Python MCAP protobuf support

This package provides protobuf support for the Python MCAP file format reader.

Installation

Install via Pipenv by adding mcap-protobuf-support to your Pipfile or via the command line:

pipenv install mcap-protobuf-support

MCAP protobuf writing example

First, compile each of your message definitions:

protoc complex_message.proto --python_out . -o complex_message.fds
protoc simple_message.proto --python_out . -o simple_message.fds

Next, use those message definitions to register schema in the mcap file. Note that the names of the messages in the proto files must correspond to the schema names in the mcap file. For example, the message type SimpleMessage in the .proto file must be registered as an mcap schema with the name SimpleMessage.

from pathlib import Path
from typing import IO, Any

from mcap.mcap0.writer import Writer as McapWriter

from .complex_message_pb2 import ComplexMessage
from .simple_message_pb2 import SimpleMessage

output = open("example.mcap", "wb")
mcap_writer = McapWriter(output)
mcap_writer.start(profile="protobuf", library="test")

simple_schema_id = mcap_writer.register_schema(
    name="SimpleMessage",
    encoding="protobuf",
    data=(Path(__file__).parent / "simple_message.fds").read_bytes(),
)

complex_schema_id = mcap_writer.register_schema(
    name="ComplexMessage",
    encoding="protobuf",
    data=(Path(__file__).parent / "complex_message.fds").read_bytes(),
)

simple_channel_id = mcap_writer.register_channel(
    topic="/simple_message",
    message_encoding="protobuf",
    schema_id=simple_schema_id,
)

complex_channel_id = mcap_writer.register_channel(
    topic="/complex_message",
    message_encoding="protobuf",
    schema_id=complex_schema_id,
)

for i in range(1, 11):
    simple_message = SimpleMessage(data=f"Hello MCAP protobuf world #{i}!")
    mcap_writer.add_message(
        channel_id=simple_channel_id,
        log_time=i * 1000,
        data=simple_message.SerializeToString(),
        publish_time=i * 1000,
    )
    complex_message = ComplexMessage(fieldA=f"Field A {i}", fieldB="Field B {i}")
    mcap_writer.add_message(
        channel_id=complex_channel_id,
        log_time=i * 1000,
        data=complex_message.SerializeToString(),
        publish_time=i * 1000,
    )

mcap_writer.finish()
output.close()

MCAP protobuf decoding example

from mcap.mcap0.stream_reader import StreamReader
from mcap_protobuf.decoder import Decoder

reader = StreamReader("my_data.mcap")
decoder = Decoder(reader)
for m in decoder.messages:
    print(m)

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