bpo-41092: Request actual filesize from 'os.path.getsize'

Provide the ability to get the actual, on-disk size of a file using
'os.path.getsize'. This is helpful when dealing with things like sparse
files.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

Doc/library/os.path.rst

@@ -234,14 +234,22 @@ the :mod:`glob` module.)
       Accepts a :term:`path-like object`.
 
 
-.. function:: getsize(path)
+.. function:: getsize(path, apparent=True)
 
    Return the size, in bytes, of *path*.  Raise :exc:`OSError` if the file does
-   not exist or is inaccessible.
+   not exist or is inaccessible.  If *apparent* is ``True``, the apparent size
+   (number of bytes) of the file is returned.  If ``False``, the actual size
+   (disk space occupied) is returned. The actual size reflects the block size,
+   meaning it will typically be larger than the apparent size. However, the
+   inverse may also be true due to holes in ("sparse") files, internal
+   fragmentation, indirect blocks, etc.
 
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
 
+   .. versionchanged:: 3.10
+      Add the optional *apparent* parameter.
+
 
 .. function:: isabs(path)
 

Lib/genericpath.py

@@ -45,9 +45,20 @@ def isdir(s):
     return stat.S_ISDIR(st.st_mode)
 
 
-def getsize(filename):
-    """Return the size of a file, reported by os.stat()."""
-    return os.stat(filename).st_size
+def getsize(filename, apparent=True):
+    """Return the size of a file, reported by os.stat().
+
+    If 'apparent' is True (default), the apparent size (number of bytes) of the
+    file is returned. If False, the actual size (disk space occupied) is
+    returned. The actual size reflects the block size, meaning it will
+    typically be larger than the apparent size. However, the inverse may also
+    be true due to holes in ("sparse") files, internal fragmentation, indirect
+    blocks, etc.
+    """
+    if apparent:
+        return os.stat(filename).st_size
+    else:
+        return os.stat(filename).st_blocks * 512
 
 
 def getmtime(filename):

Lib/test/test_genericpath.py

@@ -102,10 +102,20 @@ def test_getsize(self):
 
         create_file(filename, b'Hello')
         self.assertEqual(self.pathmodule.getsize(filename), 5)
+        self.assertEqual(
+            self.pathmodule.getsize(filename, apparent=False), 4096)
         os.remove(filename)
 
         create_file(filename, b'Hello World!')
         self.assertEqual(self.pathmodule.getsize(filename), 12)
+        self.assertEqual(
+            self.pathmodule.getsize(filename, apparent=False), 4096)
+        os.remove(filename)
+
+        open(filename, 'xb', 0).close()
+        os.truncate(filename, 512)
+        self.assertEqual(self.pathmodule.getsize(filename), 512)
+        self.assertEqual(self.pathmodule.getsize(filename, apparent=False), 0)
 
     def test_filetime(self):
         filename = support.TESTFN