add prefer_stream arg to utils.encoding.getdefaultencoding

sometimes the stream encoding should be ignored,
and start with locale (e.g. Popen on Windows Python 2).

Default behavior is unchanged.

IPython/utils/encoding.py

@@ -1,67 +1,71 b''
 # coding: utf-8
 """
 Utilities for dealing with text encodings
 """
 
 #-----------------------------------------------------------------------------
 #  Copyright (C) 2008-2012  The IPython Development Team
 #
 #  Distributed under the terms of the BSD License.  The full license is in
 #  the file COPYING, distributed as part of this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 import sys
 import locale
 import warnings
 
 # to deal with the possibility of sys.std* not being a stream at all
 def get_stream_enc(stream, default=None):
     """Return the given stream's encoding or a default.
 
     There are cases where ``sys.std*`` might not actually be a stream, so
     check for the encoding attribute prior to returning it, and return
     a default if it doesn't exist or evaluates as False. ``default``
     is None if not provided.
     """
     if not hasattr(stream, 'encoding') or not stream.encoding:
         return default
     else:
         return stream.encoding
 
 # Less conservative replacement for sys.getdefaultencoding, that will try
 # to match the environment.
 # Defined here as central function, so if we find better choices, we
 # won't need to make changes all over IPython.
-def getdefaultencoding():
+def getdefaultencoding(prefer_stream=True):
     """Return IPython's guess for the default encoding for bytes as text.
-
-    Asks for stdin.encoding first, to match the calling Terminal, but that
-    is often None for subprocesses.  Fall back on locale.getpreferredencoding()
+    
+    If prefer_stream is True (default), asks for stdin.encoding first,
+    to match the calling Terminal, but that is often None for subprocesses.
+    
+    Then fall back on locale.getpreferredencoding(),
     which should be a sensible platform default (that respects LANG environment),
     and finally to sys.getdefaultencoding() which is the most conservative option,
-    and usually ASCII.
+    and usually ASCII on Python 2 or UTF8 on Python 3.
     """
-    enc = get_stream_enc(sys.stdin)
+    enc = None
+    if prefer_stream:
+        enc = get_stream_enc(sys.stdin)
     if not enc or enc=='ascii':
         try:
             # There are reports of getpreferredencoding raising errors
             # in some cases, which may well be fixed, but let's be conservative here.
             enc = locale.getpreferredencoding()
         except Exception:
             pass
     enc = enc or sys.getdefaultencoding()
     # On windows `cp0` can be returned to indicate that there is no code page.
     # Since cp0 is an invalid encoding return instead cp1252 which is the
     # Western European default.
     if enc == 'cp0':
         warnings.warn(
             "Invalid code page cp0 detected - using cp1252 instead."
             "If cp1252 is incorrect please ensure a valid code page "
             "is defined for the process.", RuntimeWarning)
         return 'cp1252'
     return enc
 
 DEFAULT_ENCODING = getdefaultencoding()