kubens: implement namespace switching

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

internal/kubeconfig/namespace_test.go

@@ -3,6 +3,8 @@ package kubeconfig
 import (
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
+
 	"github.com/ahmetb/kubectx/internal/testutil"
 )
 
@@ -44,3 +46,35 @@ func TestKubeconfig_NamespaceOfContext(t *testing.T) {
 		t.Fatalf("c2: expected=%q got=%q", expected, v2)
 	}
 }
+
+func TestKubeconfig_SetNamespace(t *testing.T) {
+	l := WithMockKubeconfigLoader(testutil.KC().
+		WithCtxs(
+			testutil.Ctx("c1"),
+			testutil.Ctx("c2").Ns("c2n1")).ToYAML(t))
+	kc := new(Kubeconfig).WithLoader(l)
+	if err := kc.Parse(); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := kc.SetNamespace("c3", "foo"); err == nil {
+		t.Fatalf("expected error for non-existing ctx")
+	}
+
+	if err := kc.SetNamespace("c1", "c1n1"); err != nil {
+		t.Fatal(err)
+	}
+	if err := kc.SetNamespace("c2", "c2n2"); err != nil {
+		t.Fatal(err)
+	}
+	if err := kc.Save(); err != nil {
+		t.Fatal(err)
+	}
+
+	expected := testutil.KC().WithCtxs(
+		testutil.Ctx("c1").Ns("c1n1"),
+		testutil.Ctx("c2").Ns("c2n2")).ToYAML(t)
+	if diff := cmp.Diff(l.Output(), expected); diff != "" {
+		t.Fatal(diff)
+	}
+}