@@ -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_openonly():
             buffering = -1
             line_buffering = True
         if buffering < 0:
@@ -1794,6 +1794,19 @@ def isatty(self):
         self._checkClosed()
         return os.isatty(self._fd)
 
+    def _isatty_openonly(self):
+        """Checks whether the file is a TTY using an open-only optimization.
+        Normally isatty always makes a system call. In the case of open() there
+        is a _inside the same python call_ stat result which we can use to
+        skip that system call for non-character files. Outside of that context
+        this is subject to TOCTOU issues (the FD has been returned to user code
+        and arbitrary syscalls could have happened).
+        """
+        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()."""