Fail with a proper error when tying to modify attributes pythran cons…

…iders immutable

Fix #2224

pythran/tests/test_normalize_methods.py

@@ -125,6 +125,17 @@ def test_dispatch_update(self):
                       {1}, {1:1},
                       dispatch_update=[Set[int], Dict[int,int]])
 
+    def test_supported_attr_store(self):
+        self.run_test("def supported_attr_store(s): s.real += 1; return s",
+                      numpy.ones(10, dtype=complex),
+                      supported_attr_store=[NDArray[complex, :]])
+
+    def test_unsupported_attr_store(self):
+        with self.assertRaises(pythran.syntax.PythranSyntaxError):
+            self.run_test("def unsupported_attr_store(s): s.strides = 2; return s",
+                          numpy.ones(10, dtype=complex),
+                          unsupported_attr_store=[NDArray[complex, :]])
+
     def test_invalid_method_call(self):
         code = '''
 def np_asarray7(sRate=44100):

pythran/transformations/normalize_method_calls.py

@@ -199,7 +199,11 @@ def visit_Attribute(self, node):
                 # the only situation where this arises is for real/imag of
                 # a ndarray. As a call is not valid for a store, add a slice
                 # to ends up with a valid lhs
-                assert node.attr in ('real', 'imag'), "only store to imag/real"
+                if node.attr not in ('real', 'imag'):
+                    raise PythranSyntaxError(
+                            "Unsupported store to attribute {}".format(node.attr),
+                            node)
+
                 return ast.Subscript(call,
                                      ast.Slice(None, None, None),
                                      node.ctx)