Add an example for optional field (#39)

example/client.py

@@ -8,6 +8,8 @@
 try:
     response = client.MakeHat(
     	ctx=Context(), request=haberdasher_pb2.Size(inches=12), server_path_prefix="/twirpy")
+    if not response.HasField('name'):
+        print("We didn't get a name!")
     print(response)
 except TwirpServerException as e:
     print(e.code, e.message, e.meta, e.to_dict())

example/haberdasher.proto

@@ -14,7 +14,7 @@ message Hat {
   string color = 2;
 
   // The name of a hat is it's type. Like, 'bowler', or something.
-  string name = 3;
+  optional string name = 3;
 }
 
 // Size is passed when requesting a new hat to be made. It's always

example/server.py

@@ -9,11 +9,14 @@ class HaberdasherService(object):
     def MakeHat(self, context, size):
         if size.inches <= 0:
             raise InvalidArgument(argument="inches", error="I can't make a hat that small!")
-        return haberdasher_pb2.Hat(
+        response = haberdasher_pb2.Hat(
             size=size.inches,
-            color= random.choice(["white", "black", "brown", "red", "blue"]),
-            name=random.choice(["bowler", "baseball cap", "top hat", "derby"])
+            color=random.choice(["white", "black", "brown", "red", "blue"]),
         )
+        if random.random() > 0.5:
+            response.name = random.choice(["bowler", "baseball cap", "top hat", "derby"])
+
+        return response
 
 
 service = haberdasher_twirp.HaberdasherServer(