Patch 1329 (partial) by Christian Heimes.

Add a closefd flag to open() which can be set to False to prevent closing
the file descriptor when close() is called or when the object is destroyed.
Useful to ensure that sys.std{in,out,err} keep their file descriptors open
when Python is uninitialized.  (This was always a feature in 2.x, it just
wasn't implemented in 3.0 yet.)

Lib/io.py

@@ -49,7 +49,8 @@ class BlockingIOError(IOError):
         self.characters_written = characters_written
 
 
-def open(file, mode="r", buffering=None, encoding=None, newline=None):
+def open(file, mode="r", buffering=None, encoding=None, newline=None,
+         closefd=True):
     r"""Replacement for the built-in open function.
 
     Args:
@@ -81,9 +82,12 @@ def open(file, mode="r", buffering=None, encoding=None, newline=None):
           other legal values, any `'\n'` characters written are
           translated to the given string.
 
+      closefd: optional argument to keep the underlying file descriptor
+               open when the file is closed.  It must not be false when
+               a filename is given.
+
     (*) If a file descriptor is given, it is closed when the returned
-    I/O object is closed.  If you don't want this to happen, use
-    os.dup() to create a duplicate file descriptor.
+    I/O object is closed, unless closefd=False is give.
 
     Mode strings characters:
       'r': open for reading (default)
@@ -138,7 +142,8 @@ def open(file, mode="r", buffering=None, encoding=None, newline=None):
                  (reading and "r" or "") +
                  (writing and "w" or "") +
                  (appending and "a" or "") +
-                 (updating and "+" or ""))
+                 (updating and "+" or ""),
+                 closefd)
     if buffering is None:
         buffering = -1
     if buffering < 0 and raw.isatty():