[mlir][python] Add a __contains__ method to the python bindings for DictionaryAttr.

[akuegel]

Summary

This makes it easier to check in python whether a certain attribute is there.

[ftynse]

Thanks!

mlir/lib/Bindings/Python/IRAttributes.cpp

 class PyDictAttribute : public PyConcreteAttribute<PyDictAttribute> {
 public:
   static constexpr IsAFunctionTy isaFunction = mlirAttributeIsADictionary;
   static constexpr const char *pyClassName = "DictAttr";
   using PyConcreteAttribute::PyConcreteAttribute;
 
   intptr_t dunderLen() { return mlirDictionaryAttrGetNumElements(*this); }
 
+  bool dunderContains(const std::string &name) {
+    return !mlirAttributeIsNull(
+        mlirDictionaryAttrGetElementByName(*this, toMlirStringRef(name)));
+  }
+
   static void bindDerived(ClassTy &c) {
+    c.def("__contains__", &PyDictAttribute::dunderContains);
     c.def("__len__", &PyDictAttribute::dunderLen);
     c.def_static(
         "get",
         [](py::dict attributes, DefaultingPyMlirContext context) {
           SmallVector<MlirNamedAttribute> mlirNamedAttributes;
           mlirNamedAttributes.reserve(attributes.size());
           for (auto &it : attributes) {
             auto &mlir_attr = it.second.cast<PyAttribute &>();

mlir/test/python/ir/attributes.py

with Context():
     assert len(a) == 2
 
     # CHECK: 42 : i32
     print(a['integerattr'])
 
     # CHECK: "string"
     print(a['stringattr'])
 
+    # CHECK: True
+    print('stringattr' in a)
+
+    # CHECK: False
+    print('not_in_dict' in a)
+
     # Check that exceptions are raised as expected.
     try:
       _ = a['does_not_exist']
     except KeyError:
       pass
     else:
       assert False, "Exception not produced"