pythonGH-113373: Slightly speed up path parsing in pathlib

Small improvements:

- `Path('foo', '')` and `Path('foo') / ''` are improved by omitting an
  `os.path.join()` call.
- The result of `os.path.join()` is stored and re-used for subsequent
  joins.

Smaller improvements:

- Check for the common case (`len(paths) == 0`) first, and speed up the
  subsequent `elif` by avoiding a length check
- Don't bother assigning the `_parse_path()` result to local variables

Lib/pathlib/__init__.py

@@ -89,7 +89,8 @@ def __init__(self, *args):
                         "argument should be a str or an os.PathLike "
                         "object where __fspath__ returns a str, "
                         f"not {type(path).__name__!r}")
-                paths.append(path)
+                if path:
+                    paths.append(path)
         # Avoid calling super().__init__, as an optimisation
         self._raw_paths = paths
         self._resolving = False

Lib/pathlib/_abc.py

@@ -206,7 +206,11 @@ class PurePathBase:
     )
     pathmod = os.path
 
-    def __init__(self, *paths):
+    def __init__(self, *args):
+        paths = []
+        for arg in args:
+            if arg:
+                paths.append(arg)
         self._raw_paths = paths
         self._resolving = False
 
@@ -219,8 +223,6 @@ def with_segments(self, *pathsegments):
 
     @classmethod
     def _parse_path(cls, path):
-        if not path:
-            return '', '', []
         sep = cls.pathmod.sep
         altsep = cls.pathmod.altsep
         if altsep:
@@ -239,16 +241,19 @@ def _parse_path(cls, path):
 
     def _load_parts(self):
         paths = self._raw_paths
-        if len(paths) == 0:
-            path = ''
-        elif len(paths) == 1:
+        if len(paths) == 1:
             path = paths[0]
-        else:
+        elif paths:
             path = self.pathmod.join(*paths)
-        drv, root, tail = self._parse_path(path)
-        self._drv = drv
-        self._root = root
-        self._tail_cached = tail
+            # Store the joined path. This makes subsequent joins onto this
+            # path slightly faster.
+            paths.clear()
+            paths.append(path)
+        else:
+            self._drv = self._root = ''
+            self._tail_cached = []
+            return
+        self._drv, self._root, self._tail_cached = self._parse_path(path)
 
     def _from_parsed_parts(self, drv, root, tail):
         path_str = self._format_parsed_parts(drv, root, tail)

Misc/NEWS.d/next/Library/2023-12-21-19-26-35.gh-issue-113373.QLaBAS.rst

@@ -0,0 +1 @@
+Slightly speed up path parsing in :mod:`pathlib`. Patch by Barney Gale.