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