bpo-35634: Raise an error when first passed kwargs contains duplicated keys. (GH-11438)

Lib/test/test_extcall.py

@@ -316,6 +316,52 @@ not function
       ...
     TypeError: dir() got multiple values for keyword argument 'b'
 
+Test a kwargs mapping with duplicated keys.
+
+    >>> from collections.abc import Mapping
+    >>> class MultiDict(Mapping):
+    ...     def __init__(self, items):
+    ...         self._items = items
+    ...
+    ...     def __iter__(self):
+    ...         return (k for k, v in self._items)
+    ...
+    ...     def __getitem__(self, key):
+    ...         for k, v in self._items:
+    ...             if k == key:
+    ...                 return v
+    ...         raise KeyError(key)
+    ...
+    ...     def __len__(self):
+    ...         return len(self._items)
+    ...
+    ...     def keys(self):
+    ...         return [k for k, v in self._items]
+    ...
+    ...     def values(self):
+    ...         return [v for k, v in self._items]
+    ...
+    ...     def items(self):
+    ...         return [(k, v) for k, v in self._items]
+    ...
+    >>> g(**MultiDict([('x', 1), ('y', 2)]))
+    1 () {'y': 2}
+
+    >>> g(**MultiDict([('x', 1), ('x', 2)]))
+    Traceback (most recent call last):
+      ...
+    TypeError: g() got multiple values for keyword argument 'x'
+
+    >>> g(a=3, **MultiDict([('x', 1), ('x', 2)]))
+    Traceback (most recent call last):
+      ...
+    TypeError: g() got multiple values for keyword argument 'x'
+
+    >>> g(**MultiDict([('a', 3)]), **MultiDict([('x', 1), ('x', 2)]))
+    Traceback (most recent call last):
+      ...
+    TypeError: g() got multiple values for keyword argument 'x'
+
 Another helper function
 
     >>> def f2(*a, **b):

Misc/NEWS.d/next/Core and Builtins/2019-01-05-18-39-49.bpo-35634.nVP_gs.rst

@@ -0,0 +1,3 @@
+``func(**kwargs)`` will now raise an error when ``kwargs`` is a mapping
+containing multiple entries with the same key. An error was already raised
+when other keyword arguments are passed before ``**kwargs`` since Python 3.6.

Python/ceval.c

@@ -70,7 +70,7 @@ static PyObject * unicode_concatenate(PyObject *, PyObject *,
                                       PyFrameObject *, const _Py_CODEUNIT *);
 static PyObject * special_lookup(PyObject *, _Py_Identifier *);
 static int check_args_iterable(PyObject *func, PyObject *vararg);
-static void format_kwargs_mapping_error(PyObject *func, PyObject *kwargs);
+static void format_kwargs_error(PyObject *func, PyObject *kwargs);
 static void format_awaitable_error(PyTypeObject *, int);
 
 #define NAME_ERROR_MSG \
@@ -2660,37 +2660,8 @@ main_loop:
             for (i = oparg; i > 0; i--) {
                 PyObject *arg = PEEK(i);
                 if (_PyDict_MergeEx(sum, arg, 2) < 0) {
-                    PyObject *func = PEEK(2 + oparg);
-                    if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
-                        format_kwargs_mapping_error(func, arg);
-                    }
-                    else if (PyErr_ExceptionMatches(PyExc_KeyError)) {
-                        PyObject *exc, *val, *tb;
-                        PyErr_Fetch(&exc, &val, &tb);
-                        if (val && PyTuple_Check(val) && PyTuple_GET_SIZE(val) == 1) {
-                            PyObject *key = PyTuple_GET_ITEM(val, 0);
-                            if (!PyUnicode_Check(key)) {
-                                PyErr_Format(PyExc_TypeError,
-                                        "%.200s%.200s keywords must be strings",
-                                        PyEval_GetFuncName(func),
-                                        PyEval_GetFuncDesc(func));
-                            } else {
-                                PyErr_Format(PyExc_TypeError,
-                                        "%.200s%.200s got multiple "
-                                        "values for keyword argument '%U'",
-                                        PyEval_GetFuncName(func),
-                                        PyEval_GetFuncDesc(func),
-                                        key);
-                            }
-                            Py_XDECREF(exc);
-                            Py_XDECREF(val);
-                            Py_XDECREF(tb);
-                        }
-                        else {
-                            PyErr_Restore(exc, val, tb);
-                        }
-                    }
                     Py_DECREF(sum);
+                    format_kwargs_error(PEEK(2 + oparg), arg);
                     goto error;
                 }
             }
@@ -3286,17 +3257,9 @@ main_loop:
                     PyObject *d = PyDict_New();
                     if (d == NULL)
                         goto error;
-                    if (PyDict_Update(d, kwargs) != 0) {
+                    if (_PyDict_MergeEx(d, kwargs, 2) < 0) {
                         Py_DECREF(d);
-                        /* PyDict_Update raises attribute
+                        format_kwargs_error(SECOND(), kwargs);
-                         * error (percolated from an attempt
-                         * to get 'keys' attribute) instead of
-                         * a type error if its second argument
-                         * is not a mapping.
-                         */
-                        if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
-                            format_kwargs_mapping_error(SECOND(), kwargs);
-                        }
                         Py_DECREF(kwargs);
                         goto error;
                     }
@@ -5063,14 +5026,48 @@ check_args_iterable(PyObject *func, PyObject *args)
 }
 
 static void
-format_kwargs_mapping_error(PyObject *func, PyObject *kwargs)
+format_kwargs_error(PyObject *func, PyObject *kwargs)
 {
+    /* _PyDict_MergeEx raises attribute
+     * error (percolated from an attempt
+     * to get 'keys' attribute) instead of
+     * a type error if its second argument
+     * is not a mapping.
+     */
+    if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
         PyErr_Format(PyExc_TypeError,
                      "%.200s%.200s argument after ** "
                      "must be a mapping, not %.200s",
                      PyEval_GetFuncName(func),
                      PyEval_GetFuncDesc(func),
                      kwargs->ob_type->tp_name);
+    }
+    else if (PyErr_ExceptionMatches(PyExc_KeyError)) {
+        PyObject *exc, *val, *tb;
+        PyErr_Fetch(&exc, &val, &tb);
+        if (val && PyTuple_Check(val) && PyTuple_GET_SIZE(val) == 1) {
+            PyObject *key = PyTuple_GET_ITEM(val, 0);
+            if (!PyUnicode_Check(key)) {
+                PyErr_Format(PyExc_TypeError,
+                             "%.200s%.200s keywords must be strings",
+                             PyEval_GetFuncName(func),
+                             PyEval_GetFuncDesc(func));
+            } else {
+                PyErr_Format(PyExc_TypeError,
+                             "%.200s%.200s got multiple "
+                             "values for keyword argument '%U'",
+                             PyEval_GetFuncName(func),
+                             PyEval_GetFuncDesc(func),
+                             key);
+            }
+            Py_XDECREF(exc);
+            Py_XDECREF(val);
+            Py_XDECREF(tb);
+        }
+        else {
+            PyErr_Restore(exc, val, tb);
+        }
+    }
 }
 
 static void