Index: Lib/io.py
===================================================================
--- Lib/io.py	(Revision 67608)
+++ Lib/io.py	(Arbeitskopie)
@@ -58,7 +58,7 @@
 import os
 import abc
 import codecs
-import _fileio
+from _fileio import _FileIO, count_lineendings
 # Import _thread instead of threading to reduce startup cost
 try:
     from _thread import allocate_lock as Lock
@@ -602,7 +602,7 @@
         self._unsupported("write")
 
 
-class FileIO(_fileio._FileIO, RawIOBase):
+class FileIO(_FileIO, RawIOBase):
 
     """Raw I/O implementation for OS files."""
 
@@ -612,11 +612,11 @@
     # to do since _fileio.c is written in C).
 
     def __init__(self, name, mode="r", closefd=True):
-        _fileio._FileIO.__init__(self, name, mode, closefd)
+        _FileIO.__init__(self, name, mode, closefd)
         self._name = name
 
     def close(self):
-        _fileio._FileIO.close(self)
+        _FileIO.close(self)
         RawIOBase.close(self)
 
     @property
@@ -1303,9 +1303,7 @@
             self.buffer = b''
 
         # Record which newlines are read
-        crlf = output.count('\r\n')
-        cr = output.count('\r') - crlf
-        lf = output.count('\n') - crlf
+        crlf, cr, lf = count_lineendings(output)
         self.seennl |= (lf and self._LF) | (cr and self._CR) \
                     | (crlf and self._CRLF)
 
@@ -1932,9 +1930,7 @@
                 self._pending = ""
 
             # Record which newlines are read
-            crlf = input.count('\r\n')
-            cr = input.count('\r') - crlf
-            lf = input.count('\n') - crlf
+            crlf, cr, lf = count_lineendings(input)
             self._seennl |= (lf and self._LF) | (cr and self._CR) \
                          | (crlf and self._CRLF)
 
Index: Lib/test/test_fileio.py
===================================================================
--- Lib/test/test_fileio.py	(Revision 67608)
+++ Lib/test/test_fileio.py	(Arbeitskopie)
@@ -248,7 +248,29 @@
             self.assertRaises(ValueError, _fileio._FileIO, "/some/invalid/name", "rt")
             self.assertEqual(w.warnings, [])
 
+    def test_count_lineendings(self):
+        def assertCle(s, crlf, cr, lf):
+            got = _fileio.count_lineendings(s)
+            expected = (crlf, cr, lf)
+            self.assertEqual(got, expected, 
+                             "%r != %r for %r" % (got, expected, s))
 
+        assertCle("", 0, 0, 0)
+        assertCle("\r", 0, 1, 0)
+        assertCle("\n", 0, 0, 1)
+        assertCle("\r\n", 1, 0, 0)
+        assertCle("\r\r\n", 1, 1, 0)
+        assertCle("\r\n\n", 1, 0, 1)
+        assertCle("\n\r\n", 1, 0, 1)
+        assertCle("\r\n\r\n", 2, 0, 0)
+        assertCle("\n\r\n\r", 1, 1, 1)
+
+        self.assertRaises(TypeError, _fileio.count_lineendings, b'')
+        self.assertRaises(TypeError, _fileio.count_lineendings, 1)
+        self.assertRaises(TypeError, _fileio.count_lineendings, object)
+        self.assertRaises(TypeError, _fileio.count_lineendings, object())
+
+
 def test_main():
     # Historically, these tests have been sloppy about removing TESTFN.
     # So get rid of it no matter what.
Index: Modules/_fileio.c
===================================================================
--- Modules/_fileio.c	(Revision 67608)
+++ Modules/_fileio.c	(Arbeitskopie)
@@ -904,7 +904,72 @@
 	PyObject_Del,				/* tp_free */
 };
 
+
+PyObject*
+fileio_count_lineendings(PyObject *self, PyObject *arg)
+{
+	PyObject *result = NULL;
+	PyUnicodeObject *ustr;
+	Py_ssize_t crlf = 0, lf = 0, cr = 0;
+	Py_UNICODE *uc, *ulast;
+
+	ustr = (PyUnicodeObject *)PyUnicode_FromObject(arg);
+	if (ustr == NULL) {
+		goto end;
+	}
+	if (ustr->length == 0) {
+		goto success;
+	}
+
+	/* str is \0 terminated */
+	ulast = &ustr->str[ustr->length-1];
+
+	#define UNICODE_CR 0x000D
+	#define UNICODE_LF 0x000A
+	for (uc = ustr->str; uc < ulast; uc++) {
+	    switch(*uc) {
+		case(UNICODE_CR):
+			if (*(uc+1) == UNICODE_LF) {
+				crlf++;
+				uc++;
+			} else {
+				cr++;
+			}
+			break;
+		case(UNICODE_LF):
+			lf++;
+			break;
+		}
+	}
+	if (uc == ulast) {
+	    switch(*uc) {
+		case(UNICODE_CR):
+			cr++;
+			break;
+		case(UNICODE_LF):
+			lf++;
+			break;
+		}
+
+	}
+	#undef UNICODE_CR
+	#undef UNICODE_LF
+    success:
+	result = Py_BuildValue("nnn", crlf, cr, lf);
+    end:
+	Py_XDECREF(ustr);
+	return result;
+}
+
+
+PyDoc_STRVAR(fileio_count_lineendings__doc__,
+"count_lineendings(s: str) -> (crlf, cr, lf)\n\n"
+"Return count of '\\r\\n', '\\r' and '\\n' inside s. '\\r\\n' counts \n"
+"only to crlf."
+);
+
 static PyMethodDef module_methods[] = {
+	{"count_lineendings", fileio_count_lineendings, METH_O,  fileio_count_lineendings__doc__},
 	{NULL, NULL}
 };