Fixes issue #17488: Change the subprocess.Popen bufsize parameter default value
from unbuffered (0) to buffering (-1) to match the behavior existing code
expects and match the behavior of the subprocess module in Python 2 to avoid
introducing hard to track down bugs.
diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index 92b21b5..a72ca44 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -356,7 +356,7 @@
functions.
-.. class:: Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, \
+.. class:: Popen(args, bufsize=-1, executable=None, stdin=None, stdout=None, \
stderr=None, preexec_fn=None, close_fds=True, shell=False, \
cwd=None, env=None, universal_newlines=False, \
startupinfo=None, creationflags=0, restore_signals=True, \
@@ -428,17 +428,20 @@
untrusted input. See the warning under :ref:`frequently-used-arguments`
for details.
- *bufsize*, if given, has the same meaning as the corresponding argument to the
- built-in open() function: :const:`0` means unbuffered, :const:`1` means line
- buffered, any other positive value means use a buffer of (approximately) that
- size. A negative *bufsize* means to use the system default, which usually means
- fully buffered. The default value for *bufsize* is :const:`0` (unbuffered).
+ *bufsize* will be supplied as the corresponding argument to the :meth:`io.open`
+ function when creating the stdin/stdout/stderr pipe file objects:
+ :const:`0` means unbuffered (read and write are one system call and can return short),
+ :const:`1` means line buffered, any other positive value means use a buffer of
+ approximately that size. A negative bufsize (the default) means
+ the system default of io.DEFAULT_BUFFER_SIZE will be used.
- .. note::
+ .. versionchanged:: 3.2.4, 3.3.1
- If you experience performance issues, it is recommended that you try to
- enable buffering by setting *bufsize* to either -1 or a large enough
- positive value (such as 4096).
+ *bufsize* now defaults to -1 to enable buffering by default to match the
+ behavior that most code expects. In 3.2.0 through 3.2.3 and 3.3.0 it
+ incorrectly defaulted to :const:`0` which was unbuffered and allowed
+ short reads. This was unintentional and did not match the behavior of
+ Python 2 as most code expected.
The *executable* argument specifies a replacement program to execute. It
is very seldom needed. When ``shell=False``, *executable* replaces the