BUG: Prevent unsafe string concatenation

This did not handle exceptions correctly.
Changed to use python to format the exception like all the others in this file.

This also adds quotes around the ufunc name.

numpy/core/_exceptions.py

@@ -36,6 +36,22 @@ def __init__(self, ufunc):
         self.ufunc = ufunc
 
 
+@_display_as_base
+class _UFuncBinaryResolutionError(UFuncTypeError):
+    """ Thrown when a binary resolution fails """
+    def __init__(self, ufunc, dtypes):
+        super().__init__(ufunc)
+        self.dtypes = tuple(dtypes)
+        assert len(self.dtypes) == 2
+
+    def __str__(self):
+        return (
+            "ufunc {!r} cannot use operands with types {!r} and {!r}"
+        ).format(
+            self.ufunc.__name__, *self.dtypes
+        )
+
+
 @_display_as_base
 class _UFuncNoLoopError(UFuncTypeError):
     """ Thrown when a ufunc loop cannot be found """

numpy/core/src/umath/ufunc_type_resolution.c

@@ -74,18 +74,28 @@ npy_casting_to_string(NPY_CASTING casting)
  */
 static int
 raise_binary_type_reso_error(PyUFuncObject *ufunc, PyArrayObject **operands) {
-    PyObject *errmsg;
-    const char *ufunc_name = ufunc_get_name_cstr(ufunc);
-    errmsg = PyUString_FromFormat("ufunc %s cannot use operands "
-                        "with types ", ufunc_name);
-    PyUString_ConcatAndDel(&errmsg,
-            PyObject_Repr((PyObject *)PyArray_DESCR(operands[0])));
-    PyUString_ConcatAndDel(&errmsg,
-            PyUString_FromString(" and "));
-    PyUString_ConcatAndDel(&errmsg,
-            PyObject_Repr((PyObject *)PyArray_DESCR(operands[1])));
-    PyErr_SetObject(PyExc_TypeError, errmsg);
-    Py_DECREF(errmsg);
+    static PyObject *exc_type = NULL;
+    PyObject *exc_value;
+
+    npy_cache_import(
+        "numpy.core._exceptions", "_UFuncBinaryResolutionError",
+        &exc_type);
+    if (exc_type == NULL) {
+        return -1;
+    }
+
+    /* produce an error object */
+    exc_value = Py_BuildValue(
+        "O(OO)", ufunc,
+        (PyObject *)PyArray_DESCR(operands[0]),
+        (PyObject *)PyArray_DESCR(operands[1])
+    );
+    if (exc_value == NULL){
+        return -1;
+    }
+    PyErr_SetObject(exc_type, exc_value);
+    Py_DECREF(exc_value);
+
     return -1;
 }
 

numpy/core/tests/test_datetime.py

@@ -1825,7 +1825,7 @@ def test_timedelta_modulus_type_resolution(self, val1, val2):
         # NOTE: some of the operations may be supported
         # in the future
         with assert_raises_regex(TypeError,
-                                 "remainder cannot use operands with types"):
+                                 "'remainder' cannot use operands with types"):
             val1 % val2
 
     def test_timedelta_arange_no_dtype(self):