[MLIR] [Python] Allow 'operation.parent' to return 'None'

This is more Pythonic and better matches the C++ and C APIs.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D108183

mlir/lib/Bindings/Python/IRCore.cpp

@@ -868,22 +868,23 @@ py::object PyOperationBase::getAsm(bool binary,
   return fileObject.attr("getvalue")();
 }
 
-PyOperationRef PyOperation::getParentOperation() {
+llvm::Optional<PyOperationRef> PyOperation::getParentOperation() {
   checkValid();
   if (!isAttached())
     throw SetPyError(PyExc_ValueError, "Detached operations have no parent");
   MlirOperation operation = mlirOperationGetParentOperation(get());
   if (mlirOperationIsNull(operation))
-    throw SetPyError(PyExc_ValueError, "Operation has no parent.");
+    return {};
   return PyOperation::forOperation(getContext(), operation);
 }
 
 PyBlock PyOperation::getBlock() {
   checkValid();
-  PyOperationRef parentOperation = getParentOperation();
+  llvm::Optional<PyOperationRef> parentOperation = getParentOperation();
   MlirBlock block = mlirOperationGetBlock(get());
   assert(!mlirBlockIsNull(block) && "Attached operation has null parent");
-  return PyBlock{std::move(parentOperation), block};
+  assert(parentOperation && "Operation has no parent");
+  return PyBlock{std::move(*parentOperation), block};
 }
 
 py::object PyOperation::getCapsule() {
@@ -2121,8 +2122,11 @@ void mlir::python::populateIRCore(py::module &m) {
                   py::arg("loc") = py::none(), py::arg("ip") = py::none(),
                   kOperationCreateDocstring)
       .def_property_readonly("parent",
-                             [](PyOperation &self) {
-                               return self.getParentOperation().getObject();
+                             [](PyOperation &self) -> py::object {
+                               auto parent = self.getParentOperation();
+                               if (parent)
+                                 return parent->getObject();
+                               return py::none();
                              })
       .def("erase", &PyOperation::erase)
       .def_property_readonly(MLIR_PYTHON_CAPI_PTR_ATTR,

mlir/lib/Bindings/Python/IRModule.h

@@ -18,6 +18,7 @@
 #include "mlir-c/IR.h"
 #include "mlir-c/IntegerSet.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Optional.h"
 
 namespace mlir {
 namespace python {
@@ -452,7 +453,7 @@ public:
 
   /// Gets the parent operation or raises an exception if the operation has
   /// no parent.
-  PyOperationRef getParentOperation();
+  llvm::Optional<PyOperationRef> getParentOperation();
 
   /// Gets a capsule wrapping the void* within the MlirOperation.
   pybind11::object getCapsule();