Implement intern() builtin function.

src/runtime/builtin_modules/builtins.cpp

@@ -958,6 +958,16 @@ Box* input(Box* prompt) {
     throwCAPIException();
 }
 
+Box* intern(Box* str) {
+    if (!PyString_Check(str)) {
+        raiseExcHelper(TypeError, "must be string, not %s", getTypeName(str));
+    }
+    if (!PyString_CheckExact(str)) {
+        raiseExcHelper(TypeError, "can't intern subclass of string");
+    }
+    return PyString_InternFromString(PyString_AsString(str));
+}
+
 Box* builtinRound(Box* _number, Box* _ndigits) {
     if (!isSubclass(_number->cls, float_cls))
         raiseExcHelper(TypeError, "a float is required");
@@ -1104,6 +1114,9 @@ void setupBuiltins() {
     builtins_module->giveAttr("__import__", new BoxedBuiltinFunctionOrMethod(import_func, "__import__",
                                                                              { None, None, None, new BoxedInt(-1) }));
 
+    Box* intern_obj = new BoxedBuiltinFunctionOrMethod(boxRTFunction((void*)intern, UNKNOWN, 1), "intern");
+    builtins_module->giveAttr("intern", intern_obj);
+
     enumerate_cls = BoxedHeapClass::create(type_cls, object_cls, &BoxedEnumerate::gcHandler, 0, 0,
                                            sizeof(BoxedEnumerate), false, "enumerate");
     enumerate_cls->giveAttr(

test/tests/intern.py

+try:
+    print intern(123)
+except TypeError:
+    print "caught expected TypeError"
+
+print intern("abcd")
+
+class StringSubclass(str):
+    pass
+
+# CPython does not allow interning on subclasses of str
+try:
+    print intern(StringSubclass())
+except TypeError:
+    print "caught expected TypeError from subclassing"