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.

2 files changed, 37 insertions, 9 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 773f3e8..689046e 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -25,7 +25,7 @@ Using the subprocess module
 ===========================
 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 @@ not all MS Windows applications interpret the command line the same
 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.
@@ -711,7 +711,7 @@ _PLATFORM_DEFAULT_CLOSE_FDS = object()
 
 
 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,
@@ -725,7 +725,7 @@ class Popen(object):
         self._input = None
         self._communication_started = 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 e8e74ed..dd720fe 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -82,6 +82,34 @@ class PopenExecuteChildRaises(subprocess.Popen):
 
 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.stdin.close()
+            p.stdout.close()
+            p.stderr.close()
+            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.stdin.close()
+            p.stdout.close()
+            p.stderr.close()
+            p.wait()
+
     def test_call_seq(self):
         # call() function with sequence argument
         rc = subprocess.call([sys.executable, "-c",