Revert the fix for #1548891, it broke backwards compatibility with arbitrary read buffers.

Fixes #1730114.

Doc/lib/libstringio.tex

@@ -78,6 +78,10 @@ Unlike the memory files implemented by the \refmodule{StringIO}
 module, those provided by this module are not able to accept Unicode
 strings that cannot be encoded as plain \ASCII{} strings.
 
+Calling \function{StringIO()} with a Unicode string parameter populates
+the object with the buffer representation of the Unicode string, instead of
+encoding the string.
+
 Another difference from the \refmodule{StringIO} module is that calling
 \function{StringIO()} with a string parameter creates a read-only object.
 Unlike an object created without a string parameter, it does not have

Lib/test/test_StringIO.py

@@ -121,28 +121,6 @@ def test_unicode(self):
 class TestcStringIO(TestGenericStringIO):
     MODULE = cStringIO
 
-    def test_unicode(self):
-
-        if not test_support.have_unicode: return
-
-        # The cStringIO module converts Unicode strings to character
-        # strings when writing them to cStringIO objects.
-        # Check that this works.
-
-        f = self.MODULE.StringIO()
-        f.write(unicode(self._line[:5]))
-        s = f.getvalue()
-        self.assertEqual(s, 'abcde')
-        self.assertEqual(type(s), types.StringType)
-
-        f = self.MODULE.StringIO(unicode(self._line[:5]))
-        s = f.getvalue()
-        self.assertEqual(s, 'abcde')
-        self.assertEqual(type(s), types.StringType)
-
-        self.assertRaises(UnicodeEncodeError, self.MODULE.StringIO,
-                          unicode('\xf4', 'latin-1'))
-
 import sys
 if sys.platform.startswith('java'):
     # Jython doesn't have a buffer object, so we just do a useless

Misc/NEWS

@@ -782,10 +782,6 @@ Extension Modules
 
 - Patch #1576166: Support os.utime for directories on Windows NT+.
 
-- Bug #1548891: The cStringIO.StringIO() constructor now encodes unicode
-  arguments with the system default encoding just like the write()
-  method does, instead of converting it to a raw buffer.
-
 - Patch #1572724: fix typo ('=' instead of '==') in _msi.c.
 
 - Bug #1572832: fix a bug in ISO-2022 codecs which may cause segfault

Modules/cStringIO.c

@@ -673,8 +673,11 @@ newIobject(PyObject *s) {
   char *buf;
   Py_ssize_t size;
 
-  if (PyObject_AsCharBuffer(s, (const char **)&buf, &size) != 0)
-      return NULL;
+  if (PyObject_AsReadBuffer(s, (const char **)&buf, &size)) {
+    PyErr_Format(PyExc_TypeError, "expected read buffer, %.200s found",
+                 s->ob_type->tp_name);
+    return NULL;
+  }
 
   self = PyObject_New(Iobject, &Itype);
   if (!self) return NULL;