The bufsize argument to Popen() should accept None meaning the default (0).

Lib/subprocess.py

@@ -465,6 +465,8 @@ class Popen(object):
         _cleanup()
 
         self._child_created = False
+        if bufsize is None:
+            bufsize = 0  # Restore default
         if not isinstance(bufsize, int):
             raise TypeError("bufsize must be an integer")
 

Lib/test/test_subprocess.py

@@ -455,6 +455,14 @@ class ProcessTestCase(unittest.TestCase):
         else:
             self.fail("Expected TypeError")
 
+    def test_bufsize_is_none(self):
+        # bufsize=None should be the same as bufsize=0.
+        p = subprocess.Popen([sys.executable, "-c", "pass"], None)
+        self.assertEqual(p.wait(), 0)
+        # Again with keyword arg
+        p = subprocess.Popen([sys.executable, "-c", "pass"], bufsize=None)
+        self.assertEqual(p.wait(), 0)
+
     #
     # POSIX tests
     #