AckCmdReader

class lsst.ts.salobj.topics.AckCmdReader(salinfo: SalInfo, queue_len: int = 100)

Bases: lsst.ts.salobj.topics.read_topic.ReadTopic

ackcmd (command acknowledgement) topic reader.

Parameters

salinfoSalInfo

SAL component information.

queue_lenint

Number of elements that can be queued for get_oldest.

Notes

The same ackcmd topic is used for all command topics from a given SAL component.

Attributes Summary

DataType	The type (class) for a message of this topic.
allow_multiple_callbacks	Can callbacks can run simultaneously?
callback	Callback function, or None if there is not one.
has_callback	Return True if there is a callback function.
has_data	Has any data ever been seen for this topic?
max_history
metadata	Get topic metadata as a TopicMetadata, if available,else None.
nqueued	Return the number of messages in the Python queue.
volatile	Does this topic have volatile durability?

Methods Summary

aget([timeout])	Get the most recent message, or wait for data if no data has ever been seen (has_data False).
basic_close()	A synchronous and possibly less thorough version of close.
close()	Shut down and release resources.
flush()	Flush the queue used by get_oldest and next.
get()	Get the most recent message, or None if no data has ever been seen (has_data False).
get_oldest()	Pop and return the oldest message from the queue, or None if the queue is empty.
next(*, flush[, timeout])	Pop and return the oldest message from the queue, waiting for data if the queue is empty.

Attributes Documentation

DataType

The type (class) for a message of this topic.

When you read or write a message for this topic you are reading or writing an instance of DataType.

Notes

The preferred way to write a message for a topic is:

• RemoteCommand.start to start a command.

• CommandEvent.write to write an event.

• CommandTelemetry.write to write a telemetry message.

allow_multiple_callbacks

Can callbacks can run simultaneously?

Notes

Ignored for synchronous callbacks because those block while running. In particular, if the callback is synchronous but launches one or more background jobs then the number of those jobs cannot be limited by this class.

callback

Callback function, or None if there is not one.

The callback function is called when a new message is received; it receives one argument: the message (an object of type topics.BaseTopic.DataType).

Raises

TypeError

When setting a new callback if the callback is not None and is not callable.

Notes

The callback function can be synchronous or asynchronous (e.g. defined with async def).

Setting a callback flushes the queue, and it will remain empty as long as there is a callback.

get_oldest and next are prohibited if there is a callback function. Technically they could both work, but get_oldest would always return None and next would miss messages if they arrived while waiting for something else. It seems safer to raise an exception.

has_callback

Return True if there is a callback function.

has_data

Has any data ever been seen for this topic?

Raises

RuntimeError

If the salinfo has not started reading.

max_history

metadata

Get topic metadata as a TopicMetadata, if available,else None.

nqueued

Return the number of messages in the Python queue.

volatile

Does this topic have volatile durability?

Methods Documentation

async aget(timeout: Optional[float] = None) → lsst.ts.salobj.type_hints.BaseMsgType

Get the most recent message, or wait for data if no data has ever been seen (has_data False).

This method does not change which message will be returned by any other method (except for the fact that new data will arrive while waiting).

Parameters

timeoutfloat, optional

Time limit, in seconds. If None then no time limit.

Returns

dataDataType

The current or next message.

Raises

RuntimeError

If a callback function is present, or if the salinfo has not started reading.

Notes

Do not modify the returned data. To make a copy that you can safely modify, use copy.copy(data).

basic_close() → None

A synchronous and possibly less thorough version of close.

Intended for exit handlers and constructor error handlers.

async close() → None

Shut down and release resources.

Intended to be called by SalInfo.close(), since that tracks all topics.

flush() → None

Flush the queue used by get_oldest and next.

This makes get_oldest return None and next wait, until a new message arrives. It does not change which message will be returned by aget or get.

Raises

RuntimeError

If a callback function is present.

get() → Optional[lsst.ts.salobj.type_hints.BaseMsgType]

Get the most recent message, or None if no data has ever been seen (has_data False).

This method does not change which message will be returned by aget, get_oldest, and next.

Returns

dataself.DataType or None

Return self.data if data has been read, else None.

Raises

RuntimeError

If the salinfo has not started reading.

get_oldest() → Optional[lsst.ts.salobj.type_hints.BaseMsgType]

Pop and return the oldest message from the queue, or None if the queue is empty.

This is a variant of next that does not wait for a new message. This method affects which message will be returned by next, but not which message will be returned by aget or get.

Returns

dataself.DataType or None

The oldest message found on the queue, if any, else None.

Raises

RuntimeError

If a callback function is present, or if the salinfo has not started reading.

Notes

Use with caution when mixing with next, since that also consumes data from the queue.

async next(*, flush: bool, timeout: Optional[float] = None) → lsst.ts.salobj.type_hints.BaseMsgType

Pop and return the oldest message from the queue, waiting for data if the queue is empty.

This method affects the data returned by get_oldest, but not the data returned by aget or get.

Parameters

flushbool

If True then flush the queue before starting a read. This guarantees that the method will wait for a new message. If False and there is data on the queue, then pop and return the oldest message from the queue, without waiting; if queue is empty then wait for a new message.

timeoutfloat, optional

Time limit, in seconds. If None then no time limit.

Returns

dataDataType

The message data.

Raises

RuntimeError

If a callback function is present, or if the salinfo has not started reading.

Notes

Do not modify the returned data. To make a copy that you can safely modify, use copy.copy(data).