Add fallback for __mod__ magic method (#1934)

* Add fallback for `__mod__` magic method * Add 'CHANGELOG' entry * Complete tests
PyO3 · Oct 19, 2021 · 7349513 · 7349513
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -53,6 +53,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix incorrect linking to version-specific DLL instead of `python3.dll` when cross-compiling to Windows with `abi3`. [#1880](https://github.com/PyO3/pyo3/pull/1880)
 - Fix panic in generated `#[derive(FromPyObject)]` for enums. [#1888](https://github.com/PyO3/pyo3/pull/1888)
 - Fix cross-compiling to Python 3.7 builds with the "m" abi flag. [#1908](https://github.com/PyO3/pyo3/pull/1908)
+- Fix `__mod__` magic method fallback to `__rmod__`. [#1934](https://github.com/PyO3/pyo3/pull/1934).
 
 ## [0.14.5] - 2021-09-05
 

diff --git a/pyo3-macros-backend/src/defs.rs b/pyo3-macros-backend/src/defs.rs
@@ -528,7 +528,9 @@ pub const NUM: Proto = Proto {
         SlotDef::new(&["__mul__", "__rmul__"], "Py_nb_multiply", "mul_rmul"),
         SlotDef::new(&["__mul__"], "Py_nb_multiply", "mul"),
         SlotDef::new(&["__rmul__"], "Py_nb_multiply", "rmul"),
+        SlotDef::new(&["__mod__", "__rmod__"], "Py_nb_remainder", "mod_rmod"),
         SlotDef::new(&["__mod__"], "Py_nb_remainder", "mod_"),
+        SlotDef::new(&["__rmod__"], "Py_nb_remainder", "rmod"),
         SlotDef::new(
             &["__divmod__", "__rdivmod__"],
             "Py_nb_divmod",

diff --git a/src/class/number.rs b/src/class/number.rs
@@ -614,7 +614,14 @@ py_binary_fallback_num_func!(
 );
 py_binary_num_func!(mul, PyNumberMulProtocol, T::__mul__);
 py_binary_reversed_num_func!(rmul, PyNumberRMulProtocol, T::__rmul__);
+py_binary_fallback_num_func!(
+    mod_rmod,
+    T,
+    PyNumberModProtocol::__mod__,
+    PyNumberRModProtocol::__rmod__
+);
 py_binary_num_func!(mod_, PyNumberModProtocol, T::__mod__);
+py_binary_reversed_num_func!(rmod, PyNumberRModProtocol, T::__rmod__);
 py_binary_fallback_num_func!(
     divmod_rdivmod,
     T,

diff --git a/tests/test_arithmetics_protos.rs b/tests/test_arithmetics_protos.rs
@@ -152,6 +152,10 @@ impl PyNumberProtocol for BinaryArithmetic {
         format!("{:?} - {:?}", lhs, rhs)
     }
 
+    fn __mod__(lhs: &PyAny, rhs: &PyAny) -> String {
+        format!("{:?} % {:?}", lhs, rhs)
+    }
+
     fn __mul__(lhs: &PyAny, rhs: &PyAny) -> String {
         format!("{:?} * {:?}", lhs, rhs)
     }
@@ -195,6 +199,8 @@ fn binary_arithmetic() {
     py_run!(py, c, "assert 1 - c == '1 - BA'");
     py_run!(py, c, "assert c * 1 == 'BA * 1'");
     py_run!(py, c, "assert 1 * c == '1 * BA'");
+    py_run!(py, c, "assert c % 1 == 'BA % 1'");
+    py_run!(py, c, "assert 1 % c == '1 % BA'");
 
     py_run!(py, c, "assert c << 1 == 'BA << 1'");
     py_run!(py, c, "assert 1 << c == '1 << BA'");
@@ -225,6 +231,10 @@ impl PyNumberProtocol for RhsArithmetic {
         format!("{:?} - RA", other)
     }
 
+    fn __rmod__(&self, other: &PyAny) -> String {
+        format!("{:?} % RA", other)
+    }
+
     fn __rmul__(&self, other: &PyAny) -> String {
         format!("{:?} * RA", other)
     }
@@ -264,6 +274,8 @@ fn rhs_arithmetic() {
     py_run!(py, c, "assert 1 + c == '1 + RA'");
     py_run!(py, c, "assert c.__rsub__(1) == '1 - RA'");
     py_run!(py, c, "assert 1 - c == '1 - RA'");
+    py_run!(py, c, "assert c.__rmod__(1) == '1 % RA'");
+    py_run!(py, c, "assert 1 % c == '1 % RA'");
     py_run!(py, c, "assert c.__rmul__(1) == '1 * RA'");
     py_run!(py, c, "assert 1 * c == '1 * RA'");
     py_run!(py, c, "assert c.__rlshift__(1) == '1 << RA'");
@@ -299,6 +311,10 @@ impl PyNumberProtocol for LhsAndRhs {
         format!("{:?} - {:?}", lhs, rhs)
     }
 
+    fn __mod__(lhs: PyRef<Self>, rhs: &PyAny) -> String {
+        format!("{:?} % {:?}", lhs, rhs)
+    }
+
     fn __mul__(lhs: PyRef<Self>, rhs: &PyAny) -> String {
         format!("{:?} * {:?}", lhs, rhs)
     }
@@ -339,6 +355,10 @@ impl PyNumberProtocol for LhsAndRhs {
         format!("{:?} - RA", other)
     }
 
+    fn __rmod__(&self, other: &PyAny) -> String {
+        format!("{:?} % RA", other)
+    }
+
     fn __rmul__(&self, other: &PyAny) -> String {
         format!("{:?} * RA", other)
     }
@@ -388,6 +408,7 @@ fn lhs_fellback_to_rhs() {
     // If the light hand value is `LhsAndRhs`, LHS is used.
     py_run!(py, c, "assert c + 1 == 'LR + 1'");
     py_run!(py, c, "assert c - 1 == 'LR - 1'");
+    py_run!(py, c, "assert c % 1 == 'LR % 1'");
     py_run!(py, c, "assert c * 1 == 'LR * 1'");
     py_run!(py, c, "assert c << 1 == 'LR << 1'");
     py_run!(py, c, "assert c >> 1 == 'LR >> 1'");
@@ -399,6 +420,7 @@ fn lhs_fellback_to_rhs() {
     // Fellback to RHS because of type mismatching
     py_run!(py, c, "assert 1 + c == '1 + RA'");
     py_run!(py, c, "assert 1 - c == '1 - RA'");
+    py_run!(py, c, "assert 1 % c == '1 % RA'");
     py_run!(py, c, "assert 1 * c == '1 * RA'");
     py_run!(py, c, "assert 1 << c == '1 << RA'");
     py_run!(py, c, "assert 1 >> c == '1 >> RA'");