Skip to main content

Reading and writing ROS 2

To start writing Python code that reads ROS 2 data from MCAP, You will need a supported ROS2 distro installed, along with the rosbag2 and rosbag2_storage_mcap packages. To get set up, make sure you've followed the ROS2 installation guide, then:

$ sudo apt-get install ros-$ROS_DISTRO-rosbag2 ros-$ROS_DISTRO-rosbag2-storage-mcap


We’ll start with our imports:

import argparse
from rclpy.serialization import deserialize_message
from rosidl_runtime_py.utilities import get_message
from std_msgs.msg import String
import rosbag2_py

Next, we'll write a function for reading our ROS 2 messages from an MCAP file:

def read_messages(input_bag: str):
reader = rosbag2_py.SequentialReader()
rosbag2_py.StorageOptions(uri=input_bag, storage_id="mcap"),
input_serialization_format="cdr", output_serialization_format="cdr"

topic_types = reader.get_all_topics_and_types()

def typename(topic_name):
for topic_type in topic_types:
if == topic_name:
return topic_type.type
raise ValueError(f"topic {topic_name} not in bag")

while reader.has_next():
topic, data, timestamp = reader.read_next()
msg_type = get_message(typename(topic))
msg = deserialize_message(data, msg_type)
yield topic, msg, timestamp
del reader

Finally, we'll print out each message with its topic, data type, timestamp, and contents:

def main():
parser = argparse.ArgumentParser(description=__doc__)
"input", help="input bag path (folder or filepath) to read from"

args = parser.parse_args()
for topic, msg, timestamp in read_messages(args.input):
if isinstance(msg, String):
print(f"{topic} [{timestamp}]: '{}'")
print(f"{topic} [{timestamp}]: ({type(msg).__name__})")

if __name__ == "__main__":


Writing MCAP using the rosbag2_py API is simple, starting with some imports:

from rclpy.serialization import serialize_message
from std_msgs.msg import String
import rosbag2_py

Then we'll open a new bag using the mcap storage plugin:

writer = rosbag2_py.SequentialWriter()
rosbag2_py.StorageOptions(uri="output.mcap", storage_id="mcap"),
input_serialization_format="cdr", output_serialization_format="cdr"

Now we can create a topic and add some messages to it:

name="/chatter", type="std_msgs/msg/String", serialization_format="cdr"

start_time = 0
for i in range(10):
msg = String() = f"Chatter #{i}"
timestamp = start_time + (i * 100)
writer.write("/chatter", serialize_message(msg), timestamp)

Finally, we delete the writer to close the MCAP:

del writer