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

[nineteendo]

Benchmark:

ntpath.py

script

::test.bat
@echo off
echo 1 item && python -m timeit -s "import before.ntpath" "before.ntpath.join('foo')" && python -m timeit -s "import after.ntpath" "after.ntpath.join('foo')"
echo 10 items && python -m timeit -s "import before.ntpath; paths = ['foo'] * 10" "before.ntpath.join(*paths)" && python -m timeit -s "import after.ntpath; paths = ['foo'] * 10" "after.ntpath.join(*paths)"
echo 100 items && python -m timeit -s "import before.ntpath; paths = ['foo'] * 100" "before.ntpath.join(*paths)" && python -m timeit -s "import after.ntpath; paths = ['foo'] * 100" "after.ntpath.join(*paths)"

1 item
500000 loops, best of 5: 699 nsec per loop # before
500000 loops, best of 5: 648 nsec per loop # after
# -> 1.08x faster
10 items
50000 loops, best of 5: 5.48 usec per loop # before
50000 loops, best of 5: 5.52 usec per loop # after
# -> no difference
100 items
5000 loops, best of 5: 54.1 usec per loop # before
5000 loops, best of 5: 54 usec per loop # after
# -> no difference

posixpath.py

script

# test.sh
echo 1 item && python -m timeit -s "import before.posixpath" "before.posixpath.join('foo')" && python -m timeit -s "import after.posixpath" "after.posixpath.join('foo')"
echo 10 items && python -m timeit -s "import before.posixpath; paths = ['foo'] * 10" "before.posixpath.join(*paths)" && python -m timeit -s "import after.posixpath; paths = ['foo'] * 10" "after.posixpath.join(*paths)"
echo 100 items && python -m timeit -s "import before.posixpath; paths = ['foo'] * 100" "before.posixpath.join(*paths)" && python -m timeit -s "import after.posixpath; paths = ['foo'] * 100" "after.posixpath.join(*paths)"

1 item
1000000 loops, best of 5: 335 nsec per loop # before
1000000 loops, best of 5: 271 nsec per loop # after
# -> 1.24x faster
10 items
50000 loops, best of 5: 3.43 usec per loop # before
100000 loops, best of 5: 3.35 usec per loop # after
# -> 1.02x faster
100 items
10000 loops, best of 5: 34.4 usec per loop # before
10000 loops, best of 5: 34.1 usec per loop # after
# -> no difference

• Issue: Remove redundant type check in os.path.join() #117636

[nineteendo]

@serhiy-storchaka, you're the one who added this check, so you can probably best evaluate if it's still needed.

[serhiy-storchaka]

The first check was for cases like join(None) or join(['a', 'b']). The second checks was for cases like join('', b'a'), join(b'', 'a'), join(None, 'a'), or something like this.

It seems that after adding fspath() they are no longer needed.

[nineteendo]

Apparently this unnecessary concatenation has been here since the first iteration of posixpath! So it wasn't even intended as a type check:

cpython/Lib/posixpath.py

Lines 11 to 14 in c636014

	def cat(a, b):
	if b[:1] = '/': return b
	if a = '' or a[-1:] = '/': return a + b
	return a + '/' + b

[hauntsaninja]

Thank you!

[nineteendo]

@serhiy-storchaka, can this be merged?