```Python hl_lines="14-19"
{!../../../docs_src/events/tutorial003.py!}
```

The first part of the function, before the `yield`, will be executed **before** the application starts.

And the part after the `yield` will be executed **after** the application has finished.

### Async Context Manager

// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Use of public API is ok.

package main

import (
	"iter"
	"unique"
)

func seq(yield func(int) bool) {
	yield(123)
}

var s = "hello"

func main() {
	h := unique.Make(s)
	next, stop := iter.Pull(seq)
	defer stop()
	println(h.Value())
	println(next())
	println(next())

  OpBuilder builder(&island.GetBody(), island.GetBody().begin());
  tf_executor::YieldOp yield = island.GetYield();
  if (yield.getNumOperands() == 0) {
    builder.create<TF::NoOp>(island.getLoc(), TypeRange{}, ValueRange{});
  } else if (yield.getNumOperands() == 1) {
    Value operand = yield.getOperand(0);
    auto identity = builder.create<TF::IdentityOp>(island.getLoc(),

    %1 = "tf.WhileRegion"(%0) ({
       // Condition region
       // CHECK: ^bb
       // CHECK: "tf.Yield"
       ^bb0(%carg0: tensor<i32>):
          %c0 = "tf.Const"() {value = dense<0> : tensor<i32>} : () -> tensor<i32>
          %c1 = "tf.GreaterEqual"(%carg0, %0) {T = i32, device = ""} : (tensor<i32>, tensor<i32>) -> tensor<i1>
          "tf.Yield"(%c1) : (tensor<i1>) -> ()
      }, {
       // Body region
       // CHECK: ^bb0

// Linkname coroswitch is not allowed, even if iter.Pull
// is instantiated in the same package.

package main

import (
	"iter"
	"unsafe"
)

func seq(yield func(int) bool) {
	yield(123)
}

func main() {
	next, stop := iter.Pull(seq)
	next()
	stop()
	coroswitch(nil)
}

//go:linkname coroswitch runtime.coroswitch

            "tf.Yield"(%u0, %id0, %id0) : (!tf_res, !tf_res, !tf_res) -> ()
          }, {
            // expected-remark@below {{Result #0, ID 3 : 0, 1, 3, 4, 5}}
            %id0 = "tf.Identity"(%vh0) : (!tf_res) -> !tf_res
            "tf.Yield"(%id0, %id0, %arg0) : (!tf_res, !tf_res, !tf_res) -> ()
          }, {
            "tf.Yield"(%vh0, %arg1, %arg1) : (!tf_res, !tf_res, !tf_res) -> ()

      %control = tf_executor.island {
        "tf.StatefulPartitionedCall"(%arg0) <{config = "", config_proto = "", executor_type = "", f = @partitioned}> : (tensor<*x!tf_type.resource>) -> ()
        tf_executor.yield
      }
      tf_executor.fetch %control : !tf_executor.control
    }
    return
  }

}

mlir::LogicalResult IslandOp::verify() {
  IslandOp island = *this;
  if (!island.GetBody().args_empty())
    return island.emitOpError() << "expects body without any arguments";

  Operation &yield = island.GetBody().back();
  if (!isa<YieldOp>(yield))
    return yield.emitOpError()
           << "invalid tf_executor.island terminator, yield expected";

	informerFactory.Start(ctx.Done())
	if !cache.WaitForCacheSync(ctx.Done(), ctbInformer.Informer().HasSynced) {
		t.Fatalf("Timed out waiting for informer to sync")
	}

	t.Run("foo should yield the first certificate", func(t *testing.T) {
		gotBundle, err := ctbManager.GetTrustAnchorsByName("foo", false)
		if err != nil {
			t.Fatalf("Got error while calling GetTrustAnchorsBySigner: %v", err)
		}

       // CHECK: "tf.Yield"(%[[CONST]])
       %constant = "tf.Const"() {value = dense<0.0> : tensor<4xf32>} : () -> tensor<4xf32>
       "tf.AssignVariableOp"(%0, %constant) : (tensor<*x!tf_type.resource<tensor<4xf32>>>, tensor<4xf32>) -> ()
       "tf.Yield"() : () -> ()
      }, {
       // CHECK: "tf.Yield"(%[[READ]])
       "tf.Yield"() : () -> ()