Proto nested messages and repeated fields in Python

Today I was having some problems populating a proto repeated message in Python with a nested message definition, and it took me a while to figure out how to do it.

In reality it is pretty simple. Let’s make an example.

syntax = "proto3";

package test;

message Trajectory2d {
    message Point2d {
      float x = 1;
      float y = 2;
    }
    repeated Point2d points = 1;
  }

Let’s save our test.proto and generate the Python code.

protoc --proto_path=. --python_out=. test.proto

Now if we want to create an element of Trajectory2d type and add points to it, we can just use the add() function. The function will create a new message object, append it to the list of repeated objects, and return it for the caller to fill. In addition it will forward keyword arguments to the class.

from test_pb2 import Trajectory2d

trajectory = Trajectory2d()
trajectory.points.add(x=10, y=30)
trajectory.points.add(x=14, y=22)

assert len(trajectory.points) == 2

assert trajectory.points[0].x == 10
assert trajectory.points[0].y == 30

assert trajectory.points[1].x == 14
assert trajectory.points[1].y == 22