refactor: Change mlirApiObjectToCapsule to return std::optional

Instead of raising a `nanobind::type_error()`. This is necessary to honor the
nanobind type caster API contract, which requires `from_python` and `from_cpp` methods
to be marked `noexcept`.

Author: nicholasjng

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

@@ -20,6 +20,7 @@
 #define MLIR_BINDINGS_PYTHON_NANOBINDADAPTORS_H
 
 #include <cstdint>
+#include <optional>
 
 #include "mlir-c/Diagnostics.h"
 #include "mlir-c/IR.h"
@@ -43,18 +44,14 @@ namespace detail {
 /// with a raw handle (unowned). The returned object's lifetime may not extend
 /// beyond the apiObject handle without explicitly having its refcount increased
 /// (i.e. on return).
-static nanobind::object mlirApiObjectToCapsule(nanobind::handle apiObject) {
+static std::optional<nanobind::object>
+mlirApiObjectToCapsule(nanobind::handle apiObject) {
   if (PyCapsule_CheckExact(apiObject.ptr()))
     return nanobind::borrow<nanobind::object>(apiObject);
   nanobind::object api =
       nanobind::getattr(apiObject, MLIR_PYTHON_CAPI_PTR_ATTR, nanobind::none());
-  if (api.is_none()) {
-    std::string repr = nanobind::cast<std::string>(nanobind::repr(apiObject));
-    throw nanobind::type_error(
-        (llvm::Twine("Expected an MLIR object (got ") + repr + ").")
-            .str()
-            .c_str());
-  }
+  if (api.is_none())
+    return std::nullopt;
   return api;
 }
 
@@ -68,11 +65,10 @@ template <>
 struct type_caster<MlirAffineMap> {
   NB_TYPE_CASTER(MlirAffineMap, const_name("MlirAffineMap"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToAffineMap(capsule.ptr());
-    if (mlirAffineMapIsNull(value)) {
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
       return false;
-    }
+    value = mlirPythonCapsuleToAffineMap(capsule.value().ptr());
     return !mlirAffineMapIsNull(value);
   }
   static handle from_cpp(MlirAffineMap v, rv_policy,
@@ -91,8 +87,10 @@ template <>
 struct type_caster<MlirAttribute> {
   NB_TYPE_CASTER(MlirAttribute, const_name("MlirAttribute"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToAttribute(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToAttribute(capsule.value().ptr());
     return !mlirAttributeIsNull(value);
   }
   static handle from_cpp(MlirAttribute v, rv_policy,
@@ -112,8 +110,10 @@ template <>
 struct type_caster<MlirBlock> {
   NB_TYPE_CASTER(MlirBlock, const_name("MlirBlock"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToBlock(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToBlock(capsule.value().ptr());
     return !mlirBlockIsNull(value);
   }
 };
@@ -132,8 +132,8 @@ struct type_caster<MlirContext> {
                 .attr("Context")
                 .attr("current");
     }
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToContext(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    value = mlirPythonCapsuleToContext(capsule.value().ptr());
     return !mlirContextIsNull(value);
   }
 };
@@ -143,8 +143,10 @@ template <>
 struct type_caster<MlirDialectRegistry> {
   NB_TYPE_CASTER(MlirDialectRegistry, const_name("MlirDialectRegistry"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToDialectRegistry(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToDialectRegistry(capsule.value().ptr());
     return !mlirDialectRegistryIsNull(value);
   }
   static handle from_cpp(MlirDialectRegistry v, rv_policy,
@@ -169,8 +171,8 @@ struct type_caster<MlirLocation> {
                 .attr("Location")
                 .attr("current");
     }
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToLocation(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    value = mlirPythonCapsuleToLocation(capsule.value().ptr());
     return !mlirLocationIsNull(value);
   }
   static handle from_cpp(MlirLocation v, rv_policy,
@@ -189,8 +191,10 @@ template <>
 struct type_caster<MlirModule> {
   NB_TYPE_CASTER(MlirModule, const_name("MlirModule"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToModule(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToModule(capsule.value().ptr());
     return !mlirModuleIsNull(value);
   }
   static handle from_cpp(MlirModule v, rv_policy,
@@ -210,8 +214,10 @@ struct type_caster<MlirFrozenRewritePatternSet> {
   NB_TYPE_CASTER(MlirFrozenRewritePatternSet,
                  const_name("MlirFrozenRewritePatternSet"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToFrozenRewritePatternSet(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToFrozenRewritePatternSet(capsule.value().ptr());
     return value.ptr != nullptr;
   }
   static handle from_cpp(MlirFrozenRewritePatternSet v, rv_policy,
@@ -230,8 +236,10 @@ template <>
 struct type_caster<MlirOperation> {
   NB_TYPE_CASTER(MlirOperation, const_name("MlirOperation"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToOperation(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToOperation(capsule.value().ptr());
     return !mlirOperationIsNull(value);
   }
   static handle from_cpp(MlirOperation v, rv_policy,
@@ -252,8 +260,10 @@ template <>
 struct type_caster<MlirValue> {
   NB_TYPE_CASTER(MlirValue, const_name("MlirValue"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToValue(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToValue(capsule.value().ptr());
     return !mlirValueIsNull(value);
   }
   static handle from_cpp(MlirValue v, rv_policy,
@@ -275,8 +285,10 @@ template <>
 struct type_caster<MlirPassManager> {
   NB_TYPE_CASTER(MlirPassManager, const_name("MlirPassManager"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToPassManager(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToPassManager(capsule.value().ptr());
     return !mlirPassManagerIsNull(value);
   }
 };
@@ -286,8 +298,10 @@ template <>
 struct type_caster<MlirTypeID> {
   NB_TYPE_CASTER(MlirTypeID, const_name("MlirTypeID"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToTypeID(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToTypeID(capsule.value().ptr());
     return !mlirTypeIDIsNull(value);
   }
   static handle from_cpp(MlirTypeID v, rv_policy,
@@ -308,8 +322,10 @@ template <>
 struct type_caster<MlirType> {
   NB_TYPE_CASTER(MlirType, const_name("MlirType"))
   bool from_python(handle src, uint8_t flags, cleanup_list *cleanup) noexcept {
-    nanobind::object capsule = mlirApiObjectToCapsule(src);
-    value = mlirPythonCapsuleToType(capsule.ptr());
+    std::optional<nanobind::object> capsule = mlirApiObjectToCapsule(src);
+    if (!capsule)
+      return false;
+    value = mlirPythonCapsuleToType(capsule.value().ptr());
     return !mlirTypeIsNull(value);
   }
   static handle from_cpp(MlirType t, rv_policy,

mlir/test/python/lib/PythonTestModuleNanobind.cpp

@@ -113,8 +113,10 @@ NB_MODULE(_mlirPythonTestNanobind, m) {
       .attr(MLIR_PYTHON_CAPI_VALUE_CASTER_REGISTER_ATTR)(
           mlirRankedTensorTypeID)(
           nanobind::cpp_function([valueCls](const nb::object &valueObj) {
-            nb::object capsule = mlirApiObjectToCapsule(valueObj);
-            MlirValue v = mlirPythonCapsuleToValue(capsule.ptr());
+            std::optional<nb::object> capsule =
+                mlirApiObjectToCapsule(valueObj);
+            // TODO(nicholasjng): Can this capsule be std::nullopt?
+            MlirValue v = mlirPythonCapsuleToValue(capsule.value().ptr());
             MlirType t = mlirValueGetType(v);
             // This is hyper-specific in order to exercise/test registering a
             // value caster from cpp (but only for a single test case; see