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

class rclpy.executors.Executor(*, context=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 (Optional[Context]) – The context to be associated with, or None for the default global context.
add_node(node)

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

Parameters:node (Node) – The node to add to the executor.
Return type:bool
Returns:True if the node was added, False otherwise.
can_execute(entity)

Determine if a callback for an entity can be executed.

Parameters:entity (~WaitableEntityType) – Subscription, Timer, Guard condition, etc
Return type:bool
Returns:True if the entity callback can be executed, False otherwise.
context

Get the context associated with the executor.

Return type:Context
create_task(callback, *args, **kwargs)

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 (Union[Callable, Coroutine[+T_co, -T_contra, +V_co]]) – A callback to be run in the executor.
Return type:Task
get_nodes()

Return nodes that have been added to this executor.

Return type:List[_Forwardref]
remove_node(node)

Stop managing this node’s callbacks.

Parameters:node (Node) – The node to remove from the executor.
Return type:None
shutdown(timeout_sec=None)

Stop executing callbacks and wait for their completion.

Parameters:timeout_sec (Optional[float]) – Seconds to wait. Block forever if None or negative. Don’t wait if 0.
Return type:bool
Returns:True if all outstanding callbacks finished executing, or False if the timeot expires before all outstanding work is done.
spin()

Execute callbacks until shutdown.

Return type:None
spin_once(timeout_sec=None)

Wait for and execute a single callback.

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

Parameters:timeout_sec (Optional[float]) – Seconds to wait. Block forever if None or negative. Don’t wait if 0.
Return type:None
spin_until_future_complete(future, timeout_sec=None)

Execute callbacks until a given future is done or a timeout occurs.

Return type:None
wait_for_ready_callbacks(*args, **kwargs)

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=None, nodes=None)

Yield callbacks that are ready to be executed.

Raises:
Parameters:
  • timeout_sec (float) – Seconds to wait. Block forever if None or negative. Don’t wait if 0.
  • nodes (List[_Forwardref]) – A list of nodes to wait on. Wait on all nodes if None.
Return type:

Generator[Tuple[Task, ~WaitableEntityType, _Forwardref], None, None]
Return type:Tuple[Task, ~WaitableEntityType, _Forwardref]
wake()

Wake the executor because something changed.

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

Return type:None
class rclpy.executors.MultiThreadedExecutor(num_threads=None, *, context=None)

Runs callbacks in a pool of threads.

Parameters:
  • num_threads (Optional[int]) – number of worker threads in the pool. If None, the number of threads will use multiprocessing.cpu_count(). If that’s not implemented the number of threads defaults to 1.
  • context (Optional[Context]) – The context associated with the executor.
spin_once(timeout_sec=None)

Wait for and execute a single callback.

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

Parameters:timeout_sec (Optional[float]) – Seconds to wait. Block forever if None or negative. Don’t wait if 0.
Return type:None
exception rclpy.executors.ShutdownException

Signal that executor was shut down.

class rclpy.executors.SingleThreadedExecutor(*, context=None)

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

spin_once(timeout_sec=None)

Wait for and execute a single callback.

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

Parameters:timeout_sec (Optional[float]) – Seconds to wait. Block forever if None or negative. Don’t wait if 0.
Return type:None
exception rclpy.executors.TimeoutException

Signal that a timeout occurred.

rclpy.executors.await_or_execute(callback, *args)

Await a callback if it is a coroutine, else execute it.

Return type:Any

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)

Add an entity to the callback group.

Parameters:entity – a subscription, timer, client, service, or waitable instance.
Return type:None
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.
Return type:bool
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.
Return type:bool
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.
Return type:None
has_entity(entity)

Determine if an entity has been added to this group.

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