Fix MLIR Python binding for arith.constant after argument has been changed to an interface

e179532 removed the Type field from attributes and
arith::ConstantOp argument is now a TypedAttrInterface which isn't
supported by the python generator.
This patch temporarily restore the functionality for arith.constant but
won't generalize: we need to work on the generator instead.

Differential Revision: https://reviews.llvm.org/D130878

Author: joker-eph

mlir/python/mlir/dialects/_arith_ops_ext.py

@@ -60,6 +60,10 @@ def create_index(cls, value: int, *, loc=None, ip=None):
   def type(self):
     return self.results[0].type
 
+  @property
+  def value(self):
+    return Attribute(self.operation.attributes["value"])
+
   @property
   def literal_value(self) -> Union[int, float]:
     if _is_integer_like_type(self.type):

mlir/test/python/dialects/arith_dialect.py

@@ -0,0 +1,19 @@
+# RUN: %PYTHON %s | FileCheck %s
+
+from mlir.ir import *
+import mlir.dialects.func as func
+import mlir.dialects.arith as arith
+
+def run(f):
+  print("\nTEST:", f.__name__)
+  f()
+
+# CHECK-LABEL: TEST: testConstantOp
+@run
+def testConstantOps():
+  with Context() as ctx, Location.unknown():
+    module = Module.create()
+    with InsertionPoint(module.body):
+      arith.ConstantOp(value=42.42, result=F32Type.get())
+    # CHECK:         %cst = arith.constant 4.242000e+01 : f32
+    print(module)