title: How to create a list of messages in a message with protobuf in python
body: <p>I have a protobuf structure defined in the following way:</p>
  
  <pre><code>    syntax = "proto3";
  
      message Register {
  
          string name = 1;
          Access access = 2;
          uint64 deafult_value = 3;
          uint64 value = 4;
          uint64 offset = 5;
          int32 index = 6;
          string description = 7;
          int32 register_size = 8;
          repeated Field fields_list = 9;
      }
  
      message Field {
  
          string name = 1;
          string regName = 2;
          Access access = 3;
          int32 offset = 4;
          int32 length = 5;
          string description = 6;
          uint64 value = 7;
      }
  
      message Access {
          bool read = 1;
          bool write = 2;
      }
  </code></pre>
  
  <p>now I want to create a new Register with a list of fields in python and i've tried the following thing:</p>
  
  <pre><code>    proto_reg = DataStructs.Register()
      proto_field = DataStructs.Field()
      proto_reg.name ="test"
      proto_reg.fields_list.extend(proto_field)
  </code></pre>
  
  <p>but im getting an exception:</p>
  
  <pre><code>   TypeError: Value must be iterable
  </code></pre>
  
  <p>what am i doing wrong?</p>