diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index ccd6ce8..11348b2 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -277,8 +277,9 @@
     may raise a IOError when operations they do not support are called.
 
     The basic type used for binary data read from or written to a file is
-    bytes. bytearrays are accepted too, and in some cases (such as
-    readinto) needed. Text I/O classes work with str data.
+    the bytes type. Method arguments may also be bytearray or memoryview of
+    arrays of bytes. In some cases, such as readinto, a writable object such
+    as bytearray is required. Text I/O classes work with unicode data.
 
     Note that calling any method (even inquiries) on a closed stream is
     undefined. Implementations may raise IOError in this case.
@@ -649,7 +650,6 @@
         Raises BlockingIOError if the underlying raw stream has no
         data at the moment.
         """
-        # XXX This ought to work with anything that supports the buffer API
         data = self.read(len(b))
         n = len(data)
         try:
@@ -664,8 +664,7 @@
     def write(self, b):
         """Write the given buffer to the IO stream.
 
-        Return the number of bytes written, which is never less than
-        len(b).
+        Return the number of bytes written, which is always len(b).
 
         Raises BlockingIOError if the buffer is full and the
         underlying raw stream cannot accept more data at the moment.
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index b7f6046..e26ffba 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -54,6 +54,9 @@
 __metaclass__ = type
 bytes = support.py3k_bytes
 
+def byteslike(*pos, **kw):
+    return memoryview(bytearray(*pos, **kw))
+
 def _default_chunk_size():
     """Get the default TextIOWrapper chunk size"""
     with io.open(__file__, "r", encoding="latin1") as f:
@@ -273,7 +276,9 @@
         self.assertEqual(f.tell(), 6)
         self.assertEqual(f.seek(-1, 1), 5)
         self.assertEqual(f.tell(), 5)
-        self.assertEqual(f.write(bytearray(b" world\n\n\n")), 9)
+        buffer = bytearray(b" world\n\n\n")
+        self.assertEqual(f.write(buffer), 9)
+        buffer[:] = b"*" * 9  # Overwrite our copy of the data
         self.assertEqual(f.seek(0), 0)
         self.assertEqual(f.write(b"h"), 1)
         self.assertEqual(f.seek(-1, 2), 13)
@@ -286,20 +291,21 @@
     def read_ops(self, f, buffered=False):
         data = f.read(5)
         self.assertEqual(data, b"hello")
-        data = bytearray(data)
+        data = byteslike(data)
         self.assertEqual(f.readinto(data), 5)
-        self.assertEqual(data, b" worl")
+        self.assertEqual(data.tobytes(), b" worl")
+        data = bytearray(5)
         self.assertEqual(f.readinto(data), 2)
         self.assertEqual(len(data), 5)
         self.assertEqual(data[:2], b"d\n")
         self.assertEqual(f.seek(0), 0)
         self.assertEqual(f.read(20), b"hello world\n")
         self.assertEqual(f.read(1), b"")
-        self.assertEqual(f.readinto(bytearray(b"x")), 0)
+        self.assertEqual(f.readinto(byteslike(b"x")), 0)
         self.assertEqual(f.seek(-6, 2), 6)
         self.assertEqual(f.read(5), b"world")
         self.assertEqual(f.read(0), b"")
-        self.assertEqual(f.readinto(bytearray()), 0)
+        self.assertEqual(f.readinto(byteslike()), 0)
         self.assertEqual(f.seek(-6, 1), 5)
         self.assertEqual(f.read(5), b" worl")
         self.assertEqual(f.tell(), 10)
@@ -649,6 +655,16 @@
             support.gc_collect()
         self.assertEqual(recorded, [])
 
+    def test_buffered_readinto_mixin(self):
+        # Test the implementation provided by BufferedIOBase
+        class Stream(self.BufferedIOBase):
+            def read(self, size):
+                return b"12345"
+        stream = Stream()
+        buffer = byteslike(5)
+        self.assertEqual(stream.readinto(buffer), 5)
+        self.assertEqual(buffer.tobytes(), b"12345")
+
 
 class CIOTest(IOTest):
 
@@ -1111,6 +1127,11 @@
         bufio = self.tp(writer, 8)
         bufio.write(b"abc")
         self.assertFalse(writer._write_stack)
+        buffer = bytearray(b"def")
+        bufio.write(buffer)
+        buffer[:] = b"***"  # Overwrite our copy of the data
+        bufio.flush()
+        self.assertEqual(b"".join(writer._write_stack), b"abcdef")
 
     def test_write_overflow(self):
         writer = self.MockRawIO()
@@ -1440,9 +1461,9 @@
     def test_readinto(self):
         pair = self.tp(self.BytesIO(b"abcdef"), self.MockRawIO())
 
-        data = bytearray(5)
+        data = byteslike(5)
         self.assertEqual(pair.readinto(data), 5)
-        self.assertEqual(data, b"abcde")
+        self.assertEqual(data.tobytes(), b"abcde")
 
     def test_write(self):
         w = self.MockRawIO()
@@ -1450,7 +1471,9 @@
 
         pair.write(b"abc")
         pair.flush()
-        pair.write(b"def")
+        buffer = bytearray(b"def")
+        pair.write(buffer)
+        buffer[:] = b"***"  # Overwrite our copy of the data
         pair.flush()
         self.assertEqual(w._write_stack, [b"abc", b"def"])
 
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 0eb9961..2c18ad7 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -396,6 +396,7 @@
 
 
 class PyBytesIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
+    # Test _pyio.BytesIO; class also inherited for testing C implementation
 
     UnsupportedOperation = pyio.UnsupportedOperation