[Mlir-commits] [mlir] [mlir][Python] downcast Value to BlockArgument or OpResult (PR #175264)

Maksim Levental llvmlistbot at llvm.org
Mon Jan 12 09:04:59 PST 2026


https://github.com/makslevental updated https://github.com/llvm/llvm-project/pull/175264

>From 7c2c7f0ac6e721e42445d45f3ec3fff1e1368aa3 Mon Sep 17 00:00:00 2001
From: makslevental <maksim.levental at gmail.com>
Date: Fri, 9 Jan 2026 15:02:44 -0800
Subject: [PATCH 1/6] [mlir][Python] downcast ir.Value to BlockArgument or
 OpResult

---
 mlir/include/mlir/Bindings/Python/IRCore.h    | 13 ++--
 mlir/include/mlir/Bindings/Python/Nanobind.h  |  1 +
 .../mlir/Bindings/Python/NanobindUtils.h      |  2 +-
 mlir/lib/Bindings/Python/IRAffine.cpp         |  2 +-
 mlir/lib/Bindings/Python/IRCore.cpp           | 61 +++++++++----------
 mlir/test/python/dialects/python_test.py      |  2 +-
 mlir/test/python/ir/value.py                  | 22 +++----
 7 files changed, 50 insertions(+), 53 deletions(-)

diff --git a/mlir/include/mlir/Bindings/Python/IRCore.h b/mlir/include/mlir/Bindings/Python/IRCore.h
index 330318683c15e..bfc35ca5b9d50 100644
--- a/mlir/include/mlir/Bindings/Python/IRCore.h
+++ b/mlir/include/mlir/Bindings/Python/IRCore.h
@@ -36,20 +36,21 @@
 namespace mlir {
 namespace python {
 namespace MLIR_BINDINGS_PYTHON_DOMAIN {
-
+class DefaultingPyLocation;
+class DefaultingPyMlirContext;
 class PyBlock;
+class PyBlockArgument;
 class PyDiagnostic;
 class PyDiagnosticHandler;
 class PyInsertionPoint;
 class PyLocation;
-class DefaultingPyLocation;
 class PyMlirContext;
-class DefaultingPyMlirContext;
 class PyModule;
+class PyOpResult;
 class PyOperation;
 class PyOperationBase;
-class PyType;
 class PySymbolTable;
+class PyType;
 class PyValue;
 
 /// Wrapper for the global LLVM debugging flag.
@@ -1188,7 +1189,9 @@ class MLIR_PYTHON_API_EXPORTED PyValue {
   /// Gets a capsule wrapping the void* within the MlirValue.
   nanobind::object getCapsule();
 
-  nanobind::typed<nanobind::object, PyValue> maybeDownCast();
+  nanobind::typed<nanobind::object,
+                  std::variant<PyBlockArgument, PyOpResult, PyValue>>
+  maybeDownCast();
 
   /// Creates a PyValue from the MlirValue wrapped by a capsule. Ownership of
   /// the underlying MlirValue is still tied to the owning operation.
diff --git a/mlir/include/mlir/Bindings/Python/Nanobind.h b/mlir/include/mlir/Bindings/Python/Nanobind.h
index 8dc8a0d063d70..f7cf8fd38981d 100644
--- a/mlir/include/mlir/Bindings/Python/Nanobind.h
+++ b/mlir/include/mlir/Bindings/Python/Nanobind.h
@@ -29,6 +29,7 @@
 #include <nanobind/stl/string.h>
 #include <nanobind/stl/string_view.h>
 #include <nanobind/stl/tuple.h>
+#include <nanobind/stl/variant.h>
 #include <nanobind/stl/vector.h>
 #include <nanobind/typing.h>
 #if defined(__clang__) || defined(__GNUC__)
diff --git a/mlir/include/mlir/Bindings/Python/NanobindUtils.h b/mlir/include/mlir/Bindings/Python/NanobindUtils.h
index aea195fecae82..215daf245b902 100644
--- a/mlir/include/mlir/Bindings/Python/NanobindUtils.h
+++ b/mlir/include/mlir/Bindings/Python/NanobindUtils.h
@@ -277,7 +277,7 @@ class Sliceable {
   /// Returns the element at the given slice index. Supports negative indices
   /// by taking elements in inverse order. Returns a nullptr object if out
   /// of bounds.
-  nanobind::object getItem(intptr_t index) {
+  nanobind::typed<nanobind::object, ElementTy> getItem(intptr_t index) {
     // Negative indices mean we count from the end.
     index = wrapIndex(index);
     if (index < 0) {
diff --git a/mlir/lib/Bindings/Python/IRAffine.cpp b/mlir/lib/Bindings/Python/IRAffine.cpp
index 3c2da03181e6a..2e760e6e6f830 100644
--- a/mlir/lib/Bindings/Python/IRAffine.cpp
+++ b/mlir/lib/Bindings/Python/IRAffine.cpp
@@ -614,7 +614,7 @@ void populateIRAffine(nb::module_ &m) {
              return PyAffineExpr(self.getContext(),
                                  mlirAffineExprCompose(self, other));
            })
-      .def("maybe_downcast", &PyAffineExpr::maybeDownCast)
+      .def(MLIR_PYTHON_MAYBE_DOWNCAST_ATTR, &PyAffineExpr::maybeDownCast)
       .def(
           "shift_dims",
           [](PyAffineExpr &self, uint32_t numDims, uint32_t shift,
diff --git a/mlir/lib/Bindings/Python/IRCore.cpp b/mlir/lib/Bindings/Python/IRCore.cpp
index 19db41fae4fe2..646490bba366b 100644
--- a/mlir/lib/Bindings/Python/IRCore.cpp
+++ b/mlir/lib/Bindings/Python/IRCore.cpp
@@ -10,6 +10,7 @@
 #include "mlir/Bindings/Python/Globals.h"
 #include "mlir/Bindings/Python/IRCore.h"
 #include "mlir/Bindings/Python/NanobindUtils.h"
+#include "mlir/Bindings/Python/NanobindAdaptors.h"
 #include "mlir-c/Bindings/Python/Interop.h" // This is expected after nanobind.
 // clang-format on
 #include "mlir-c/BuiltinAttributes.h"
@@ -17,10 +18,6 @@
 #include "mlir-c/Diagnostics.h"
 #include "mlir-c/IR.h"
 #include "mlir-c/Support.h"
-#include "mlir/Bindings/Python/Nanobind.h"
-#include "mlir/Bindings/Python/NanobindAdaptors.h"
-#include "nanobind/nanobind.h"
-#include "nanobind/typing.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallVector.h"
 
@@ -2003,7 +2000,22 @@ nb::object PyValue::getCapsule() {
   return nb::steal<nb::object>(mlirPythonValueToCapsule(get()));
 }
 
-nb::typed<nb::object, PyValue> PyValue::maybeDownCast() {
+static PyOperationRef getValueOwnerRef(MlirValue value) {
+  MlirOperation owner;
+  if (mlirValueIsAOpResult(value))
+    owner = mlirOpResultGetOwner(value);
+  else if (mlirValueIsABlockArgument(value))
+    owner = mlirBlockGetParentOperation(mlirBlockArgumentGetOwner(value));
+  else
+    assert(false && "Value must be an block arg or op result.");
+  if (mlirOperationIsNull(owner))
+    throw nb::python_error();
+  MlirContext ctx = mlirOperationGetContext(owner);
+  return PyOperation::forOperation(PyMlirContext::forContext(ctx), owner);
+}
+
+nb::typed<nb::object, std::variant<PyBlockArgument, PyOpResult, PyValue>>
+PyValue::maybeDownCast() {
   MlirType type = mlirValueGetType(get());
   MlirTypeID mlirTypeID = mlirTypeGetTypeID(type);
   assert(!mlirTypeIDIsNull(mlirTypeID) &&
@@ -2012,26 +2024,23 @@ nb::typed<nb::object, PyValue> PyValue::maybeDownCast() {
       PyGlobals::get().lookupValueCaster(mlirTypeID, mlirTypeGetDialect(type));
   // nb::rv_policy::move means use std::move to move the return value
   // contents into a new instance that will be owned by Python.
-  nb::object thisObj = nb::cast(this, nb::rv_policy::move);
-  if (!valueCaster)
-    return thisObj;
-  return valueCaster.value()(thisObj);
+  nb::object thisObj;
+  if (mlirValueIsAOpResult(value))
+    thisObj = nb::cast<PyOpResult>(*this, nb::rv_policy::move);
+  else if (mlirValueIsABlockArgument(value))
+    thisObj = nb::cast<PyBlockArgument>(*this, nb::rv_policy::move);
+  else
+    assert(false && "Value must be an block arg or op result.");
+  if (valueCaster)
+    return valueCaster.value()(thisObj);
+  return thisObj;
 }
 
 PyValue PyValue::createFromCapsule(nb::object capsule) {
   MlirValue value = mlirPythonCapsuleToValue(capsule.ptr());
   if (mlirValueIsNull(value))
     throw nb::python_error();
-  MlirOperation owner;
-  if (mlirValueIsAOpResult(value))
-    owner = mlirOpResultGetOwner(value);
-  if (mlirValueIsABlockArgument(value))
-    owner = mlirBlockGetParentOperation(mlirBlockArgumentGetOwner(value));
-  if (mlirOperationIsNull(owner))
-    throw nb::python_error();
-  MlirContext ctx = mlirOperationGetContext(owner);
-  PyOperationRef ownerRef =
-      PyOperation::forOperation(PyMlirContext::forContext(ctx), owner);
+  PyOperationRef ownerRef = getValueOwnerRef(value);
   return PyValue(ownerRef, value);
 }
 
@@ -2279,15 +2288,7 @@ intptr_t PyOpOperandList::getRawNumElements() {
 
 PyValue PyOpOperandList::getRawElement(intptr_t pos) {
   MlirValue operand = mlirOperationGetOperand(operation->get(), pos);
-  MlirOperation owner;
-  if (mlirValueIsAOpResult(operand))
-    owner = mlirOpResultGetOwner(operand);
-  else if (mlirValueIsABlockArgument(operand))
-    owner = mlirBlockGetParentOperation(mlirBlockArgumentGetOwner(operand));
-  else
-    assert(false && "Value must be an block arg or op result.");
-  PyOperationRef pyOwner =
-      PyOperation::forOperation(operation->getContext(), owner);
+  PyOperationRef pyOwner = getValueOwnerRef(operand);
   return PyValue(pyOwner, operand);
 }
 
@@ -4679,9 +4680,7 @@ void populateIRCore(nb::module_ &m) {
           "with_"_a, "exceptions"_a, kValueReplaceAllUsesExceptDocstring)
       .def(
           MLIR_PYTHON_MAYBE_DOWNCAST_ATTR,
-          [](PyValue &self) -> nb::typed<nb::object, PyValue> {
-            return self.maybeDownCast();
-          },
+          [](PyValue &self) { return self.maybeDownCast(); },
           "Downcasts the `Value` to a more specific kind if possible.")
       .def_prop_ro(
           "location",
diff --git a/mlir/test/python/dialects/python_test.py b/mlir/test/python/dialects/python_test.py
index 9c0966d2d8798..8814f4d66fdf9 100644
--- a/mlir/test/python/dialects/python_test.py
+++ b/mlir/test/python/dialects/python_test.py
@@ -862,7 +862,7 @@ def values(lst):
                 ]
                 is Value
             )
-            assert type(variadic_operands.non_variadic) is Value
+            assert type(variadic_operands.non_variadic) is OpResult
 
             # CHECK: ['Value(%{{.*}} = arith.constant 0 : i32)', 'Value(%{{.*}} = arith.constant 1 : i32)']
             print(values(variadic_operands.variadic1))
diff --git a/mlir/test/python/ir/value.py b/mlir/test/python/ir/value.py
index 45efb880bab44..4cd1c2fa96c27 100644
--- a/mlir/test/python/ir/value.py
+++ b/mlir/test/python/ir/value.py
@@ -347,13 +347,6 @@ def __init__(self, v):
         def __str__(self):
             return super().__str__().replace(Value.__name__, NOPResult.__name__)
 
-    class NOPValue(Value):
-        def __init__(self, v):
-            super().__init__(v)
-
-        def __str__(self):
-            return super().__str__().replace(Value.__name__, NOPValue.__name__)
-
     class NOPBlockArg(BlockArgument):
         def __init__(self, v):
             super().__init__(v)
@@ -362,14 +355,12 @@ def __str__(self):
             return super().__str__().replace(Value.__name__, NOPBlockArg.__name__)
 
     @register_value_caster(IntegerType.static_typeid)
-    def cast_int(v) -> Value:
+    def cast_int(v) -> NOPResult | NOPBlockArg:
         print("in caster", v.__class__.__name__)
         if isinstance(v, OpResult):
             return NOPResult(v)
         if isinstance(v, BlockArgument):
             return NOPBlockArg(v)
-        elif isinstance(v, Value):
-            return NOPValue(v)
 
     ctx = Context()
     ctx.allow_unregistered_dialects = True
@@ -400,12 +391,15 @@ def cast_int(v) -> Value:
             # CHECK: "custom.op2"(%0#0, %0#1) : (i32, i32) -> ()
             print(op1)
 
-            # CHECK: in caster Value
-            # CHECK: operand 0 NOPValue(%0:2 = "custom.op1"() : () -> (i32, i32))
+            # CHECK: in caster OpResult
+            # CHECK: operand 0 NOPResult(%0:2 = "custom.op1"() : () -> (i32, i32))
             print("operand 0", op1.operands[0])
-            # CHECK: in caster Value
-            # CHECK: operand 1 NOPValue(%0:2 = "custom.op1"() : () -> (i32, i32))
+            assert isinstance(op1.operands[0], Value)
+            assert isinstance(op1.operands[0], OpResult)
+            # CHECK: in caster OpResult
+            # CHECK: operand 1 NOPResult(%0:2 = "custom.op1"() : () -> (i32, i32))
             print("operand 1", op1.operands[1])
+            assert isinstance(op1.operands[1], OpResult)
 
             # CHECK: in caster BlockArgument
             # CHECK: in caster BlockArgument

>From 8424f9c96dd5d88cae92f4a8901ad0932bac03eb Mon Sep 17 00:00:00 2001
From: makslevental <maksim.levental at gmail.com>
Date: Fri, 9 Jan 2026 20:28:47 -0800
Subject: [PATCH 2/6] fix __Str__

---
 mlir/include/mlir/Bindings/Python/IRCore.h |  8 ++++++
 mlir/test/python/dialects/python_test.py   |  6 ++---
 mlir/test/python/dialects/rocdl.py         |  2 +-
 mlir/test/python/dialects/ub.py            |  2 +-
 mlir/test/python/ir/value.py               | 31 +++++++++++-----------
 5 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/mlir/include/mlir/Bindings/Python/IRCore.h b/mlir/include/mlir/Bindings/Python/IRCore.h
index bfc35ca5b9d50..b0a097f4beda8 100644
--- a/mlir/include/mlir/Bindings/Python/IRCore.h
+++ b/mlir/include/mlir/Bindings/Python/IRCore.h
@@ -1570,6 +1570,14 @@ class MLIR_PYTHON_API_EXPORTED PyConcreteValue : public PyValue {
         [](DerivedTy &self) -> nanobind::typed<nanobind::object, DerivedTy> {
           return self.maybeDownCast();
         });
+    cls.def("__str__", [](PyValue &self) {
+      PyPrintAccumulator printAccum;
+      printAccum.parts.append(std::string(DerivedTy::pyClassName) + "(");
+      mlirValuePrint(self.get(), printAccum.getCallback(),
+                     printAccum.getUserData());
+      printAccum.parts.append(")");
+      return printAccum.join();
+    });
 
     if (DerivedTy::getTypeIdFunction) {
       PyGlobals::get().registerValueCaster(
diff --git a/mlir/test/python/dialects/python_test.py b/mlir/test/python/dialects/python_test.py
index 8814f4d66fdf9..d4b394b8c7e06 100644
--- a/mlir/test/python/dialects/python_test.py
+++ b/mlir/test/python/dialects/python_test.py
@@ -854,7 +854,7 @@ def values(lst):
             variadic_operands = test.SameVariadicOperandSizeOp(
                 [zero, one], two, [three, four]
             )
-            # CHECK: Value(%{{.*}} = arith.constant 2 : i32)
+            # CHECK: OpResult(%{{.*}} = arith.constant 2 : i32)
             print(variadic_operands.non_variadic)
             assert (
                 typing.get_type_hints(test.SameVariadicOperandSizeOp.non_variadic.fget)[
@@ -864,7 +864,7 @@ def values(lst):
             )
             assert type(variadic_operands.non_variadic) is OpResult
 
-            # CHECK: ['Value(%{{.*}} = arith.constant 0 : i32)', 'Value(%{{.*}} = arith.constant 1 : i32)']
+            # CHECK: ['OpResult(%{{.*}} = arith.constant 0 : i32)', 'OpResult(%{{.*}} = arith.constant 1 : i32)']
             print(values(variadic_operands.variadic1))
             assert (
                 typing.get_type_hints(test.SameVariadicOperandSizeOp.variadic1.fget)[
@@ -874,7 +874,7 @@ def values(lst):
             )
             assert type(variadic_operands.variadic1) is OpOperandList
 
-            # CHECK: ['Value(%{{.*}} = arith.constant 3 : i32)', 'Value(%{{.*}} = arith.constant 4 : i32)']
+            # CHECK: ['OpResult(%{{.*}} = arith.constant 3 : i32)', 'OpResult(%{{.*}} = arith.constant 4 : i32)']
             print(values(variadic_operands.variadic2))
             assert type(variadic_operands.variadic2) is OpOperandList
 
diff --git a/mlir/test/python/dialects/rocdl.py b/mlir/test/python/dialects/rocdl.py
index c73a536e03820..ed83504f3ac5c 100644
--- a/mlir/test/python/dialects/rocdl.py
+++ b/mlir/test/python/dialects/rocdl.py
@@ -34,7 +34,7 @@ def testSmoke():
     # CHECK: %{{.*}} = "rocdl.wmma.f16.16x16x16.f16"
     print(c_frag)
     assert isinstance(c_frag, OpView)
-    # CHECK: Value(%{{.*}} = "rocdl.wmma.f16.16x16x16.f16"
+    # CHECK: OpResult(%{{.*}} = "rocdl.wmma.f16.16x16x16.f16"
     c_frag = rocdl.wmma_f16_16x16x16_f16_(v16f32, a_frag, b_frag, c_frag, opsel=False)
     print(c_frag)
     assert isinstance(c_frag, Value)
diff --git a/mlir/test/python/dialects/ub.py b/mlir/test/python/dialects/ub.py
index 0d88da82c5e7b..cbbd49d2ec6cc 100644
--- a/mlir/test/python/dialects/ub.py
+++ b/mlir/test/python/dialects/ub.py
@@ -20,7 +20,7 @@ def constructAndPrintInModule(f):
 # CHECK-LABEL: testSmoke
 @constructAndPrintInModule
 def testSmoke():
-    # CHECK: Value(%{{.*}} = ub.poison : f32
+    # CHECK: OpResult(%{{.*}} = ub.poison : f32
     f32 = F32Type.get()
     poison = ub.poison(f32)
     print(poison)
diff --git a/mlir/test/python/ir/value.py b/mlir/test/python/ir/value.py
index 4cd1c2fa96c27..18fd1ba0ac775 100644
--- a/mlir/test/python/ir/value.py
+++ b/mlir/test/python/ir/value.py
@@ -229,11 +229,11 @@ def testValuePrintAsOperand():
         module = Module.create()
         with InsertionPoint(module.body):
             value = Operation.create("custom.op1", results=[i32]).results[0]
-            # CHECK: Value(%[[VAL1:.*]] = "custom.op1"() : () -> i32)
+            # CHECK: OpResult(%[[VAL1:.*]] = "custom.op1"() : () -> i32)
             print(value)
 
             value2 = Operation.create("custom.op2", results=[i32]).results[0]
-            # CHECK: Value(%[[VAL2:.*]] = "custom.op2"() : () -> i32)
+            # CHECK: OpResult(%[[VAL2:.*]] = "custom.op2"() : () -> i32)
             print(value2)
 
             topFn = func.FuncOp("test", ([i32, i32], []))
@@ -241,10 +241,10 @@ def testValuePrintAsOperand():
 
             with InsertionPoint(entry_block):
                 value3 = Operation.create("custom.op3", results=[i32]).results[0]
-                # CHECK: Value(%[[VAL3:.*]] = "custom.op3"() : () -> i32)
+                # CHECK: OpResult(%[[VAL3:.*]] = "custom.op3"() : () -> i32)
                 print(value3)
                 value4 = Operation.create("custom.op4", results=[i32]).results[0]
-                # CHECK: Value(%[[VAL4:.*]] = "custom.op4"() : () -> i32)
+                # CHECK: OpResult(%[[VAL4:.*]] = "custom.op4"() : () -> i32)
                 print(value4)
                 func.ReturnOp([])
 
@@ -306,7 +306,7 @@ def testValuePrintAsOperandNamedLocPrefix():
             named_value = Operation.create(
                 "custom.op5", results=[i32], loc=Location.name("apple")
             ).results[0]
-            # CHECK: Value(%[[VAL5:.*]] = "custom.op5"() : () -> i32)
+            # CHECK: OpResult(%[[VAL5:.*]] = "custom.op5"() : () -> i32)
             print(named_value)
 
         # CHECK: With use_name_loc_as_prefix
@@ -326,11 +326,11 @@ def testValueSetType():
         module = Module.create()
         with InsertionPoint(module.body):
             value = Operation.create("custom.op1", results=[i32]).results[0]
-            # CHECK: Value(%[[VAL1:.*]] = "custom.op1"() : () -> i32)
+            # CHECK: OpResult(%[[VAL1:.*]] = "custom.op1"() : () -> i32)
             print(value)
 
             value.set_type(i64)
-            # CHECK: Value(%[[VAL1]] = "custom.op1"() : () -> i64)
+            # CHECK: OpResult(%[[VAL1]] = "custom.op1"() : () -> i64)
             print(value)
 
             # CHECK: %[[VAL1]] = "custom.op1"() : () -> i64
@@ -345,14 +345,16 @@ def __init__(self, v):
             super().__init__(v)
 
         def __str__(self):
-            return super().__str__().replace(Value.__name__, NOPResult.__name__)
+            return super().__str__().replace(OpResult.__name__, NOPResult.__name__)
 
     class NOPBlockArg(BlockArgument):
         def __init__(self, v):
             super().__init__(v)
 
         def __str__(self):
-            return super().__str__().replace(Value.__name__, NOPBlockArg.__name__)
+            return (
+                super().__str__().replace(BlockArgument.__name__, NOPBlockArg.__name__)
+            )
 
     @register_value_caster(IntegerType.static_typeid)
     def cast_int(v) -> NOPResult | NOPBlockArg:
@@ -420,8 +422,7 @@ def reduction(arg0, arg1):
     try:
 
         @register_value_caster(IntegerType.static_typeid)
-        def dont_cast_int_shouldnt_register(v):
-            ...
+        def dont_cast_int_shouldnt_register(v): ...
 
     except RuntimeError as e:
         # CHECK: Value caster is already registered: {{.*}}cast_int
@@ -437,12 +438,12 @@ def dont_cast_int(v) -> OpResult:
         i32 = IntegerType.get_signless(32)
         module = Module.create()
         with InsertionPoint(module.body):
-            # CHECK: don't cast 0 Value(%0 = "custom.op1"() : () -> i32)
+            # CHECK: don't cast 0 OpResult(%0 = "custom.op1"() : () -> i32)
             new_value = Operation.create("custom.op1", results=[i32]).result
-            # CHECK: result 0 Value(%0 = "custom.op1"() : () -> i32)
+            # CHECK: result 0 OpResult(%0 = "custom.op1"() : () -> i32)
             print("result", new_value.result_number, new_value)
 
-            # CHECK: don't cast 0 Value(%1 = "custom.op2"() : () -> i32)
+            # CHECK: don't cast 0 OpResult(%1 = "custom.op2"() : () -> i32)
             new_value = Operation.create("custom.op2", results=[i32]).results[0]
-            # CHECK: result 0 Value(%1 = "custom.op2"() : () -> i32)
+            # CHECK: result 0 OpResult(%1 = "custom.op2"() : () -> i32)
             print("result", new_value.result_number, new_value)

>From a72da3c3f6e6d0fa1ac272e3a5567ecda6a3909c Mon Sep 17 00:00:00 2001
From: makslevental <maksim.levental at gmail.com>
Date: Fri, 9 Jan 2026 20:30:55 -0800
Subject: [PATCH 3/6] format

---
 mlir/test/python/ir/value.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mlir/test/python/ir/value.py b/mlir/test/python/ir/value.py
index 18fd1ba0ac775..380aee724ccbc 100644
--- a/mlir/test/python/ir/value.py
+++ b/mlir/test/python/ir/value.py
@@ -422,7 +422,8 @@ def reduction(arg0, arg1):
     try:
 
         @register_value_caster(IntegerType.static_typeid)
-        def dont_cast_int_shouldnt_register(v): ...
+        def dont_cast_int_shouldnt_register(v):
+            ...
 
     except RuntimeError as e:
         # CHECK: Value caster is already registered: {{.*}}cast_int

>From f882d34d31b1ecbd644d02818c3ee05123c6dd9d Mon Sep 17 00:00:00 2001
From: makslevental <maksim.levental at gmail.com>
Date: Fri, 9 Jan 2026 20:36:52 -0800
Subject: [PATCH 4/6] add test

---
 mlir/test/python/ir/value.py | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/mlir/test/python/ir/value.py b/mlir/test/python/ir/value.py
index 380aee724ccbc..6df32fedfb954 100644
--- a/mlir/test/python/ir/value.py
+++ b/mlir/test/python/ir/value.py
@@ -340,6 +340,24 @@ def testValueSetType():
 # CHECK-LABEL: TEST: testValueCasters
 @run
 def testValueCasters():
+    # check before registering casters
+    ctx = Context()
+    ctx.allow_unregistered_dialects = True
+    with Location.unknown(ctx):
+        i32 = IntegerType.get_signless(32)
+        module = Module.create()
+        with InsertionPoint(module.body):
+            value = Operation.create("custom.op0", results=[i32]).result
+            # CHECK: value OpResult(%0 = "custom.op0"() : () -> i32)
+            print("value", value)
+
+            @func.FuncOp.from_py_func(i32, i32)
+            def reduction(arg0, arg1):
+                # CHECK: arg0 BlockArgument(<block argument> of type 'i32' at index: 0)
+                print("arg0", arg0)
+                # CHECK: arg1 BlockArgument(<block argument> of type 'i32' at index: 1)
+                print("arg1", arg1)
+
     class NOPResult(OpResult):
         def __init__(self, v):
             super().__init__(v)
@@ -407,6 +425,7 @@ def cast_int(v) -> NOPResult | NOPBlockArg:
             # CHECK: in caster BlockArgument
             @func.FuncOp.from_py_func(i32, i32)
             def reduction(arg0, arg1):
+                print("arg0", arg0)
                 # CHECK: as func arg 0 NOPBlockArg
                 print("as func arg", arg0.arg_number, arg0.__class__.__name__)
                 # CHECK: as func arg 1 NOPBlockArg

>From 2da195c0187035e314007be223391a41b65394d8 Mon Sep 17 00:00:00 2001
From: makslevental <maksim.levental at gmail.com>
Date: Fri, 9 Jan 2026 20:42:37 -0800
Subject: [PATCH 5/6] restore predecls

---
 mlir/include/mlir/Bindings/Python/IRCore.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mlir/include/mlir/Bindings/Python/IRCore.h b/mlir/include/mlir/Bindings/Python/IRCore.h
index b0a097f4beda8..533a5bf4e7d0b 100644
--- a/mlir/include/mlir/Bindings/Python/IRCore.h
+++ b/mlir/include/mlir/Bindings/Python/IRCore.h
@@ -36,21 +36,20 @@
 namespace mlir {
 namespace python {
 namespace MLIR_BINDINGS_PYTHON_DOMAIN {
-class DefaultingPyLocation;
-class DefaultingPyMlirContext;
+
 class PyBlock;
-class PyBlockArgument;
 class PyDiagnostic;
 class PyDiagnosticHandler;
 class PyInsertionPoint;
 class PyLocation;
+class DefaultingPyLocation;
 class PyMlirContext;
+class DefaultingPyMlirContext;
 class PyModule;
-class PyOpResult;
 class PyOperation;
 class PyOperationBase;
-class PySymbolTable;
 class PyType;
+class PySymbolTable;
 class PyValue;
 
 /// Wrapper for the global LLVM debugging flag.
@@ -1171,6 +1170,8 @@ class MLIR_PYTHON_API_EXPORTED PyStringAttribute
 /// value. For block argument values, this is the operation that contains the
 /// block to which the value is an argument (blocks cannot be detached in Python
 /// bindings so such operation always exists).
+class PyBlockArgument;
+class PyOpResult;
 class MLIR_PYTHON_API_EXPORTED PyValue {
 public:
   // The virtual here is "load bearing" in that it enables RTTI

>From a70559847da7dc5c3deec6795ae6854c149a5520 Mon Sep 17 00:00:00 2001
From: makslevental <maksim.levental at gmail.com>
Date: Mon, 12 Jan 2026 09:04:42 -0800
Subject: [PATCH 6/6] address comment

---
 mlir/test/python/ir/value.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mlir/test/python/ir/value.py b/mlir/test/python/ir/value.py
index 6df32fedfb954..7761c2eb4a424 100644
--- a/mlir/test/python/ir/value.py
+++ b/mlir/test/python/ir/value.py
@@ -381,6 +381,7 @@ def cast_int(v) -> NOPResult | NOPBlockArg:
             return NOPResult(v)
         if isinstance(v, BlockArgument):
             return NOPBlockArg(v)
+        raise ValueError(f"expected OpResult or BlockArgument; got {v=}")
 
     ctx = Context()
     ctx.allow_unregistered_dialects = True



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