restore singleton property of empty frozenset

Cython/Compiler/Optimize.py

@@ -2027,15 +2027,17 @@ class OptimizeBuiltinCalls(Visitor.MethodDispatcherTransform):
         ])
 
     def _handle_simple_function_frozenset(self, node, function, pos_args):
-        if len(pos_args) != 1:
+        if not pos_args:
+            pos_args = [ExprNodes.NullNode(node.pos)]
+        elif len(pos_args) > 1:
             return node
         # PyFrozenSet_New(it) is better than a generic Python call to frozenset(it)
         return ExprNodes.PythonCapiCallNode(
-            node.pos, "PyFrozenSet_New",
+            node.pos, "__Pyx_PyFrozenSet_New",
             self.PyFrozenSet_New_func_type,
             args=pos_args,
             is_temp=node.is_temp,
-            utility_code=UtilityCode.load_cached('pyset_compat', 'Builtins.c'),
+            utility_code=UtilityCode.load_cached('pyfrozenset_new', 'Builtins.c'),
             py_name="frozenset")
 
     PyObject_AsDouble_func_type = PyrexTypes.CFuncType(

Cython/Utility/Builtins.c

@@ -388,6 +388,9 @@ static CYTHON_INLINE PyObject* __Pyx_PyDict_ViewItems(PyObject* d) {
 #define PySet_Size(anyset) \
     PyObject_Size((anyset))
 
+#define PySet_GET_SIZE(anyset) \
+    PyObject_Size((anyset))
+
 #define PySet_Contains(anyset, key) \
     PySequence_Contains((anyset), (key))
 
@@ -414,3 +417,25 @@ static CYTHON_INLINE int PySet_Add(PyObject *set, PyObject *key) {
 
 #endif /* PyAnySet_CheckExact (<= Py2.4) */
 #endif /* < Py2.5  */
+
+//////////////////// pyfrozenset_new.proto ////////////////////
+//@substitute: naming
+//@requires: pyset_compat
+
+static CYTHON_INLINE PyObject* __Pyx_PyFrozenSet_New(PyObject* it) {
+    if (it) {
+        PyObject* result = PyFrozenSet_New(it);
+        if (unlikely(!result))
+            return NULL;
+        if (likely(PySet_GET_SIZE(result)))
+            return result;
+        // empty frozenset is a singleton
+        // seems wasteful, but CPython does the same
+        Py_DECREF(result);
+    }
+    #if CYTHON_COMPILING_IN_CPYTHON
+    return PyFrozenSet_Type.tp_new(&PyFrozenSet_Type, $empty_tuple, NULL);
+    #else
+    return PyObject_Call((PyObject*)&PyFrozenSet_Type, $empty_tuple, NULL);
+    #endif
+}

tests/run/set.pyx

@@ -5,6 +5,8 @@ _frozenset = frozenset
 
 cimport cython
 
+import sys
+
 
 def cython_set():
     """
@@ -284,6 +286,37 @@ def test_frozenset_of_iterable(x):
     return frozenset(x)
 
 
+@cython.test_assert_path_exists("//PythonCapiCallNode")
+@cython.test_fail_if_path_exists(
+    "//SimpleCallNode",
+    "//SetNode"
+)
+def test_empty_frozenset():
+    """
+    >>> s = test_empty_frozenset()
+    >>> isinstance(s, _frozenset)
+    True
+    >>> len(s)
+    0
+    >>> sys.version_info < (2,5) or s is frozenset()   # singleton!
+    True
+    """
+    return frozenset()
+
+
+def test_singleton_empty_frozenset():
+    """
+    >>> test_singleton_empty_frozenset()  # from CPython's test_set.py
+    1
+    """
+    f = frozenset()
+    efs = [frozenset(), frozenset([]), frozenset(()), frozenset(''),
+           frozenset(), frozenset([]), frozenset(()), frozenset(''),
+           frozenset(range(0)), frozenset(frozenset()),
+           frozenset(f), f]
+    return len(set(map(id, efs))) if sys.version_info >= (2,5) else 1
+
+
 def sorted(it):
     # Py3 can't compare different types
     chars = []