Support __rmod__ for RHS subclasses of str in % string formatting ope…

…rations. When you use `'%s' % SubClassOfStr()`, where `SubClassOfStr.__rmod__` exists, the reverse operation is ignored as normally such string formatting operations use the `PyUnicode_Format()` fast path. This patch tests for subclasses of `str` first and picks the slow path in that case. Fixes issue28598.
python · serhiy-storchaka · Feb 23, 2017 · Feb 12, 2017 · Feb 12, 2017 · Feb 23, 2017
commit 1ef5f557c3512b7f6ddfb7ddc6fa9507573713fe
diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
@@ -1448,6 +1448,15 @@ def test_formatting_huge_precision(self):
         with self.assertRaises(ValueError):
             result = format_string % 2.34
 
+    def test_issue28598_strsubclass_rhs(self):
+        # A subclass of str with an __rmod__ method should be able to hook
+        # into the % operator
+        class SubclassedStr(str):
+            def __rmod__(self, other):
+                return 'Success, self.__rmod__({!r}) was called'.format(other)
+        self.assertEqual('lhs %% %r' % SubclassedStr('rhs'),
+                         "Success, self.__rmod__('lhs %% %r') was called")
+
     @support.cpython_only
     def test_formatting_huge_precision_c_limits(self):
         from _testcapi import INT_MAX

diff --git a/Misc/NEWS b/Misc/NEWS
@@ -10,6 +10,8 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #28598: Support __rmod__ for subclasses of str being called before str.__mod__.
+
 - bpo-29438: Fixed use-after-free problem in key sharing dict.
 
 - Issue #29319: Prevent RunMainFromImporter overwriting sys.path[0].

diff --git a/Python/ceval.c b/Python/ceval.c
@@ -1355,9 +1355,15 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
         TARGET(BINARY_MODULO) {
             PyObject *divisor = POP();
             PyObject *dividend = TOP();
-            PyObject *res = PyUnicode_CheckExact(dividend) ?
-                PyUnicode_Format(dividend, divisor) :
-                PyNumber_Remainder(dividend, divisor);
+            PyObject *res;
+            if (PyUnicode_CheckExact(dividend) && !(
+                  PyUnicode_Check(divisor) && !PyUnicode_CheckExact(divisor))) {
+              // fast path; string formatting, but not if the RHS is a str subclass
+              // (see issue28598)
+              res = PyUnicode_Format(dividend, divisor);
+            } else {
+              res = PyNumber_Remainder(dividend, divisor);
+            }
             Py_DECREF(divisor);
             Py_DECREF(dividend);
             SET_TOP(res);