gh-94909: fix joining of absolute and relative Windows paths in pathlib

Author: barneygale

Lib/pathlib.py

@@ -65,10 +65,8 @@ def parse_parts(self, parts):
         sep = self.sep
         altsep = self.altsep
         drv = root = ''
-        it = reversed(parts)
-        for part in it:
-            if not part:
-                continue
+        if parts:
+            part = self.pathmod.join(*parts)
             if altsep:
                 part = part.replace(altsep, sep)
             drv, root, rel = self.splitroot(part)
@@ -79,20 +77,6 @@ def parse_parts(self, parts):
             else:
                 if rel and rel != '.':
                     parsed.append(sys.intern(rel))
-            if drv or root:
-                if not drv:
-                    # If no drive is present, try to find one in the previous
-                    # parts. This makes the result of parsing e.g.
-                    # ("C:", "/", "a") reasonably intuitive.
-                    for part in it:
-                        if not part:
-                            continue
-                        if altsep:
-                            part = part.replace(altsep, sep)
-                        drv = self.splitroot(part)[0]
-                        if drv:
-                            break
-                break
         if drv or root:
             parsed.append(drv + root)
         parsed.reverse()

Lib/test/test_pathlib.py

@@ -136,6 +136,10 @@ def test_parse_parts(self):
         check(['a', '/b', 'c'],         ('', '\\', ['\\', 'b', 'c']))
         check(['Z:/a', '/b', 'c'],      ('Z:', '\\', ['Z:\\', 'b', 'c']))
         check(['//?/Z:/a', '/b', 'c'],  ('\\\\?\\Z:', '\\', ['\\\\?\\Z:\\', 'b', 'c']))
+        # Joining with the same drive => the first path is appended to if
+        # the second path is relative.
+        check(['c:/a/b', 'c:x/y'], ('c:', '\\', ['c:\\', 'a', 'b', 'x', 'y']))
+        check(['c:/a/b', 'c:/x/y'], ('c:', '\\', ['c:\\', 'x', 'y']))
 
     def test_splitroot(self):
         f = self.flavour.splitroot

Misc/NEWS.d/next/Library/2022-07-29-20-58-37.gh-issue-94909.YjMusj.rst

@@ -0,0 +1,2 @@
+Fix incorrect joining of relative Windows paths with drives in
+:class:`pathlib.PurePath` initializer.