py/runtime: Implement dispatch for "reverse op" special methods.

If, for class X, X.__add__(Y) doesn't exist (or returns NotImplemented),
try Y.__radd__(X) instead.

This patch could be simpler, but requires undoing operand swap and
operation switch to get non-confusing error message in case __radd__
doesn't exist.

4 files changed, 47 insertions, 2 deletions

diff --git a/py/mpconfig.h b/py/mpconfig.h
index 38cf4b560..63438a846 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -759,11 +759,19 @@ typedef double mp_float_t;
 #endif
 
 // Whether to support complete set of special methods
-// for user classes, otherwise only the most used
+// for user classes, or only the most used ones. "Reverse"
+// methods are controlled by MICROPY_PY_REVERSE_SPECIAL_METHODS
+// below.
 #ifndef MICROPY_PY_ALL_SPECIAL_METHODS
 #define MICROPY_PY_ALL_SPECIAL_METHODS (0)
 #endif
 
+// Whether to support reverse arithmetic operarions methods
+// (__radd__, etc.)
+#ifndef MICROPY_PY_REVERSE_SPECIAL_METHODS
+#define MICROPY_PY_REVERSE_SPECIAL_METHODS (0)
+#endif
+
 // Whether to support compile function
 #ifndef MICROPY_PY_BUILTINS_COMPILE
 #define MICROPY_PY_BUILTINS_COMPILE (0)
diff --git a/py/objtype.c b/py/objtype.c
index 8ce593dba..cf9311d5c 100644
--- a/py/objtype.c
+++ b/py/objtype.c
@@ -441,6 +441,13 @@ const qstr mp_binary_op_method_name[] = {
     MP_BINARY_OP_INPLACE_TRUE_DIVIDE,
     MP_BINARY_OP_INPLACE_MODULO,
     MP_BINARY_OP_INPLACE_POWER,*/
+
+    #if MICROPY_PY_REVERSE_SPECIAL_METHODS
+    [MP_BINARY_OP_REVERSE_ADD] = MP_QSTR___radd__,
+    [MP_BINARY_OP_REVERSE_SUBTRACT] = MP_QSTR___rsub__,
+    [MP_BINARY_OP_REVERSE_MULTIPLY] = MP_QSTR___rmul__,
+    #endif
+
     [MP_BINARY_OP_LESS] = MP_QSTR___lt__,
     [MP_BINARY_OP_MORE] = MP_QSTR___gt__,
     [MP_BINARY_OP_EQUAL] = MP_QSTR___eq__,
diff --git a/py/runtime.c b/py/runtime.c
index 21ef42577..c533558da 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -555,7 +555,20 @@ generic_binary_op:
         }
     }
 
-    // TODO implement dispatch for reverse binary ops
+#if MICROPY_PY_REVERSE_SPECIAL_METHODS
+    if (op >= MP_BINARY_OP_OR && op <= MP_BINARY_OP_REVERSE_POWER) {
+        mp_obj_t t = rhs;
+        rhs = lhs;
+        lhs = t;
+        if (op <= MP_BINARY_OP_POWER) {
+            op += MP_BINARY_OP_REVERSE_OR - MP_BINARY_OP_OR;
+            goto generic_binary_op;
+        }
+
+        // Convert __rop__ back to __op__ for error message
+        op -= MP_BINARY_OP_REVERSE_OR - MP_BINARY_OP_OR;
+    }
+#endif
 
 unsupported_op:
     if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
diff --git a/py/runtime0.h b/py/runtime0.h
index 3cf5e530d..a3e9d46b9 100644
--- a/py/runtime0.h
+++ b/py/runtime0.h
@@ -101,6 +101,23 @@ typedef enum {
     // Operations below this line don't appear in bytecode, they
     // just identify special methods.
 
+    // MP_BINARY_OP_REVERSE_* must follow immediately after MP_BINARY_OP_*
+#if MICROPY_PY_REVERSE_SPECIAL_METHODS
+    MP_BINARY_OP_REVERSE_OR,
+    MP_BINARY_OP_REVERSE_XOR,
+    MP_BINARY_OP_REVERSE_AND,
+    MP_BINARY_OP_REVERSE_LSHIFT,
+    MP_BINARY_OP_REVERSE_RSHIFT,
+    MP_BINARY_OP_REVERSE_ADD,
+
+    MP_BINARY_OP_REVERSE_SUBTRACT,
+    MP_BINARY_OP_REVERSE_MULTIPLY,
+    MP_BINARY_OP_REVERSE_FLOOR_DIVIDE,
+    MP_BINARY_OP_REVERSE_TRUE_DIVIDE,
+    MP_BINARY_OP_REVERSE_MODULO,
+    MP_BINARY_OP_REVERSE_POWER,
+#endif
+
     MP_BINARY_OP_DIVMOD, // not emitted by the compiler but supported by the runtime
 
     MP_BINARY_OP_LAST,