Use set comprehension for `posixpath.commonpath()`

[nineteendo]

Feature or enhancement

Proposal:

We can use a set comprehension to check if no absolute and relative paths are mixed:

-try:
-    isabs, = set(p[:1] == sep for p in paths)
-except ValueError:
-    raise ValueError("Can't mix absolute and relative paths") from None
+if len({p.startswith(sep) for p in paths}) != 1:
+    raise ValueError("Can't mix absolute and relative paths")

-prefix = sep if isabs else sep[:0]
+prefix = sep if paths[0].startswith(sep) else sep[:0]

This is faster and more readable.

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-117641: Use set comprehension for posixpath.commonpath() #117652