bpo-32557: allow shutil.disk_usage to take a file path on Windows also (GH-9372)

https://bugs.python.org/issue32557

Doc/library/shutil.rst

@@ -343,11 +343,14 @@ Directory and files operations
 
    Return disk usage statistics about the given path as a :term:`named tuple`
    with the attributes *total*, *used* and *free*, which are the amount of
-   total, used and free space, in bytes.  On Windows, *path* must be a
-   directory; on Unix, it can be a file or directory.
+   total, used and free space, in bytes. *path* may be a file or a
+   directory.
 
    .. versionadded:: 3.3
 
+   .. versionchanged:: 3.8
+     On Windows, *path* can now be a file or directory.
+
    Availability: Unix, Windows.
 
 .. function:: chown(path, user=None, group=None)

Lib/test/test_shutil.py

@@ -1363,6 +1363,7 @@ class TestShutil(unittest.TestCase):
                          "disk_usage not available on this platform")
     def test_disk_usage(self):
         usage = shutil.disk_usage(os.path.dirname(__file__))
+        self.assertEqual(usage, shutil.disk_usage(__file__))
         self.assertGreater(usage.total, 0)
         self.assertGreater(usage.used, 0)
         self.assertGreaterEqual(usage.free, 0)

Misc/NEWS.d/next/Windows/2018-09-25-10-39-27.bpo-32557.Rs1bf9.rst

@@ -0,0 +1 @@
+Allow shutil.disk_usage to take a file path on Windows

Modules/posixmodule.c

@@ -10079,13 +10079,38 @@ os__getdiskusage_impl(PyObject *module, path_t *path)
 {
     BOOL retval;
     ULARGE_INTEGER _, total, free;
+    DWORD err = 0;
 
     Py_BEGIN_ALLOW_THREADS
     retval = GetDiskFreeSpaceExW(path->wide, &_, &total, &free);
     Py_END_ALLOW_THREADS
-    if (retval == 0)
-        return PyErr_SetFromWindowsErr(0);
+    if (retval == 0) {
+        if (GetLastError() == ERROR_DIRECTORY) {
+            wchar_t *dir_path = NULL;
 
+            dir_path = PyMem_New(wchar_t, path->length + 1);
+            if (dir_path == NULL) {
+                return PyErr_NoMemory();
+            }
+
+            wcscpy_s(dir_path, path->length + 1, path->wide);
+
+            if (_dirnameW(dir_path) != -1) {
+                Py_BEGIN_ALLOW_THREADS
+                retval = GetDiskFreeSpaceExW(dir_path, &_, &total, &free);
+                Py_END_ALLOW_THREADS
+            }
+            /* Record the last error in case it's modified by PyMem_Free. */
+            err = GetLastError();
+            PyMem_Free(dir_path);
+            if (retval) {
+                goto success;
+            }
+        }
+        return PyErr_SetFromWindowsErr(err);
+    }
+
+success:
     return Py_BuildValue("(LL)", total.QuadPart, free.QuadPart);
 }
 #endif /* MS_WINDOWS */