Issue #20699: Document that “io” methods accept bytes-like objects

This matches the usage of ZipFile and BufferedWriter. This still requires
return values to be bytes() objects.

Also document and test that the write() methods should only access their
argument before they return.

1 files changed, 27 insertions, 16 deletions

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index cd81cf9..53d6f27 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -66,7 +66,8 @@ The text stream API is described in detail in the documentation of
 Binary I/O
 ^^^^^^^^^^
 
-Binary I/O (also called *buffered I/O*) expects and produces :class:`bytes`
+Binary I/O (also called *buffered I/O*) expects
+:term:`bytes-like objects <bytes-like object>` and produces :class:`bytes`
 objects.  No encoding, decoding, or newline translation is performed.  This
 category of streams can be used for all kinds of non-text data, and also when
 manual control over the handling of text data is desired.
@@ -227,9 +228,10 @@ I/O Base Classes
    when operations they do not support are called.
 
    The basic type used for binary data read from or written to a file is
-   :class:`bytes`.  :class:`bytearray`\s are accepted too, and in some cases
-   (such as :meth:`readinto`) required.  Text I/O classes work with
-   :class:`str` data.
+   :class:`bytes`.  Other :term:`bytes-like objects <bytes-like object>` are
+   accepted as method arguments too.  In some cases, such as
+   :meth:`~RawIOBase.readinto`, a writable object such as :class:`bytearray`
+   is required.  Text I/O classes work with :class:`str` data.
 
    Note that calling any method (even inquiries) on a closed stream is
    undefined.  Implementations may raise :exc:`ValueError` in this case.
@@ -393,18 +395,22 @@ I/O Base Classes
 
    .. method:: readinto(b)
 
-      Read up to ``len(b)`` bytes into :class:`bytearray` *b* and return the
+      Read bytes into a pre-allocated, writable
+      :term:`bytes-like object` *b*, and return the
       number of bytes read.  If the object is in non-blocking mode and no bytes
       are available, ``None`` is returned.
 
    .. method:: write(b)
 
-      Write the given :class:`bytes` or :class:`bytearray` object, *b*, to the
-      underlying raw stream and return the number of bytes written.  This can
-      be less than ``len(b)``, depending on specifics of the underlying raw
+      Write the given :term:`bytes-like object`, *b*, to the
+      underlying raw stream, and return the number of
+      bytes written.  This can be less than the length of *b* in
+      bytes, depending on specifics of the underlying raw
       stream, and especially if it is in non-blocking mode.  ``None`` is
       returned if the raw stream is set not to block and no single byte could
-      be readily written to it.
+      be readily written to it.  The caller may release or mutate *b* after
+      this method returns, so the implementation should only access *b*
+      during the method call.
 
 
 .. class:: BufferedIOBase
@@ -476,8 +482,8 @@ I/O Base Classes
 
    .. method:: readinto(b)
 
-      Read up to ``len(b)`` bytes into bytearray *b* and return the number of
-      bytes read.
+      Read bytes into a pre-allocated, writable
+      :term:`bytes-like object` *b* and return the number of bytes read.
 
       Like :meth:`read`, multiple reads may be issued to the underlying raw
       stream, unless the latter is interactive.
@@ -487,7 +493,8 @@ I/O Base Classes
 
    .. method:: readinto1(b)
 
-      Read up to ``len(b)`` bytes into bytearray *b*, using at most one call to
+      Read bytes into a pre-allocated, writable
+      :term:`bytes-like object` *b*, using at most one call to
       the underlying raw stream's :meth:`~RawIOBase.read` (or
       :meth:`~RawIOBase.readinto`) method. Return the number of bytes read.
 
@@ -498,8 +505,8 @@ I/O Base Classes
 
    .. method:: write(b)
 
-      Write the given :class:`bytes` or :class:`bytearray` object, *b* and
-      return the number of bytes written (never less than ``len(b)``, since if
+      Write the given :term:`bytes-like object`, *b*, and return the number
+      of bytes written (always equal to the length of *b* in bytes, since if
       the write fails an :exc:`OSError` will be raised).  Depending on the
       actual implementation, these bytes may be readily written to the
       underlying stream, or held in a buffer for performance and latency
@@ -509,6 +516,9 @@ I/O Base Classes
       data needed to be written to the raw stream but it couldn't accept
       all the data without blocking.
 
+      The caller may release or mutate *b* after this method returns,
+      so the implementation should only access *b* during the method call.
+
 
 Raw File I/O
 ^^^^^^^^^^^^
@@ -584,7 +594,8 @@ than raw I/O does.
    :class:`BufferedIOBase`.  The buffer is discarded when the
    :meth:`~IOBase.close` method is called.
 
-   The argument *initial_bytes* contains optional initial :class:`bytes` data.
+   The optional argument *initial_bytes* is a :term:`bytes-like object` that
+   contains initial data.
 
    :class:`BytesIO` provides or overrides these methods in addition to those
    from :class:`BufferedIOBase` and :class:`IOBase`:
@@ -682,7 +693,7 @@ than raw I/O does.
 
    .. method:: write(b)
 
-      Write the :class:`bytes` or :class:`bytearray` object, *b* and return the
+      Write the :term:`bytes-like object`, *b*, and return the
       number of bytes written.  When in non-blocking mode, a
       :exc:`BlockingIOError` is raised if the buffer needs to be written out but
       the raw stream blocks.