Question about example ROS publisher code

Question

def __init__(self):
    super().__init__('minimal_publisher')
    self.publisher_ = self.create_publisher(String, 'topic', 10)
    timer_period = 0.5  # seconds
    self.timer = self.create_timer(timer_period, self.timer_callback)
    self.i = 0

def timer_callback(self):
    msg = String()
    msg.data = 'Hello World: %d' % self.i
    self.publisher_.publish(msg)
    self.get_logger().info('Publishing: "%s"' % msg.data)
    self.i += 1

There was a example publisher code on the ROS site and I tried to study it, but I was confused at the def __init__(self).

If you see the self.timer_callback in def __init__(self) part, the codes needs self.i but the self.i is bellow the self.timer, and it works without errors. I don't know how it works.

Could anyone help me understanding this.

Answer 1

The only possible explanation is that the self.create_timer doesn't call the callback self.timer_callback instantly. It might wait for the timer_period (or some other time) before it calls the callback the first time, that's why the line self.i = 0 executes before the self.timer_callback executes for the first time.