Remove redundant type check in `os.path.join()`

[nineteendo]

Feature or enhancement

Proposal:

These type checks were introduced a long time ago in 5bfc03f and became redundant in 3f9183b with the addition of os.fspath(). They can be safely removed, slightly speeding up os.path.join() in the process:

 def join(path, *paths):
     path = os.fspath(path)
     if isinstance(path, bytes):
         sep = b'\\'
         seps = b'\\/'
         colon_seps = b':\\/'
     else:
         sep = '\\'
         seps = '\\/'
         colon_seps = ':\\/'
     try:
-        if not paths:
-            path[:0] + sep  #23780: Ensure compatible data type even if p is null.

 def join(a, *p):
     """Join two or more pathname components, inserting '/' as needed.
     If any component is an absolute path, all previous path components
     will be discarded.  An empty last part will result in a path that
     ends with a separator."""
     a = os.fspath(a)
     sep = _get_sep(a)
     path = a
     try:
-        if not p:
-            path[:0] + sep  #23780: Ensure compatible data type even if p is null.

I also noticed we're concatenating b to an empty string in posixpath.join() in case path has a length of 0. Which we can fix quite easily:

-if b.startswith(sep):
+if b.startswith(sep) or not path:
     path = b
-elif not path or path.endswith(sep):
+elif path.endswith(sep):
     path += b

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

• gh-117349: Speedup os.path #117610

Linked PRs

• gh-117636: Remove redundant type check in os.path.join() #117638