1 files changed, 16 insertions, 1 deletions
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 18849b309b8..2a1d2a33d02 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -238,7 +238,7 @@ def open(file, mode="r", buffering=-1, encoding=None, errors=None,
     result = raw
     try:
         line_buffering = False
-        if buffering == 1 or buffering < 0 and raw.isatty():
+        if buffering == 1 or buffering < 0 and raw._isatty_open_only():
             buffering = -1
             line_buffering = True
         if buffering < 0:
@@ -1794,6 +1794,21 @@ class FileIO(RawIOBase):
         self._checkClosed()
         return os.isatty(self._fd)
 
+    def _isatty_open_only(self):
+        """Checks whether the file is a TTY using an open-only optimization.
+
+        TTYs are always character devices. If the interpreter knows a file is
+        not a character device when it would call ``isatty``, can skip that
+        call. Inside ``open()``  there is a fresh stat result that contains that
+        information. Use the stat result to skip a system call. Outside of that
+        context TOCTOU issues (the fd could be arbitrarily modified by
+        surrounding code).
+        """
+        if (self._stat_atopen is not None
+            and not stat.S_ISCHR(self._stat_atopen.st_mode)):
+            return True
+        return os.isatty(self._fd)
+
     @property
     def closefd(self):
         """True if the file descriptor will be closed by close()."""