refactor: Assign in if statements everywhere, adjust test expectations

Modeled after a patch posted in the conversation in PR #143866:
https://gist.github.com/makslevental/b224ffca7f15e273a4897975cda28b4c.

Author: nicholasjng

mlir/include/mlir/Bindings/Python/NanobindAdaptors.h

@@ -51,7 +51,7 @@ mlirApiObjectToCapsule(nanobind::handle apiObject) {
   nanobind::object api =
       nanobind::getattr(apiObject, MLIR_PYTHON_CAPI_PTR_ATTR, nanobind::none());
   if (api.is_none())
-    return std::nullopt;
+    return {};
   return api;
 }
 
@@ -65,10 +65,8 @@ template <>
 struct type_caster<MlirAffineMap> {
   NB_TYPE_CASTER(MlirAffineMap, const_name("MlirAffineMap"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToAffineMap(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToAffineMap(capsule->ptr());
     return !mlirAffineMapIsNull(value);
   }
   static handle from_cpp(MlirAffineMap v, rv_policy,
@@ -87,10 +85,8 @@ template <>
 struct type_caster<MlirAttribute> {
   NB_TYPE_CASTER(MlirAttribute, const_name("MlirAttribute"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToAttribute(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToAttribute(capsule->ptr());
     return !mlirAttributeIsNull(value);
   }
   static handle from_cpp(MlirAttribute v, rv_policy,
@@ -110,10 +106,8 @@ template <>
 struct type_caster<MlirBlock> {
   NB_TYPE_CASTER(MlirBlock, const_name("MlirBlock"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToBlock(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToBlock(capsule->ptr());
     return !mlirBlockIsNull(value);
   }
 };
@@ -133,7 +127,7 @@ struct type_caster<MlirContext> {
                 .attr("current");
     }
     std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToContext(capsule.value().ptr());
+    value = mlirPythonCapsuleToContext(capsule->ptr());
     return !mlirContextIsNull(value);
   }
 };
@@ -143,10 +137,8 @@ template <>
 struct type_caster<MlirDialectRegistry> {
   NB_TYPE_CASTER(MlirDialectRegistry, const_name("MlirDialectRegistry"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToDialectRegistry(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToDialectRegistry(capsule->ptr());
     return !mlirDialectRegistryIsNull(value);
   }
   static handle from_cpp(MlirDialectRegistry v, rv_policy,
@@ -171,8 +163,8 @@ struct type_caster<MlirLocation> {
                 .attr("Location")
                 .attr("current");
     }
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToLocation(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToLocation(capsule->ptr());
     return !mlirLocationIsNull(value);
   }
   static handle from_cpp(MlirLocation v, rv_policy,
@@ -191,10 +183,8 @@ template <>
 struct type_caster<MlirModule> {
   NB_TYPE_CASTER(MlirModule, const_name("MlirModule"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToModule(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToModule(capsule->ptr());
     return !mlirModuleIsNull(value);
   }
   static handle from_cpp(MlirModule v, rv_policy,
@@ -214,10 +204,8 @@ struct type_caster<MlirFrozenRewritePatternSet> {
   NB_TYPE_CASTER(MlirFrozenRewritePatternSet,
                  const_name("MlirFrozenRewritePatternSet"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToFrozenRewritePatternSet(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToFrozenRewritePatternSet(capsule->ptr());
     return value.ptr != nullptr;
   }
   static handle from_cpp(MlirFrozenRewritePatternSet v, rv_policy,
@@ -236,10 +224,8 @@ template <>
 struct type_caster<MlirOperation> {
   NB_TYPE_CASTER(MlirOperation, const_name("MlirOperation"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToOperation(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToOperation(capsule->ptr());
     return !mlirOperationIsNull(value);
   }
   static handle from_cpp(MlirOperation v, rv_policy,
@@ -260,10 +246,8 @@ template <>
 struct type_caster<MlirValue> {
   NB_TYPE_CASTER(MlirValue, const_name("MlirValue"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToValue(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToValue(capsule->ptr());
     return !mlirValueIsNull(value);
   }
   static handle from_cpp(MlirValue v, rv_policy,
@@ -285,10 +269,8 @@ template <>
 struct type_caster<MlirPassManager> {
   NB_TYPE_CASTER(MlirPassManager, const_name("MlirPassManager"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToPassManager(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToPassManager(capsule->ptr());
     return !mlirPassManagerIsNull(value);
   }
 };
@@ -298,10 +280,8 @@ template <>
 struct type_caster<MlirTypeID> {
   NB_TYPE_CASTER(MlirTypeID, const_name("MlirTypeID"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToTypeID(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToTypeID(capsule->ptr());
     return !mlirTypeIDIsNull(value);
   }
   static handle from_cpp(MlirTypeID v, rv_policy,
@@ -322,10 +302,8 @@ template <>
 struct type_caster<MlirType> {
   NB_TYPE_CASTER(MlirType, const_name("MlirType"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
-    if (!capsule)
-      return false;
-    value = mlirPythonCapsuleToType(capsule.value().ptr());
+    if (auto capsule = mlirApiObjectToCapsule(src))
+      value = mlirPythonCapsuleToType(capsule->ptr());
     return !mlirTypeIsNull(value);
   }
   static handle from_cpp(MlirType t, rv_policy,

mlir/test/python/dialects/python_test.py

@@ -361,7 +361,9 @@ def testCustomAttribute():
         try:
             TestAttr(42)
         except TypeError as e:
-            assert "Expected an MLIR object" in str(e)
+            assert "Expected an MLIR object (got 42)" in str(e)
+        except ValueError as e:
+            assert "Cannot cast attribute to TestAttr (from 42)" in str(e)
         else:
             raise
 
@@ -406,7 +408,9 @@ def testCustomType():
         try:
             TestType(42)
         except TypeError as e:
-            assert "Expected an MLIR object" in str(e)
+            assert "Expected an MLIR object (got 42)" in str(e)
+        except ValueError as e:
+            assert "Cannot cast type to TestType (from 42)" in str(e)
         else:
             raise
 

mlir/test/python/lib/PythonTestModuleNanobind.cpp

@@ -115,7 +115,7 @@ NB_MODULE(_mlirPythonTestNanobind, m) {
           nanobind::cpp_function([valueCls](const nb::object &valueObj) {
             std::optional<nb::object> capsule =
                 mlirApiObjectToCapsule(valueObj);
-            // TODO(nicholasjng): Can this capsule be std::nullopt?
+            assert(capsule.has_value() && "capsule is not null");
             MlirValue v = mlirPythonCapsuleToValue(capsule.value().ptr());
             MlirType t = mlirValueGetType(v);
             // This is hyper-specific in order to exercise/test registering a