bpo-32236: open() emits RuntimeWarning if buffering=1 for binary mode (GH-4842)

If buffering=1 is specified for open() in binary mode, it is silently
treated as buffering=-1 (i.e., the default buffer size).
Coupled with the fact that line buffering is always supported in Python 2,
such behavior caused several issues (e.g., bpo-10344, bpo-21332).

Warn that line buffering is not supported if open() is called with
binary mode and buffering=1.

1 files changed, 10 insertions, 1 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index c827113a93..5db6f0cc24 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -743,12 +743,21 @@ class Popen(object):
 
         self._closed_child_pipe_fds = False
 
+        if self.text_mode:
+            if bufsize == 1:
+                line_buffering = True
+                # Use the default buffer size for the underlying binary streams
+                # since they don't support line buffering.
+                bufsize = -1
+            else:
+                line_buffering = False
+
         try:
             if p2cwrite != -1:
                 self.stdin = io.open(p2cwrite, 'wb', bufsize)
                 if self.text_mode:
                     self.stdin = io.TextIOWrapper(self.stdin, write_through=True,
-                            line_buffering=(bufsize == 1),
+                            line_buffering=line_buffering,
                             encoding=encoding, errors=errors)
             if c2pread != -1:
                 self.stdout = io.open(c2pread, 'rb', bufsize)