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 1c8aafd..59ee13f 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -284,7 +284,7 @@
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, \
@@ -356,17 +356,20 @@
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
- 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 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
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 7cfe5df..7255645 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -25,7 +25,7 @@
===========================
This module defines one class called Popen:
-class Popen(args, bufsize=0, executable=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,
@@ -56,12 +56,12 @@
way: The list2cmdline is designed for applications using the same
rules as the MS C runtime.
-bufsize, if given, has the same meaning as the corresponding argument
-to the built-in open() function: 0 means unbuffered, 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 0 (unbuffered).
+bufsize will be supplied as the corresponding argument to the io.open()
+function when creating the stdin/stdout/stderr pipe file objects:
+0 means unbuffered (read & write are one system call and can return short),
+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.
stdin, stdout and stderr specify the executed programs' standard
input, standard output and standard error file handles, respectively.
@@ -638,7 +638,7 @@
class Popen(object):
- def __init__(self, args, bufsize=0, executable=None,
+ def __init__(self, args, bufsize=-1, executable=None,
stdin=None, stdout=None, stderr=None,
preexec_fn=None, close_fds=_PLATFORM_DEFAULT_CLOSE_FDS,
shell=False, cwd=None, env=None, universal_newlines=False,
@@ -650,7 +650,7 @@
self._child_created = False
if bufsize is None:
- bufsize = 0 # Restore default
+ bufsize = -1 # Restore default
if not isinstance(bufsize, int):
raise TypeError("bufsize must be an integer")
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index b1e9027..1a50de3 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -79,6 +79,28 @@
class ProcessTestCase(BaseTestCase):
+ def test_io_buffered_by_default(self):
+ p = subprocess.Popen([sys.executable, "-c", "import sys; sys.exit(0)"],
+ stdin=subprocess.PIPE, stdout=subprocess.PIPE,
+ stderr=subprocess.PIPE)
+ try:
+ self.assertIsInstance(p.stdin, io.BufferedIOBase)
+ self.assertIsInstance(p.stdout, io.BufferedIOBase)
+ self.assertIsInstance(p.stderr, io.BufferedIOBase)
+ finally:
+ p.wait()
+
+ def test_io_unbuffered_works(self):
+ p = subprocess.Popen([sys.executable, "-c", "import sys; sys.exit(0)"],
+ stdin=subprocess.PIPE, stdout=subprocess.PIPE,
+ stderr=subprocess.PIPE, bufsize=0)
+ try:
+ self.assertIsInstance(p.stdin, io.RawIOBase)
+ self.assertIsInstance(p.stdout, io.RawIOBase)
+ self.assertIsInstance(p.stderr, io.RawIOBase)
+ finally:
+ p.wait()
+
def test_call_seq(self):
# call() function with sequence argument
rc = subprocess.call([sys.executable, "-c",
diff --git a/Misc/NEWS b/Misc/NEWS
index 6a36c3a..e4d04b1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -233,6 +233,11 @@
Library
-------
+- 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.
+
- Issue #17521: Corrected non-enabling of logger following two calls to
fileConfig().