Optimize contruction of Path and PurePath objects

Lib/pathlib.py

@@ -658,6 +658,8 @@ class PurePath(object):
         """
         if cls is PurePath:
             cls = PureWindowsPath if os.name == 'nt' else PurePosixPath
+        if len(args) == 1 and type(args[0]) is cls:
+            return args[0]
         return cls._from_parts(args)
 
     def __reduce__(self):
@@ -1064,6 +1066,8 @@ class Path(PurePath):
     def __new__(cls, *args, **kwargs):
         if cls is Path:
             cls = WindowsPath if os.name == 'nt' else PosixPath
+        if len(args) == 1 and len(kwargs) == 0 and type(args[0]) is cls:
+            return args[0]
         self = cls._from_parts(args, init=False)
         if not self._flavour.is_supported:
             raise NotImplementedError("cannot instantiate %r on your system"

Lib/test/test_pathlib.py

@@ -214,6 +214,12 @@ class _BasePurePathTest(object):
         for part in p.parts:
             self.assertIs(type(part), str)
 
+    def test_init(self):
+        # See bpo-39783, creating a PurePath or a Path object from an other
+        # PurePath instance return the existing instance.
+        p = self.cls('foo')
+        self.assertIs(p, self.cls(p))
+
     def test_str_subclass_common(self):
         self._check_str_subclass('')
         self._check_str_subclass('.')
@@ -1402,6 +1408,10 @@ class _BasePathTest(object):
         self.assertIs(type(p), type(q))
         self.assertTrue(p.is_absolute())
 
+    def test_init(self):
+        p = self.cls('foo')
+        self.assertIs(p, self.cls(p))
+
     def test_cwd(self):
         p = self.cls.cwd()
         self._test_cwd(p)

Misc/NEWS.d/next/Library/2020-05-21-13-41-44.bpo-39783.Eq-m0w.rst

@@ -0,0 +1,3 @@
+:class:`pathlib.PurePath`, :class:`pathlib.Path` and their subclasses now
+return the same object when their argument is already a Path. Patch
+contributed by Rémi Lapeyre.