mcap.writer module

class mcap.writer.CompressionType(value)

Bases: Enum

An enumeration.

LZ4 = 2

NONE = 1

ZSTD = 3

class mcap.writer.IndexType(value)

Bases: Flag

Determines what indexes should be written to the MCAP file. If in doubt, choose ALL.

ALL = 30

ATTACHMENT = 2

CHUNK = 4

MESSAGE = 8

METADATA = 16

NONE = 1

class mcap.writer.Writer(output: str | IO[Any] | BufferedWriter, chunk_size: int = 1048576, compression: CompressionType = CompressionType.ZSTD, index_types: IndexType = IndexType.ALL, repeat_channels: bool = True, repeat_schemas: bool = True, use_chunking: bool = True, use_statistics: bool = True, use_summary_offsets: bool = True, enable_crcs: bool = True, enable_data_crcs: bool = False)

Bases: object

Writes MCAP data.

Parameters:

• output – A filename or stream to write to.

• chunk_size – The maximum size of individual data chunks in a chunked file.

• compression – Compression to apply to chunk data, if any.

• index_types – Indexes to write to the file. See IndexType for possibilities.

• repeat_channels – Repeat channel information at the end of the file.

• repeat_schemas – Repeat schemas at the end of the file.

• use_chunking – Group data in chunks.

• use_statistics – Write statistics record.

• use_summary_offsets – Write summary offset records.

add_attachment(create_time: int, log_time: int, name: str, media_type: str, data: bytes)

Adds an attachment to the file.

Parameters:

• log_time – Time at which the attachment was recorded.

• create_time – Time at which the attachment was created. If not available, must be set to zero.

• name – Name of the attachment, e.g “scene1.jpg”.

• media_type – Media Type (e.g “text/plain”).

• data – Attachment data.

add_message(channel_id: int, log_time: int, data: bytes, publish_time: int, sequence: int = 0)

Adds a new message to the file. If chunking is enabled the message will be added to the current chunk.

Parameters:

• channel_id – The id of the channel to which the message should be added.

• sequence – Optional message counter assigned by publisher.

• log_time – Time at which the message was recorded.

• publish_time – Time at which the message was published. If not available, must be set to the log time.

• data – Message data, to be decoded according to the schema of the channel.

add_metadata(name: str, data: Dict[str, str])

Adds key-value metadata to the file.

Parameters:

• name – A name to associate with the metadata.

• data – Key-value metadata.

finish()

Writes any final indexes, summaries etc to the file. Note that it does not close the underlying output stream.

register_channel(topic: str, message_encoding: str, schema_id: int, metadata: Dict[str, str] = {}) → int

Registers a new message channel. Returns the numeric id of the new channel.

Parameters:

• schema_id – The schema for messages on this channel. A schema_id of 0 indicates there is no schema for this channel.

• topic – The channel topic.

• message_encoding – Encoding for messages on this channel. See the list of well-known message encodings for common values.

• metadata – Metadata about this channel.

register_schema(name: str, encoding: str, data: bytes)

Registers a new message schema. Returns the new integer schema id.

Parameters:

• name – An identifier for the schema.

• encoding – Format for the schema. See the list of well-known schema encodings for common values. An empty string indicates no schema is available.

• data – Schema data. Must conform to the schema encoding. If encoding is an empty string, data should be 0 length.

start(profile: str = '', library: str = 'python mcap 1.1.1')

Starts writing to the output stream.

Parameters:

• profile – The profile is used for indicating requirements for fields throughout the file (encoding, user_data, etc).

• library – Free-form string for writer to specify its name, version, or other information for use in debugging.