@@ -407,11 +407,15 @@ efficiently (see :issue:`33671`).
 "fast-copy" means that the copying operation occurs within the kernel, avoiding
 the use of userspace buffers in Python as in "``outfd.write(infd.read())``".
 
-On OSX `fcopyfile`_ is used to copy the file content (not metadata).
+On macOS `fcopyfile`_ is used to copy the file content (not metadata).
 
 On Linux, Solaris and other POSIX platforms where :func:`os.sendfile` supports
 copies between 2 regular file descriptors :func:`os.sendfile` is used.
 
+On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
+instead of 16 KiB) and a :func:`memoryview`-based variant of
+:func:`shutil.copyfileobj` is used.
+
 If the fast-copy operation fails and no data was written in the destination
 file then shutil will silently fallback on using less efficient
 :func:`copyfileobj` function internally.
 
@@ -95,20 +95,18 @@ Optimizations
 
 * :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
   :func:`shutil.copytree` and :func:`shutil.move` use platform-specific
-  "fast-copy" syscalls on Linux, OSX and Solaris in order to copy the file more
-  efficiently.
+  "fast-copy" syscalls on Linux, macOS and Solaris in order to copy the file
+  more efficiently.
   "fast-copy" means that the copying operation occurs within the kernel,
   avoiding the use of userspace buffers in Python as in
   "``outfd.write(infd.read())``".
-  All other platforms not using such technique will rely on a faster
-  :func:`shutil.copyfile` implementation using :func:`memoryview`,
-  :class:`bytearray` and
-  :meth:`BufferedIOBase.readinto() <io.BufferedIOBase.readinto>`.
-  Finally, :func:`shutil.copyfile` default buffer size on Windows was increased
-  from 16KB to 1MB.
-  The speedup for copying a 512MB file within the same partition is about +26%
-  on Linux, +50% on OSX and +38% on Windows. Also, much less CPU cycles are
-  consumed.
+  On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
+  instead of 16 KiB) and a :func:`memoryview`-based variant of
+  :func:`shutil.copyfileobj` is used.
+  The speedup for copying a 512 MiB file within the same partition is about
+  +26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles
+  are consumed.
+  See :ref:`shutil-platform-dependent-efficient-copy-operations` section.
   (Contributed by Giampaolo Rodola' in :issue:`25427`.)
 
 * The default protocol in the :mod:`pickle` module is now Protocol 4,
@@ -179,6 +177,14 @@ Changes in the Python API
 * The :class:`cProfile.Profile` class can now be used as a context
   manager. (Contributed by Scott Sanderson in :issue:`29235`.)
 
+* :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
+  :func:`shutil.copytree` and :func:`shutil.move` use platform-specific
+  "fast-copy" syscalls (see
+  :ref:`shutil-platform-dependent-efficient-copy-operations` section).
+
+* :func:`shutil.copyfile` default buffer size on Windows was changed from
+  16 KiB to 1 MiB.
+
 CPython bytecode changes
 ------------------------
 
 
@@ -43,15 +43,16 @@
 except ImportError:
     getgrnam = None
 
+_WINDOWS = os.name == 'nt'
 posix = nt = None
 if os.name == 'posix':
     import posix
-elif os.name == 'nt':
+elif _WINDOWS:
     import nt
 
-COPY_BUFSIZE = 1024 * 1024 if os.name == 'nt' else 16 * 1024
+COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 16 * 1024
 _HAS_SENDFILE = posix and hasattr(os, "sendfile")
-_HAS_FCOPYFILE = posix and hasattr(posix, "_fcopyfile")  # OSX
+_HAS_FCOPYFILE = posix and hasattr(posix, "_fcopyfile")  # macOS
 
 __all__ = ["copyfileobj", "copyfile", "copymode", "copystat", "copy", "copy2",
            "copytree", "move", "rmtree", "Error", "SpecialFileError",
@@ -88,9 +89,9 @@ class _GiveupOnFastCopy(Exception):
     file copy when fast-copy functions fail to do so.
     """
 
-def _fastcopy_osx(fsrc, fdst, flags):
+def _fastcopy_fcopyfile(fsrc, fdst, flags):
     """Copy a regular file content or metadata by using high-performance
-    fcopyfile(3) syscall (OSX).
+    fcopyfile(3) syscall (macOS).
     """
     try:
         infd = fsrc.fileno()
@@ -168,8 +169,11 @@ def _fastcopy_sendfile(fsrc, fdst):
                 break  # EOF
             offset += sent
 
-def _copybinfileobj(fsrc, fdst, length=COPY_BUFSIZE):
-    """Copy 2 regular file objects open in binary mode."""
+def _copyfileobj_readinto(fsrc, fdst, length=COPY_BUFSIZE):
+    """readinto()/memoryview() based variant of copyfileobj().
+    *fsrc* must support readinto() method and both files must be
+    open in binary mode.
+    """
     # Localize variable access to minimize overhead.
     fsrc_readinto = fsrc.readinto
     fdst_write = fdst.write
@@ -179,28 +183,21 @@ def _copybinfileobj(fsrc, fdst, length=COPY_BUFSIZE):
             if not n:
                 break
             elif n < length:
-                fdst_write(mv[:n])
+                with mv[:n] as smv:
+                    fdst.write(smv)
             else:
                 fdst_write(mv)
 
-def _is_binary_files_pair(fsrc, fdst):
-    return hasattr(fsrc, 'readinto') and \
-        isinstance(fsrc, io.BytesIO) or 'b' in getattr(fsrc, 'mode', '') and \
-        isinstance(fdst, io.BytesIO) or 'b' in getattr(fdst, 'mode', '')
-
 def copyfileobj(fsrc, fdst, length=COPY_BUFSIZE):
     """copy data from file-like object fsrc to file-like object fdst"""
-    if _is_binary_files_pair(fsrc, fdst):
-        _copybinfileobj(fsrc, fdst, length=length)
-    else:
-        # Localize variable access to minimize overhead.
-        fsrc_read = fsrc.read
-        fdst_write = fdst.write
-        while 1:
-            buf = fsrc_read(length)
-            if not buf:
-                break
-            fdst_write(buf)
+    # Localize variable access to minimize overhead.
+    fsrc_read = fsrc.read
+    fdst_write = fdst.write
+    while True:
+        buf = fsrc_read(length)
+        if not buf:
+            break
+        fdst_write(buf)
 
 def _samefile(src, dst):
     # Macintosh, Unix.
@@ -215,7 +212,7 @@ def _samefile(src, dst):
             os.path.normcase(os.path.abspath(dst)))
 
 def copyfile(src, dst, *, follow_symlinks=True):
-    """Copy data from src to dst.
+    """Copy data from src to dst in the most efficient way possible.
 
     If follow_symlinks is not set and src is a symbolic link, a new
     symlink will be created instead of copying the file it points to.
@@ -224,7 +221,8 @@ def copyfile(src, dst, *, follow_symlinks=True):
     if _samefile(src, dst):
         raise SameFileError("{!r} and {!r} are the same file".format(src, dst))
 
-    for fn in [src, dst]:
+    file_size = 0
+    for i, fn in enumerate([src, dst]):
         try:
             st = os.stat(fn)
         except OSError:
@@ -234,26 +232,34 @@ def copyfile(src, dst, *, follow_symlinks=True):
             # XXX What about other special files? (sockets, devices...)
             if stat.S_ISFIFO(st.st_mode):
                 raise SpecialFileError("`%s` is a named pipe" % fn)
+            if _WINDOWS and i == 0:
+                file_size = st.st_size
 
     if not follow_symlinks and os.path.islink(src):
         os.symlink(os.readlink(src), dst)
     else:
         with open(src, 'rb') as fsrc, open(dst, 'wb') as fdst:
-            if _HAS_SENDFILE:
+            # macOS
+            if _HAS_FCOPYFILE:
                 try:
-                    _fastcopy_sendfile(fsrc, fdst)
+                    _fastcopy_fcopyfile(fsrc, fdst, posix._COPYFILE_DATA)
                     return dst
                 except _GiveupOnFastCopy:
                     pass
-
-            if _HAS_FCOPYFILE:
+            # Linux / Solaris
+            elif _HAS_SENDFILE:
                 try:
-                    _fastcopy_osx(fsrc, fdst, posix._COPYFILE_DATA)
+                    _fastcopy_sendfile(fsrc, fdst)
                     return dst
                 except _GiveupOnFastCopy:
                     pass
+            # Windows, see:
+            # https://github.com/python/cpython/pull/7160#discussion_r195405230
+            elif _WINDOWS and file_size > 0:
+                _copyfileobj_readinto(fsrc, fdst, min(file_size, COPY_BUFSIZE))
+                return dst
 
-            _copybinfileobj(fsrc, fdst)
+            copyfileobj(fsrc, fdst)
 
     return dst
 
@@ -1147,7 +1153,7 @@ def disk_usage(path):
         used = (st.f_blocks - st.f_bfree) * st.f_frsize
         return _ntuple_diskusage(total, used, free)
 
-elif os.name == 'nt':
+elif _WINDOWS:
 
     __all__.append('disk_usage')
     _ntuple_diskusage = collections.namedtuple('usage', 'total used free')
 
@@ -33,7 +33,7 @@
 from test.support import TESTFN, FakePath
 
 TESTFN2 = TESTFN + "2"
-OSX = sys.platform.startswith("darwin")
+MACOS = sys.platform.startswith("darwin")
 try:
     import grp
     import pwd
@@ -1808,7 +1808,7 @@ def _open(filename, mode='r'):
 
         self.assertRaises(OSError, shutil.copyfile, 'srcfile', 'destfile')
 
-    @unittest.skipIf(OSX, "skipped on OSX")
+    @unittest.skipIf(MACOS, "skipped on macOS")
     def test_w_dest_open_fails(self):
 
         srcfile = self.Faux()
@@ -1828,7 +1828,7 @@ def _open(filename, mode='r'):
         self.assertEqual(srcfile._exited_with[1].args,
                          ('Cannot open "destfile"',))
 
-    @unittest.skipIf(OSX, "skipped on OSX")
+    @unittest.skipIf(MACOS, "skipped on macOS")
     def test_w_dest_close_fails(self):
 
         srcfile = self.Faux()
@@ -1851,7 +1851,7 @@ def _open(filename, mode='r'):
         self.assertEqual(srcfile._exited_with[1].args,
                          ('Cannot close',))
 
-    @unittest.skipIf(OSX, "skipped on OSX")
+    @unittest.skipIf(MACOS, "skipped on macOS")
     def test_w_source_close_fails(self):
 
         srcfile = self.Faux(True)
@@ -1892,6 +1892,80 @@ def test_move_dir_caseinsensitive(self):
             os.rmdir(dst_dir)
 
 
+class TestCopyFileObj(unittest.TestCase):
+    FILESIZE = 2 * 1024 * 1024
+
+    @classmethod
+    def setUpClass(cls):
+        write_test_file(TESTFN, cls.FILESIZE)
+
+    @classmethod
+    def tearDownClass(cls):
+        support.unlink(TESTFN)
+        support.unlink(TESTFN2)
+
+    def tearDown(self):
+        support.unlink(TESTFN2)
+
+    @contextlib.contextmanager
+    def get_files(self):
+        with open(TESTFN, "rb") as src:
+            with open(TESTFN2, "wb") as dst:
+                yield (src, dst)
+
+    def assert_files_eq(self, src, dst):
+        with open(src, 'rb') as fsrc:
+            with open(dst, 'rb') as fdst:
+                self.assertEqual(fsrc.read(), fdst.read())
+
+    def test_content(self):
+        with self.get_files() as (src, dst):
+            shutil.copyfileobj(src, dst)
+        self.assert_files_eq(TESTFN, TESTFN2)
+
+    def test_file_not_closed(self):
+        with self.get_files() as (src, dst):
+            shutil.copyfileobj(src, dst)
+            assert not src.closed
+            assert not dst.closed
+
+    def test_file_offset(self):
+        with self.get_files() as (src, dst):
+            shutil.copyfileobj(src, dst)
+            self.assertEqual(src.tell(), self.FILESIZE)
+            self.assertEqual(dst.tell(), self.FILESIZE)
+
+    @unittest.skipIf(os.name != 'nt', "Windows only")
+    def test_win_impl(self):
+        # Make sure alternate Windows implementation is called.
+        with unittest.mock.patch("shutil._copyfileobj_readinto") as m:
+            shutil.copyfile(TESTFN, TESTFN2)
+        assert m.called
+
+        # File size is 2 MiB but max buf size should be 1 MiB.
+        self.assertEqual(m.call_args[0][2], 1 * 1024 * 1024)
+
+        # If file size < 1 MiB memoryview() length must be equal to
+        # the actual file size.
+        with tempfile.NamedTemporaryFile(delete=False) as f:
+            f.write(b'foo')
+        fname = f.name
+        self.addCleanup(support.unlink, fname)
+        with unittest.mock.patch("shutil._copyfileobj_readinto") as m:
+            shutil.copyfile(fname, TESTFN2)
+        self.assertEqual(m.call_args[0][2], 3)
+
+        # Empty files should not rely on readinto() variant.
+        with tempfile.NamedTemporaryFile(delete=False) as f:
+            pass
+        fname = f.name
+        self.addCleanup(support.unlink, fname)
+        with unittest.mock.patch("shutil._copyfileobj_readinto") as m:
+            shutil.copyfile(fname, TESTFN2)
+        assert not m.called
+        self.assert_files_eq(fname, TESTFN2)
+
+
 class _ZeroCopyFileTest(object):
     """Tests common to all zero-copy APIs."""
     FILESIZE = (10 * 1024 * 1024)  # 10 MiB
@@ -2111,12 +2185,12 @@ def test_file2file_not_supported(self):
             shutil._HAS_SENDFILE = True
 
 
-@unittest.skipIf(not OSX, 'OSX only')
-class TestZeroCopyOSX(_ZeroCopyFileTest, unittest.TestCase):
+@unittest.skipIf(not MACOS, 'macOS only')
+class TestZeroCopyMACOS(_ZeroCopyFileTest, unittest.TestCase):
     PATCHPOINT = "posix._fcopyfile"
 
     def zerocopy_fun(self, src, dst):
-        return shutil._fastcopy_osx(src, dst, posix._COPYFILE_DATA)
+        return shutil._fastcopy_fcopyfile(src, dst, posix._COPYFILE_DATA)
 
 
 class TermsizeTests(unittest.TestCase):
 
@@ -1,11 +1,10 @@
 :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`,
 :func:`shutil.copytree` and :func:`shutil.move` use platform-specific
-fast-copy syscalls on Linux, Solaris and OSX in order to copy the file
-more efficiently. All other platforms not using such technique will rely on a
-faster :func:`shutil.copyfile` implementation using :func:`memoryview`,
-:class:`bytearray` and
-:meth:`BufferedIOBase.readinto() <io.BufferedIOBase.readinto>`.
-Finally, :func:`shutil.copyfile` default buffer size on Windows was increased
-from 16KB to 1MB. The speedup for copying a 512MB file is about +26% on Linux,
-+50% on OSX and +38% on Windows. Also, much less CPU cycles are consumed
+fast-copy syscalls on Linux, Solaris and macOS in order to copy the file
+more efficiently.
+On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB
+instead of 16 KiB) and a :func:`memoryview`-based variant of
+:func:`shutil.copyfileobj` is used.
+The speedup for copying a 512MiB file is about +26% on Linux, +50% on macOS and
++40% on Windows. Also, much less CPU cycles are consumed.
 (Contributed by Giampaolo Rodola' in :issue:`25427`.)
@@ -8774,12 +8774,12 @@ os._fcopyfile
     flags: int
     /
 
-Efficiently copy content or metadata of 2 regular file descriptors (OSX).
+Efficiently copy content or metadata of 2 regular file descriptors (macOS).
 [clinic start generated code]*/
 
 static PyObject *
 os__fcopyfile_impl(PyObject *module, int infd, int outfd, int flags)
-/*[clinic end generated code: output=8e8885c721ec38e3 input=aeb9456804eec879]*/
+/*[clinic end generated code: output=8e8885c721ec38e3 input=69e0770e600cb44f]*/
 {
     int ret;
Original file line number	Diff line number	Diff line change
`@@ -8774,12 +8774,12 @@ os._fcopyfile`
`8774`	`8774`	`flags: int`
`8775`	`8775`	`/`
`8776`	`8776`
`8777`		`-Efficiently copy content or metadata of 2 regular file descriptors (OSX).`
	`8777`	`+Efficiently copy content or metadata of 2 regular file descriptors (macOS).`
`8778`	`8778`	`[clinic start generated code]*/`
`8779`	`8779`
`8780`	`8780`	`static PyObject *`
`8781`	`8781`	`os__fcopyfile_impl(PyObject *module, int infd, int outfd, int flags)`
`8782`		`-/[clinic end generated code: output=8e8885c721ec38e3 input=aeb9456804eec879]/`
	`8782`	`+/[clinic end generated code: output=8e8885c721ec38e3 input=69e0770e600cb44f]/`
`8783`	`8783`	`{`
`8784`	`8784`	`int ret;`
`8785`	`8785`