path_split
  ValueError: too many values to unpack (expected 2)

Looks like Serhiy forgot to cap the rsplit() call to a single split (which rpartition does implicitly). I'll fix it shortly.

Found another bug introduced by this patch which I will have a patch for shortly.

New changeset d98a82f4c9bd by Brett Cannon in branch 'default':
Issue #17220: two fixes for changeset 2528e4aea338.
http://hg.python.org/cpython/rev/d98a82f4c9bd