Special case for "" & "." in `posixpath.abspath()`

[nineteendo]

Feature or enhancement

Proposal:

We could add a special case for the current directory and return it without normalising. This speeds up posixpath.relpath() with one argument.

 def abspath(path):
     """Return an absolute path."""
     path = os.fspath(path)
     if isinstance(path, bytes):
-        if not path.startswith(b'/'):
-            path = join(os.getcwdb(), path)
+        sep = b'/'
+        curdir = b'.'
+        getcwd = os.getcwdb
     else:
-        if not path.startswith('/'):
-            path = join(os.getcwd(), path)
+        sep = '/'
+        curdir = '.'
+        getcwd = os.getcwd
+    if not path.startswith(sep):
+        if not path or path == curdir:
+            return getcwd()
+        path = join(getcwd(), path)
     return normpath(path)

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-117639: Special case for "" & "." in posixpath.abspath() #117640

[AlexWaygood]

How often does real code call abspath on the current working directory? I'm somewhat sceptical that making the code more complicated here is worth it for such an edge case

[nineteendo]

This speeds up posixpath.relpath() with 1 argument at no cost.

[AlexWaygood]

This speeds up posixpath.relpath() with 1 argument at no cost.

I see. That would be useful information to edit into the issue description.

[erlend-aasland]

This speeds up posixpath.relpath() with 1 argument at no cost.

How much is that speedup? According to your PR there is no speedup for the general call with one argument.

[nineteendo]

I'll do a benchmark soon. The pull request currently doesn't compare the performance of posixpath.relpath(), but it will be at most 13% faster.

[nineteendo]

This special case makes posixpath.relpath() with a single argument 4-6% faster.

[AlexWaygood]

This special case makes posixpath.relpath() with a single argument 4-6% faster.

Hmm. In my opinion, this particular optimisation is not worth it for a speedup of that magnitude. The refactor makes the code of this function look quite odd (the kind of thing that somebody keen on "refactoring" or "simplifying" the code of os.path might propose rewriting in a PR), and it's quite unobvious to a reader of the code that it's been deliberately written that way in order to speedup relpath().

[nineteendo]

OK, I was planning for a larger speedup anyway, that would apply to all relative paths: #117587.