Execution and Callbacks

There are two components that control the execution of callbacks: executors and callback groups.

Executors are responsible for the actual execution of callbacks and should extend the Executor class.

Callback groups are used to enforce concurrency rules for callbacks and should extend the CallbackGroup class.

Executors

exception rclpy.executors.ConditionReachedException: Future has been completed.

class rclpy.executors.Executor(*, context: Context | None = None)

The base class for an executor.

An executor controls the threading model used to process callbacks. Callbacks are units of work like subscription callbacks, timer callbacks, service calls, and received client responses. An executor controls which threads callbacks get executed in.

A custom executor must define spin_once(). If the executor has any cleanup then it should also define shutdown().

Parameters:: context – The context to be associated with, or None for the default global context.

add_node(node: Node) → bool

Add a node whose callbacks should be managed by this executor.

Parameters:: node – The node to add to the executor.
Returns:: True if the node was added, False otherwise.

can_execute(entity: WaitableEntityType) → bool

Determine if a callback for an entity can be executed.

Parameters:: entity – Subscription, Timer, Guard condition, etc
Returns:: True if the entity callback can be executed, False otherwise.

property context: Context: Get the context associated with the executor.

create_task(callback: Callable | Coroutine, *args, **kwargs) → Task

Add a callback or coroutine to be executed during spin() and return a Future.

Arguments to this function are passed to the callback.

Parameters:: callback – A callback to be run in the executor.

get_nodes() → List[Node]: Return nodes that have been added to this executor.

remove_node(node: Node) → None

Stop managing this node’s callbacks.

Parameters:: node – The node to remove from the executor.

shutdown(timeout_sec: float | None = None) → bool

Stop executing callbacks and wait for their completion.

Parameters:: timeout_sec – Seconds to wait. Block forever if None or negative. Don’t wait if 0.
Returns:: True if all outstanding callbacks finished executing, or False if the timeot expires before all outstanding work is done.

spin() → None: Execute callbacks until shutdown.

spin_once(timeout_sec: float | None = None) → None

Wait for and execute a single callback.

A custom executor should use wait_for_ready_callbacks() to get work.

Parameters:: timeout_sec – Seconds to wait. Block forever if None or negative. Don’t wait if 0.

spin_once_until_future_complete(future: Future, timeout_sec: float | TimeoutObject | None = None) → None

Wait for and execute a single callback.

This should behave in the same way as spin_once(). If needed by the implementation, it should awake other threads waiting.

Parameters:

future – The executor will wait until this future is done.
timeout_sec – Maximum seconds to wait. Block forever if None or negative. Don’t wait if 0.

spin_until_future_complete(future: Future, timeout_sec: float | None = None) → None: Execute callbacks until a given future is done or a timeout occurs.

wait_for_ready_callbacks(*args, **kwargs) → Tuple[Task, WaitableEntityType, Node]

Return callbacks that are ready to be executed.

The arguments to this function are passed to the internal method _wait_for_ready_callbacks() to get a generator for ready callbacks:

_wait_for_ready_callbacks(timeout_sec: float | ~rclpy.executors.TimeoutObject | None = None, nodes: ~typing.List[Node] = None, condition: ~typing.Callable[[], bool] = <function Executor.<lambda>>) → Generator[Tuple[Task, WaitableEntityType, Node], None, None]

Yield callbacks that are ready to be executed.

Raises:

TimeoutException – on timeout.
ShutdownException – on if executor was shut down.

Parameters:

timeout_sec – Seconds to wait. Block forever if None or negative. Don’t wait if 0.
nodes – A list of nodes to wait on. Wait on all nodes if None.
condition – A callable that makes the function return immediately when it evaluates to True.

wake() → None

Wake the executor because something changed.

This is used to tell the executor when entities are created or destroyed.

exception rclpy.executors.ExternalShutdownException: Context has been shutdown.

class rclpy.executors.MultiThreadedExecutor(num_threads: int | None = None, *, context: Context | None = None)

Runs callbacks in a pool of threads.

Parameters:

num_threads – number of worker threads in the pool. If None, the number of threads will be automatically set by querying the underlying OS for the CPU affinity of the process space. If the OS doesn’t provide this information, defaults to 2.
context – The context associated with the executor.

spin_once(timeout_sec: float | None = None) → None

Wait for and execute a single callback.

A custom executor should use wait_for_ready_callbacks() to get work.

Parameters:: timeout_sec – Seconds to wait. Block forever if None or negative. Don’t wait if 0.

spin_once_until_future_complete(future: Future, timeout_sec: float | TimeoutObject | None = None) → None

Wait for and execute a single callback.

This should behave in the same way as spin_once(). If needed by the implementation, it should awake other threads waiting.

Parameters:

future – The executor will wait until this future is done.
timeout_sec – Maximum seconds to wait. Block forever if None or negative. Don’t wait if 0.

exception rclpy.executors.ShutdownException: Signal that executor was shut down.

class rclpy.executors.SingleThreadedExecutor(*, context: Context | None = None)

Runs callbacks in the thread that calls Executor.spin().

spin_once(timeout_sec: float | None = None) → None

Wait for and execute a single callback.

A custom executor should use wait_for_ready_callbacks() to get work.

Parameters:: timeout_sec – Seconds to wait. Block forever if None or negative. Don’t wait if 0.

spin_once_until_future_complete(future: Future, timeout_sec: float | TimeoutObject | None = None) → None

Wait for and execute a single callback.

This should behave in the same way as spin_once(). If needed by the implementation, it should awake other threads waiting.

Parameters:

future – The executor will wait until this future is done.
timeout_sec – Maximum seconds to wait. Block forever if None or negative. Don’t wait if 0.

exception rclpy.executors.TimeoutException: Signal that a timeout occurred.

class rclpy.executors.TimeoutObject(timeout: float)

Use timeout object to save timeout.

property timeout

async rclpy.executors.await_or_execute(callback: Callable | Coroutine, *args) → Any: Await a callback if it is a coroutine, else execute it.

Callback Groups

class rclpy.callback_groups.CallbackGroup

The base class for a callback group.

A callback group controls when callbacks are allowed to be executed.

This class should not be instantiated. Instead, classes should extend it and implement can_execute(), beginning_execution(), and ending_execution().

add_entity(entity) → None

Add an entity to the callback group.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.

beginning_execution(entity) → bool

Get permission for the callback from the group to begin executing an entity.

If this returns True then CallbackGroup.ending_execution() must be called after the callback has been executed.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.
Returns:: True if the callback can be executed, False otherwise.

can_execute(entity) → bool

Determine if an entity can be executed.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.
Returns:: True if the entity can be executed, False otherwise.

ending_execution(entity) → None

Notify group that a callback has finished executing.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.

has_entity(entity) → bool

Determine if an entity has been added to this group.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.

class rclpy.callback_groups.MutuallyExclusiveCallbackGroup

Allow only one callback to be executing at a time.

beginning_execution(entity)

Get permission for the callback from the group to begin executing an entity.

If this returns True then CallbackGroup.ending_execution() must be called after the callback has been executed.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.
Returns:: True if the callback can be executed, False otherwise.

can_execute(entity)

Determine if an entity can be executed.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.
Returns:: True if the entity can be executed, False otherwise.

ending_execution(entity)

Notify group that a callback has finished executing.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.

class rclpy.callback_groups.ReentrantCallbackGroup

Allow callbacks to be executed in parallel without restriction.

beginning_execution(entity)

Get permission for the callback from the group to begin executing an entity.

If this returns True then CallbackGroup.ending_execution() must be called after the callback has been executed.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.
Returns:: True if the callback can be executed, False otherwise.

can_execute(entity)

Determine if an entity can be executed.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.
Returns:: True if the entity can be executed, False otherwise.

ending_execution(entity)

Notify group that a callback has finished executing.

Parameters:: entity – a subscription, timer, client, service, or waitable instance.