call _PyFile_SanitizeMode to strip out U (and validate) the mode before passing it along to fopen.

src/runtime/file.cpp

@@ -826,7 +826,14 @@ Box* fileNew(BoxedClass* cls, Box* s, Box* m) {
     auto fn = static_cast<BoxedString*>(s);
     auto mode = static_cast<BoxedString*>(m);
 
-    FILE* f = fopen(fn->data(), mode->data());
+    // all characters in python mode specifiers are valid in fopen calls except 'U'.  we strip it out
+    // of the string we pass to fopen, but pass it along to the BoxedFile ctor.
+    auto file_mode = std::unique_ptr<char[]>(new char[mode->size() + 1]);
+    memmove(&file_mode[0], mode->data(), mode->size() + 1);
+    _PyFile_SanitizeMode(&file_mode[0]);
+    checkAndThrowCAPIException();
+
+    FILE* f = fopen(fn->data(), &file_mode[0]);
     if (!f) {
         PyErr_SetFromErrnoWithFilename(IOError, fn->data());
         throwCAPIException();

test/tests/file.py

 import sys
+import tempfile
 
 f = open("/dev/null")
 print repr(f.read())
@@ -58,3 +59,32 @@ print f.write("H")
 print f.tell()
 print f.flush()
 print f.close()
+
+# tests for universal newlines
+fd, fn = tempfile.mkstemp()
+
+with open(fn, "wb") as f:
+    f.write("hello world!\r")
+    f.write("hello world!\r\n")
+    f.write("hello world!\n")
+    f.write("hello world!\r")
+
+with open(fn) as f:
+    print len(f.readlines())
+
+with open(fn, "rU") as f:
+    print len(f.readlines())
+
+
+fd, fn = tempfile.mkstemp()
+try:
+    with open(fn, "wU") as f:
+        print "succeeded"
+except Exception as e:
+    print e
+
+try:
+    with open(fn, "aU") as f:
+        print "succeeded"
+except Exception as e:
+    print e