bpo-28280: Make PyMapping_Keys(), PyMapping_Values() and PyMapping_Items() always return a list (#3840)

Doc/c-api/mapping.rst

@@ -50,20 +50,29 @@ Mapping Protocol
 
 .. c:function:: PyObject* PyMapping_Keys(PyObject *o)
 
-   On success, return a list or tuple of the keys in object *o*.  On failure,
-   return *NULL*.
+   On success, return a list of the keys in object *o*.  On failure, return
+   *NULL*.
+
+   .. versionchanged:: 3.7
+      Previously, the function returned a list or a tuple.
 
 
 .. c:function:: PyObject* PyMapping_Values(PyObject *o)
 
-   On success, return a list or tuple of the values in object *o*.  On failure,
-   return *NULL*.
+   On success, return a list of the values in object *o*.  On failure, return
+   *NULL*.
+
+   .. versionchanged:: 3.7
+      Previously, the function returned a list or a tuple.
 
 
 .. c:function:: PyObject* PyMapping_Items(PyObject *o)
 
-   On success, return a list or tuple of the items in object *o*, where each item
-   is a tuple containing a key-value pair.  On failure, return *NULL*.
+   On success, return a list of the items in object *o*, where each item is a
+   tuple containing a key-value pair.  On failure, return *NULL*.
+
+   .. versionchanged:: 3.7
+      Previously, the function returned a list or a tuple.
 
 
 .. c:function:: PyObject* PyMapping_GetItemString(PyObject *o, const char *key)

Doc/whatsnew/3.7.rst

@@ -404,6 +404,10 @@ Build and C API Changes
   is now of type ``const char *`` rather of ``char *``. (Contributed by Serhiy
   Storchaka in :issue:`28769`.)
 
+* The result of :c:func:`PyMapping_Keys`, :c:func:`PyMapping_Values` and
+  :c:func:`PyMapping_Items` is now always a list, rather than a list or a
+  tuple. (Contributed by Oren Milman in :issue:`28280`.)
+
 * Added functions :c:func:`PySlice_Unpack` and :c:func:`PySlice_AdjustIndices`.
   (Contributed by Serhiy Storchaka in :issue:`27867`.)
 

Lib/test/test_capi.py

@@ -1,7 +1,7 @@
 # Run the _testcapi module tests (tests for the Python/C API):  by defn,
 # these are all functions _testcapi exports whose name begins with 'test_'.
 
-from collections import namedtuple
+from collections import namedtuple, OrderedDict
 import os
 import pickle
 import platform
@@ -272,6 +272,50 @@ class C(): pass
         self.assertIn(b'MemoryError 2 20', out)
         self.assertIn(b'MemoryError 3 30', out)
 
+    def test_mapping_keys_values_items(self):
+        class Mapping1(dict):
+            def keys(self):
+                return list(super().keys())
+            def values(self):
+                return list(super().values())
+            def items(self):
+                return list(super().items())
+        class Mapping2(dict):
+            def keys(self):
+                return tuple(super().keys())
+            def values(self):
+                return tuple(super().values())
+            def items(self):
+                return tuple(super().items())
+        dict_obj = {'foo': 1, 'bar': 2, 'spam': 3}
+
+        for mapping in [{}, OrderedDict(), Mapping1(), Mapping2(),
+                        dict_obj, OrderedDict(dict_obj),
+                        Mapping1(dict_obj), Mapping2(dict_obj)]:
+            self.assertListEqual(_testcapi.get_mapping_keys(mapping),
+                                 list(mapping.keys()))
+            self.assertListEqual(_testcapi.get_mapping_values(mapping),
+                                 list(mapping.values()))
+            self.assertListEqual(_testcapi.get_mapping_items(mapping),
+                                 list(mapping.items()))
+
+    def test_mapping_keys_values_items_bad_arg(self):
+        self.assertRaises(AttributeError, _testcapi.get_mapping_keys, None)
+        self.assertRaises(AttributeError, _testcapi.get_mapping_values, None)
+        self.assertRaises(AttributeError, _testcapi.get_mapping_items, None)
+
+        class BadMapping:
+            def keys(self):
+                return None
+            def values(self):
+                return None
+            def items(self):
+                return None
+        bad_mapping = BadMapping()
+        self.assertRaises(TypeError, _testcapi.get_mapping_keys, bad_mapping)
+        self.assertRaises(TypeError, _testcapi.get_mapping_values, bad_mapping)
+        self.assertRaises(TypeError, _testcapi.get_mapping_items, bad_mapping)
+
 
 class TestPendingCalls(unittest.TestCase):
 

Misc/NEWS.d/next/C API/2017-09-30-19-41-44.bpo-28280.K_EjpO.rst

@@ -0,0 +1,2 @@
+Make `PyMapping_Keys()`, `PyMapping_Values()` and `PyMapping_Items()` always
+return a `list` (rather than a `list` or a `tuple`). Patch by Oren Milman.

Modules/_testcapimodule.c

@@ -4306,6 +4306,25 @@ py_w_stopcode(PyObject *self, PyObject *args)
 #endif
 
 
+static PyObject *
+get_mapping_keys(PyObject* self, PyObject *obj)
+{
+    return PyMapping_Keys(obj);
+}
+
+static PyObject *
+get_mapping_values(PyObject* self, PyObject *obj)
+{
+    return PyMapping_Values(obj);
+}
+
+static PyObject *
+get_mapping_items(PyObject* self, PyObject *obj)
+{
+    return PyMapping_Items(obj);
+}
+
+
 static PyObject *
 test_pythread_tss_key_state(PyObject *self, PyObject *args)
 {
@@ -4573,6 +4592,9 @@ static PyMethodDef TestMethods[] = {
 #ifdef W_STOPCODE
     {"W_STOPCODE", py_w_stopcode, METH_VARARGS},
 #endif
+    {"get_mapping_keys", get_mapping_keys, METH_O},
+    {"get_mapping_values", get_mapping_values, METH_O},
+    {"get_mapping_items", get_mapping_items, METH_O},
     {"test_pythread_tss_key_state", test_pythread_tss_key_state, METH_VARARGS},
     {NULL, NULL} /* sentinel */
 };

Objects/abstract.c

@@ -2147,55 +2147,77 @@ PyMapping_HasKey(PyObject *o, PyObject *key)
     return 0;
 }
 
+/* This function is quite similar to PySequence_Fast(), but specialized to be
+   a helper for PyMapping_Keys(), PyMapping_Items() and PyMapping_Values().
+ */
+static PyObject *
+method_output_as_list(PyObject *o, _Py_Identifier *meth_id)
+{
+    PyObject *it, *result, *meth_output;
+
+    assert(o != NULL);
+    meth_output = _PyObject_CallMethodId(o, meth_id, NULL);
+    if (meth_output == NULL || PyList_CheckExact(meth_output)) {
+        return meth_output;
+    }
+    it = PyObject_GetIter(meth_output);
+    if (it == NULL) {
+        if (PyErr_ExceptionMatches(PyExc_TypeError)) {
+            PyErr_Format(PyExc_TypeError,
+                         "%.200s.%U() returned a non-iterable (type %.200s)",
+                         Py_TYPE(o)->tp_name,
+                         meth_id->object,
+                         Py_TYPE(meth_output)->tp_name);
+        }
+        Py_DECREF(meth_output);
+        return NULL;
+    }
+    Py_DECREF(meth_output);
+    result = PySequence_List(it);
+    Py_DECREF(it);
+    return result;
+}
+
 PyObject *
 PyMapping_Keys(PyObject *o)
 {
-    PyObject *keys;
-    PyObject *fast;
     _Py_IDENTIFIER(keys);
 
-    if (PyDict_CheckExact(o))
+    if (o == NULL) {
+        return null_error();
+    }
+    if (PyDict_CheckExact(o)) {
         return PyDict_Keys(o);
-    keys = _PyObject_CallMethodId(o, &PyId_keys, NULL);
-    if (keys == NULL)
-        return NULL;
-    fast = PySequence_Fast(keys, "o.keys() are not iterable");
-    Py_DECREF(keys);
-    return fast;
+    }
+    return method_output_as_list(o, &PyId_keys);
 }
 
 PyObject *
 PyMapping_Items(PyObject *o)
 {
-    PyObject *items;
-    PyObject *fast;
     _Py_IDENTIFIER(items);
 
-    if (PyDict_CheckExact(o))
+    if (o == NULL) {
+        return null_error();
+    }
+    if (PyDict_CheckExact(o)) {
         return PyDict_Items(o);
-    items = _PyObject_CallMethodId(o, &PyId_items, NULL);
-    if (items == NULL)
-        return NULL;
-    fast = PySequence_Fast(items, "o.items() are not iterable");
-    Py_DECREF(items);
-    return fast;
+    }
+    return method_output_as_list(o, &PyId_items);
 }
 
 PyObject *
 PyMapping_Values(PyObject *o)
 {
-    PyObject *values;
-    PyObject *fast;
     _Py_IDENTIFIER(values);
 
-    if (PyDict_CheckExact(o))
+    if (o == NULL) {
+        return null_error();
+    }
+    if (PyDict_CheckExact(o)) {
         return PyDict_Values(o);
-    values = _PyObject_CallMethodId(o, &PyId_values, NULL);
-    if (values == NULL)
-        return NULL;
-    fast = PySequence_Fast(values, "o.values() are not iterable");
-    Py_DECREF(values);
-    return fast;
+    }
+    return method_output_as_list(o, &PyId_values);
 }
 
 /* isinstance(), issubclass() */