bpo-38883: Don't use POSIX `$HOME` in `pathlib.Path.home/expanduser` on Windows (GH-17961)

In bpo-36264 os.path.expanduser was changed to ignore HOME on Windows.

Path.expanduser/home still honored HOME despite being documented as behaving the same
as os.path.expanduser. This makes them also ignore HOME so that both implementations
behave the same way again.

Lib/pathlib.py

@@ -253,9 +253,7 @@ class _WindowsFlavour(_Flavour):
             return 'file:' + urlquote_from_bytes(path.as_posix().encode('utf-8'))
 
     def gethomedir(self, username):
-        if 'HOME' in os.environ:
-            userhome = os.environ['HOME']
-        elif 'USERPROFILE' in os.environ:
+        if 'USERPROFILE' in os.environ:
             userhome = os.environ['USERPROFILE']
         elif 'HOMEPATH' in os.environ:
             try:

Lib/test/test_pathlib.py

@@ -1383,8 +1383,16 @@ class _BasePathTest(object):
         self.assertTrue(p.is_absolute())
 
     def test_home(self):
-        p = self.cls.home()
-        self._test_home(p)
+        with support.EnvironmentVarGuard() as env:
+            self._test_home(self.cls.home())
+
+            env.clear()
+            env['USERPROFILE'] = os.path.join(BASE, 'userprofile')
+            self._test_home(self.cls.home())
+
+            # bpo-38883: ignore `HOME` when set on windows
+            env['HOME'] = os.path.join(BASE, 'home')
+            self._test_home(self.cls.home())
 
     def test_samefile(self):
         fileA_path = os.path.join(BASE, 'fileA')
@@ -2448,12 +2456,6 @@ class WindowsPathTest(_BasePathTest, unittest.TestCase):
                 self.assertEqual(p5.expanduser(), p5)
                 self.assertEqual(p6.expanduser(), p6)
 
-            # Test the first lookup key in the env vars.
-            env['HOME'] = 'C:\\Users\\alice'
-            check()
-
-            # Test that HOMEPATH is available instead.
-            env.pop('HOME', None)
             env['HOMEPATH'] = 'C:\\Users\\alice'
             check()
 
@@ -2466,6 +2468,10 @@ class WindowsPathTest(_BasePathTest, unittest.TestCase):
             env['USERPROFILE'] = 'C:\\Users\\alice'
             check()
 
+            # bpo-38883: ignore `HOME` when set on windows
+            env['HOME'] = 'C:\\Users\\eve'
+            check()
+
 
 class CompatiblePathTest(unittest.TestCase):
     """

Misc/NEWS.d/next/Windows/2020-01-11-22-53-55.bpo-38883.X7FRaN.rst

@@ -0,0 +1,5 @@
+:meth:`~pathlib.Path.home()` and :meth:`~pathlib.Path.expanduser()` on Windows
+now prefer :envvar:`USERPROFILE` and no longer use :envvar:`HOME`, which is not
+normally set for regular user accounts. This makes them again behave like
+:func:`os.path.expanduser`, which was changed to ignore :envvar:`HOME` in 3.8,
+see :issue:`36264`.