bpo-22671: Clarify and test default read method implementations (#4568)

Original patch written by Martin Panter, enhanced by Sanyam Khurana.

Doc/library/io.rst

@@ -205,8 +205,8 @@ ABC                        Inherits            Stub Methods              Mixin M
                                                                          ``writable``, and ``writelines``
 :class:`RawIOBase`         :class:`IOBase`     ``readinto`` and          Inherited :class:`IOBase` methods, ``read``,
                                                ``write``                 and ``readall``
-:class:`BufferedIOBase`    :class:`IOBase`     ``detach``, ``read``,     Inherited :class:`IOBase` methods, ``readinto``
-                                               ``read1``, and ``write``
+:class:`BufferedIOBase`    :class:`IOBase`     ``detach``, ``read``,     Inherited :class:`IOBase` methods, ``readinto``,
+                                               ``read1``, and ``write``  and ``readinto1``
 :class:`TextIOBase`        :class:`IOBase`     ``detach``, ``read``,     Inherited :class:`IOBase` methods, ``encoding``,
                                                ``readline``, and         ``errors``, and ``newlines``
                                                ``write``
@@ -385,14 +385,17 @@ I/O Base Classes
    .. method:: read(size=-1)
 
       Read up to *size* bytes from the object and return them.  As a convenience,
-      if *size* is unspecified or -1, :meth:`readall` is called.  Otherwise,
-      only one system call is ever made.  Fewer than *size* bytes may be
-      returned if the operating system call returns fewer than *size* bytes.
+      if *size* is unspecified or -1, all bytes until EOF are returned.
+      Otherwise, only one system call is ever made.  Fewer than *size* bytes may
+      be returned if the operating system call returns fewer than *size* bytes.
 
       If 0 bytes are returned, and *size* was not 0, this indicates end of file.
       If the object is in non-blocking mode and no bytes are available,
       ``None`` is returned.
 
+      The default implementation defers to :meth:`readall` and
+      :meth:`readinto`.
+
    .. method:: readall()
 
       Read and return all the bytes from the stream until EOF, using multiple

Lib/test/test_io.py

@@ -807,8 +807,8 @@ def test_multi_close(self):
         self.assertRaises(ValueError, f.flush)
 
     def test_RawIOBase_read(self):
-        # Exercise the default RawIOBase.read() implementation (which calls
-        # readinto() internally).
+        # Exercise the default limited RawIOBase.read(n) implementation (which
+        # calls readinto() internally).
         rawio = self.MockRawIOWithoutRead((b"abc", b"d", None, b"efg", None))
         self.assertEqual(rawio.read(2), b"ab")
         self.assertEqual(rawio.read(2), b"c")
@@ -916,6 +916,55 @@ def check_path_succeeds(path):
         with self.assertRaisesRegex(ValueError, 'read/write/append mode'):
             self.open(PathLike(support.TESTFN), 'rwxa')
 
+    def test_RawIOBase_readall(self):
+        # Exercise the default unlimited RawIOBase.read() and readall()
+        # implementations.
+        rawio = self.MockRawIOWithoutRead((b"abc", b"d", b"efg"))
+        self.assertEqual(rawio.read(), b"abcdefg")
+        rawio = self.MockRawIOWithoutRead((b"abc", b"d", b"efg"))
+        self.assertEqual(rawio.readall(), b"abcdefg")
+
+    def test_BufferedIOBase_readinto(self):
+        # Exercise the default BufferedIOBase.readinto() and readinto1()
+        # implementations (which call read() or read1() internally).
+        class Reader(self.BufferedIOBase):
+            def __init__(self, avail):
+                self.avail = avail
+            def read(self, size):
+                result = self.avail[:size]
+                self.avail = self.avail[size:]
+                return result
+            def read1(self, size):
+                """Returns no more than 5 bytes at once"""
+                return self.read(min(size, 5))
+        tests = (
+            # (test method, total data available, read buffer size, expected
+            #     read size)
+            ("readinto", 10, 5, 5),
+            ("readinto", 10, 6, 6),  # More than read1() can return
+            ("readinto", 5, 6, 5),  # Buffer larger than total available
+            ("readinto", 6, 7, 6),
+            ("readinto", 10, 0, 0),  # Empty buffer
+            ("readinto1", 10, 5, 5),  # Result limited to single read1() call
+            ("readinto1", 10, 6, 5),  # Buffer larger than read1() can return
+            ("readinto1", 5, 6, 5),  # Buffer larger than total available
+            ("readinto1", 6, 7, 5),
+            ("readinto1", 10, 0, 0),  # Empty buffer
+        )
+        UNUSED_BYTE = 0x81
+        for test in tests:
+            with self.subTest(test):
+                method, avail, request, result = test
+                reader = Reader(bytes(range(avail)))
+                buffer = bytearray((UNUSED_BYTE,) * request)
+                method = getattr(reader, method)
+                self.assertEqual(method(buffer), result)
+                self.assertEqual(len(buffer), request)
+                self.assertSequenceEqual(buffer[:result], range(result))
+                unused = (UNUSED_BYTE,) * (request - result)
+                self.assertSequenceEqual(buffer[result:], unused)
+                self.assertEqual(len(reader.avail), avail - result)
+
 
 class CIOTest(IOTest):