add text.getdefaultencoding() for central default encoding guess

This is a central location for the many places we call sys.stdin.encoding or sys.getdefaultencoding(), which
now adds locale.getpreferredencoding(False) after stdin.encoding,
which should be a better guess when stdin.encoding is None.

IPython/config/loader.py

@@ -24,7 +24,7 b' import sys'
 
 from IPython.external import argparse
 from IPython.utils.path import filefind, get_ipython_dir
-from IPython.utils import py3compat, warn
+from IPython.utils import py3compat, text, warn
 
 #-----------------------------------------------------------------------------
 # Exceptions
@@ -425,7 +425,7 b' class KeyValueConfigLoader(CommandLineConfigLoader):'
         """decode argv if bytes, using stin.encoding, falling back on default enc"""
         uargv = []
         if enc is None:
-            enc = sys.stdin.encoding or sys.getdefaultencoding()
+            enc = text.getdefaultencoding()
         for arg in argv:
             if not isinstance(arg, unicode):
                 # only decode if not already decoded
@@ -586,7 +586,8 b' class ArgParseConfigLoader(CommandLineConfigLoader):'
     def _parse_args(self, args):
         """self.parser->self.parsed_data"""
         # decode sys.argv to support unicode command-line options
-        uargs = [py3compat.cast_unicode(a) for a in args]
+        enc = text.getdefaultencoding()
+        uargs = [py3compat.cast_unicode(a, enc) for a in args]
         self.parsed_data, self.extra_args = self.parser.parse_known_args(uargs)
 
     def _convert_to_config(self):

IPython/utils/_process_win32.py

@@ -23,6 +23,7 b' from subprocess import STDOUT'
 
 # our own imports
 from ._process_common import read_no_interrupt, process_handler
+from . import text
 
 #-----------------------------------------------------------------------------
 # Function definitions
@@ -88,7 +89,7 b' def _find_cmd(cmd):'
 
 def _system_body(p):
     """Callback for _system."""
-    enc = sys.stdin.encoding or sys.getdefaultencoding()
+    enc = text.getdefaultencoding()
     for line in read_no_interrupt(p.stdout).splitlines():
         line = line.decode(enc, 'replace')
         print(line, file=sys.stdout)

IPython/utils/jsonutil.py

@@ -17,6 +17,7 b' import types'
 from datetime import datetime
 
 from IPython.utils import py3compat
+from IPython.utils import text
 next_attr_name = '__next__' if py3compat.PY3 else 'next'
 
 #-----------------------------------------------------------------------------
@@ -134,7 +135,7 b' def json_clean(obj):'
         return obj
     
     if isinstance(obj, bytes):
-        return obj.decode(sys.getdefaultencoding(), 'replace')
+        return obj.decode(text.getdefaultencoding(), 'replace')
     
     if isinstance(obj, container_to_list) or (
         hasattr(obj, '__iter__') and hasattr(obj, next_attr_name)):

IPython/utils/text.py

@@ -16,9 +16,11 b' Utilities for working with strings and text.'
 
 import __main__
 
+import locale
 import os
 import re
 import shutil
+import sys
 import textwrap
 from string import Formatter
 
@@ -31,6 +33,28 b' from IPython.utils.data import flatten'
 # Code
 #-----------------------------------------------------------------------------
 
+# 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():
+    """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()
+    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.
+    """
+    enc = sys.stdin.encoding
+    if not enc:
+        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(False)
+        except Exception:
+            pass
+    return enc or sys.getdefaultencoding()
 
 def unquote_ends(istr):
     """Remove a single pair of quotes from the endpoints of a string."""

IPython/zmq/iostream.py

@@ -4,7 +4,7 b' from io import StringIO'
 
 from session import extract_header, Message
 
-from IPython.utils import io
+from IPython.utils import io, text
 
 #-----------------------------------------------------------------------------
 # Globals
@@ -69,7 +69,7 b' class OutStream(object):'
         else:
             # Make sure that we're handling unicode
             if not isinstance(string, unicode):
-                enc = sys.stdin.encoding or sys.getdefaultencoding()
+                enc = text.getdefaultencoding()
                 string = string.decode(enc, 'replace')
             
             self._buffer.write(string)