Try to call "__rmod__()" during string %-formatting when the right side is a...

Try to call "__rmod__()" during string %-formatting when the right side is a string subtype that implements it.
See https://bugs.python.org/issue28598

CHANGES.rst

@@ -73,6 +73,10 @@ Bugs fixed
 * Multiplied string literals lost their factor when they are part of another
   constant expression (e.g. 'x' * 10 + 'y' => 'xy').
 
+* String formatting with the '%' operator didn't call the special ``__rmod__()``
+  method if the right side is a string subclass that implements it.
+  (Python issue 28598)
+
 * The directive ``language_level=3`` did not apply to the first token in the
   source file.  (Github issue #2230)
 

Cython/Compiler/ExprNodes.py

@@ -11616,13 +11616,19 @@ class ModNode(DivNode):
                     self.operand2.result())
 
     def py_operation_function(self, code):
-        if self.operand1.type is unicode_type:
-            if self.operand1.may_be_none():
+        type1, type2 = self.operand1.type, self.operand2.type
+        # ("..." % x)  must call "x.__rmod__()" for string subtypes.
+        if type1 is unicode_type:
+            if self.operand1.may_be_none() or (
+                    type2.is_extension_type and type2.subtype_of(type1) or
+                    type2 is py_object_type and not isinstance(self.operand2, CoerceToPyTypeNode)):
                 return '__Pyx_PyUnicode_FormatSafe'
             else:
                 return 'PyUnicode_Format'
-        elif self.operand1.type is str_type:
-            if self.operand1.may_be_none():
+        elif type1 is str_type:
+            if self.operand1.may_be_none() or (
+                    type2.is_extension_type and type2.subtype_of(type1) or
+                    type2 is py_object_type and not isinstance(self.operand2, CoerceToPyTypeNode)):
                 return '__Pyx_PyString_FormatSafe'
             else:
                 return '__Pyx_PyString_Format'

Cython/Utility/ModuleSetupCode.c

@@ -576,8 +576,9 @@ static CYTHON_INLINE void * PyThread_tss_get(Py_tss_t *key) {
   #define PyObject_Format(obj, fmt)  PyObject_CallMethod(obj, "__format__", "O", fmt)
 #endif
 
-#define __Pyx_PyString_FormatSafe(a, b)   ((unlikely((a) == Py_None)) ? PyNumber_Remainder(a, b) : __Pyx_PyString_Format(a, b))
-#define __Pyx_PyUnicode_FormatSafe(a, b)  ((unlikely((a) == Py_None)) ? PyNumber_Remainder(a, b) : PyUnicode_Format(a, b))
+// ("..." % x)  must call PyNumber_Remainder() if x is a string subclass that implements "__rmod__()".
+#define __Pyx_PyString_FormatSafe(a, b)   ((unlikely((a) == Py_None || (PyString_Check(b) && !PyString_CheckExact(b)))) ? PyNumber_Remainder(a, b) : __Pyx_PyString_Format(a, b))
+#define __Pyx_PyUnicode_FormatSafe(a, b)  ((unlikely((a) == Py_None || (PyUnicode_Check(b) && !PyUnicode_CheckExact(b)))) ? PyNumber_Remainder(a, b) : PyUnicode_Format(a, b))
 
 #if PY_MAJOR_VERSION >= 3
   #define __Pyx_PyString_Format(a, b)  PyUnicode_Format(a, b)

tests/run/unicode_formatting.pyx

@@ -34,3 +34,27 @@ def mix_format(a, int b, list c):
     -x-2-[1]-
     """
     return '-%s-%r-%a-' % (a, b, c)
+
+
+class PySubtype(unicode):
+    def __rmod__(self, other):
+        return f'PyRMOD({other})'
+
+
+cdef class ExtSubtype(unicode):
+    def __mod__(one, other):
+        return f'ExtMOD({one}, {other})'
+
+
+def subtypes():
+    """
+    >>> py, ext = subtypes()
+    >>> print(py)
+    PyRMOD(-%s-)
+    >>> print(ext)
+    ExtMOD(-%s-, ExtSub)
+    """
+    return [
+        '-%s-' % PySubtype("PySub"),
+        '-%s-' % ExtSubtype("ExtSub"),
+    ]