From e812d29b52c597363d8d2c2615def653701a0efc Mon Sep 17 00:00:00 2001
From: Antoine Pitrou <solipsis@pitrou.net>
Date: Sat, 19 Dec 2009 21:01:10 +0000
Subject: Issue #7545: improve documentation of the `buffering` argument in
 io.open().

---
 Doc/library/io.rst      | 18 ++++++++++++++----
 Lib/_pyio.py            | 15 +++++++++++++++
 Modules/_io/_iomodule.c | 18 ++++++++++++++----
 3 files changed, 43 insertions(+), 8 deletions(-)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 3308639..adde553 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -106,10 +106,20 @@ Module Interface
    :class:`unicode` strings, the bytes having been first decoded using a
    platform-dependent encoding or using the specified *encoding* if given.
 
-   *buffering* is an optional integer used to set the buffering policy.  By
-   default full buffering is on.  Pass 0 to switch buffering off (only allowed
-   in binary mode), 1 to set line buffering, and an integer > 1 to indicate the
-   size of the buffer.
+   *buffering* is an optional integer used to set the buffering policy.
+   Pass 0 to switch buffering off (only allowed in binary mode), 1 to select
+   line buffering (only usable in text mode), and an integer > 1 to indicate
+   the size of a fixed-size chunk buffer.  When no *buffering* argument is
+   given, the default buffering policy works as follows:
+
+   * Binary files are buffered in fixed-size chunks; the size of the buffer
+     is chosen using a heuristic trying to determine the underlying device's
+     "block size" and falling back on :attr:`DEFAULT_BUFFER_SIZE`.
+     On many systems, the buffer will typically be 4096 or 8192 bytes long.
+
+   * "Interactive" text files (files for which :meth:`isatty` returns True)
+     use line buffering.  Other text files use the policy described above
+     for binary files.
 
    *encoding* is the name of the encoding used to decode or encode the file.
    This should only be used in text mode.  The default encoding is platform
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index b0da045..5fe928b 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -92,6 +92,21 @@ def open(file, mode="r", buffering=None,
     allowed in binary mode), 1 to set line buffering, and an integer > 1
     for full buffering.
 
+    buffering is an optional integer used to set the buffering policy.
+    Pass 0 to switch buffering off (only allowed in binary mode), 1 to select
+    line buffering (only usable in text mode), and an integer > 1 to indicate
+    the size of a fixed-size chunk buffer.  When no buffering argument is
+    given, the default buffering policy works as follows:
+
+    * Binary files are buffered in fixed-size chunks; the size of the buffer
+      is chosen using a heuristic trying to determine the underlying device's
+      "block size" and falling back on `io.DEFAULT_BUFFER_SIZE`.
+      On many systems, the buffer will typically be 4096 or 8192 bytes long.
+
+    * "Interactive" text files (files for which isatty() returns True)
+      use line buffering.  Other text files use the policy described above
+      for binary files.
+
     encoding is the name of the encoding used to decode or encode the
     file. This should only be used in text mode. The default encoding is
     platform dependent, but any encoding supported by Python can be
diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
index a2a6430..ca359bc 100644
--- a/Modules/_io/_iomodule.c
+++ b/Modules/_io/_iomodule.c
@@ -219,10 +219,20 @@ PyDoc_STRVAR(open_doc,
 "returned as strings, the bytes having been first decoded using a\n"
 "platform-dependent encoding or using the specified encoding if given.\n"
 "\n"
-"buffering is an optional integer used to set the buffering policy. By\n"
-"default full buffering is on. Pass 0 to switch buffering off (only\n"
-"allowed in binary mode), 1 to set line buffering, and an integer > 1\n"
-"for full buffering.\n"
+"buffering is an optional integer used to set the buffering policy.\n"
+"Pass 0 to switch buffering off (only allowed in binary mode), 1 to select\n"
+"line buffering (only usable in text mode), and an integer > 1 to indicate\n"
+"the size of a fixed-size chunk buffer.  When no buffering argument is\n"
+"given, the default buffering policy works as follows:\n"
+"\n"
+"* Binary files are buffered in fixed-size chunks; the size of the buffer\n"
+"  is chosen using a heuristic trying to determine the underlying device's\n"
+"  \"block size\" and falling back on `io.DEFAULT_BUFFER_SIZE`.\n"
+"  On many systems, the buffer will typically be 4096 or 8192 bytes long.\n"
+"\n"
+"* \"Interactive\" text files (files for which isatty() returns True)\n"
+"  use line buffering.  Other text files use the policy described above\n"
+"  for binary files.\n"
 "\n"
 "encoding is the name of the encoding used to decode or encode the\n"
 "file. This should only be used in text mode. The default encoding is\n"
-- 
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