how do I get the tf2 transform between the turtlebot3 base\_link and the map using python in a ros node?
To get the TF2 transform between the TurtleBot3's `base_link` and the `map` frame using Python in a ROS node, you can use the `tf2` library in ROS. Here's an example code snippet to get you started:
```python
import rospy
import tf2_ros

class TurtleBot3Transformer:
    def __init__(self):
        self.tf_buffer = tf2_ros.Buffer()
        self.tf_listener = tf2_ros.TransformListener(self.tf_buffer)

    def get_transform(self):
        try:
            transform = self.tf_buffer.lookup_transform('map', 'base_link', rospy.Time())
            return transform
        except (tf2_ros.LookupException, tf2_ros.ConnectivityException, tf2_ros.ExtrapolationException):
            rospy.logwarn("Failed to get transform from base_link to map")
            return None

if __name__ == '__main__':
    rospy.init_node('turtlebot3_transformer')
    transformer = TurtleBot3Transformer()
    rate = rospy.Rate(10.0)  # 10 Hz

    while not rospy.is_shutdown():
        transform = transformer.get_transform()
        if transform:
            rospy.loginfo("Transform from base_link to map: \n{}".format(transform))
        rate.sleep()
```
Here's what's happening in the code:

1. We create a `tf2_ros.Buffer` object to store the TF2 transforms.
2. We create a `tf2_ros.TransformListener
You can get the transform between the TurtleBot3 base\_link and the map using the ROS node by first publishing the transform from the base\_link to the map and then subscribing to the transform from the map to the base\_link. You can then calculate the transform between the two using the ROS transforms API.