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pending

Classes:

PendingCancelled

Bases: ClosedResourceError

Used to indicate the pending is cancelled.

Raised by:
Source code in src/async_kernel/pending.py 
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class PendingCancelled(anyio.ClosedResourceError):
    "Used to indicate the pending is cancelled."

InvalidStateError

Bases: RuntimeError

An invalid state of the pending.

Raised by:
Source code in src/async_kernel/pending.py 
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class InvalidStateError(RuntimeError):
    "An invalid state of the pending."

PendingTracker

The base class for tracking Pending.

Subclassed by: Used by:

Methods:

  • current –

    The instance of the active tracker in the current context.

  • active_id –

    The id of the active tracker in the current context.

  • add –

    Track Pending until it is done.

Attributes:

Source code in src/async_kernel/pending.py 
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class PendingTracker:
    """
    The base class for tracking [Pending][async_kernel.pending.Pending].
    """

    _subclasses: ClassVar[tuple[type[Self], ...]] = ()
    _instances: ClassVar[weakref.WeakValueDictionary[str, Self]] = weakref.WeakValueDictionary()
    _id_contextvar: ClassVar[contextvars.ContextVar[str | None]]
    _pending: Fixed[Self, set[Pending[Any]]] = Fixed(set)

    id: Fixed[Self, str] = Fixed(lambda _: str(uuid.uuid4()))
    """The unique id of the pending tracker instance."""

    @property
    def pending(self) -> set[Pending[Any]]:
        "The pending currently associated with this instance."
        return self._pending.copy()

    def __init_subclass__(cls) -> None:
        if cls.__name__ != "PendingManager":
            PendingTracker._subclasses = (*cls._subclasses, cls)
            # Each subclass is assigned a new context variable.
            cls._id_contextvar = contextvars.ContextVar(f"{cls.__module__}.{cls.__name__}", default=None)
        return super().__init_subclass__()

    @classmethod
    def current(cls) -> Self | None:
        "The instance of the active tracker in the current context."
        if (id_ := cls._id_contextvar.get()) and (current := cls._instances.get(id_)):
            return current
        return None

    @classmethod
    def active_id(cls) -> str | None:
        "The id of the active tracker in the current context."
        return cls._id_contextvar.get()

    def __init__(self) -> None:
        self._instances[self.id] = self

    def _activate(self) -> Token[str | None]:
        try:
            return self._id_contextvar.set(self.id)
        except AttributeError as e:
            e.add_note("Pending tracker must be subclassed to use it!")
            raise

    def _deactivate(self, token: contextvars.Token[str | None]) -> None:
        self._id_contextvar.reset(token)

    def add(self, pen: Pending) -> None:
        "Track `Pending` until it is done."

        if pen not in self._pending:
            self._pending.add(pen)
            pen.add_done_callback(self._on_pending_done)

    def _on_pending_done(self, pen: Pending) -> None:
        "A done_callback that is registered with pen when it is added (don't call directly)."
        self._pending.discard(pen)

id class-attribute instance-attribute 

id: Fixed[Self, str] = Fixed(lambda _: str(uuid4()))

The unique id of the pending tracker instance.

pending property 

pending: set[Pending[Any]]

The pending currently associated with this instance.

current classmethod 

current() -> Self | None

The instance of the active tracker in the current context.

Source code in src/async_kernel/pending.py 
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@classmethod
def current(cls) -> Self | None:
    "The instance of the active tracker in the current context."
    if (id_ := cls._id_contextvar.get()) and (current := cls._instances.get(id_)):
        return current
    return None

active_id classmethod 

active_id() -> str | None

The id of the active tracker in the current context.

Source code in src/async_kernel/pending.py 
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@classmethod
def active_id(cls) -> str | None:
    "The id of the active tracker in the current context."
    return cls._id_contextvar.get()

add 

add(pen: Pending) -> None

Track Pending until it is done.

Source code in src/async_kernel/pending.py 
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def add(self, pen: Pending) -> None:
    "Track `Pending` until it is done."

    if pen not in self._pending:
        self._pending.add(pen)
        pen.add_done_callback(self._on_pending_done)

PendingManager

Bases: PendingTracker

Tracks the creation of Pending in multiple contexts.

This class must be subclassed to be useful.

For each subclass there is zero or one active trackers at a time. Activating a manager will 'replace' a previously active pending manager.

Notes:

- It must be subclassed.
- Each subclass is assigned one context variable.
    - This means that only one instance of the subclass can be active in a specific context at any time.
- It is linearly proportionally expensive to subclass PendingManager so it's usage should
    be limited to cases where it is necessary where the exact functionality is required.

Usage:

```python
class MyPendingManager(PendingManager):
    "Manages the context of ..."


m = MyPendingManager()
m2 = MyPendingManager()

# In one or more contexts
token = m.activate()
try:
    ...
    try:
        token2 = m2.activate()
        pen = m2.caller.call_soon(lambda: 1 + 1)
        assert pen in m2.pending
        assert (
            pen not in m.pending
        ), "pen is associated should only be associated with m2"
        ...
    finally:
        m2.deactivate(token2)

finally:
    m.deactivate(token)
```
Referenced by:

Methods:

  • activate –

    Start tracking Pending in the current context.

  • deactivate –

    Stop tracking.

  • remove –

    Remove a pending from the manager.

  • context –

    A context manager where the pending manager is activated.

Source code in src/async_kernel/pending.py 
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class PendingManager(PendingTracker):
    """
    Tracks the creation of `Pending` in multiple contexts.

    This class must be subclassed to be useful.

    For each subclass there is zero or one active trackers at a time. Activating a manager will 'replace' a
    previously active pending manager.

    Notes:

        - It must be subclassed.
        - Each subclass is assigned one context variable.
            - This means that only one instance of the subclass can be active in a specific context at any time.
        - It is linearly proportionally expensive to subclass PendingManager so it's usage should
            be limited to cases where it is necessary where the exact functionality is required.

    Usage:

        ```python
        class MyPendingManager(PendingManager):
            "Manages the context of ..."


        m = MyPendingManager()
        m2 = MyPendingManager()

        # In one or more contexts
        token = m.activate()
        try:
            ...
            try:
                token2 = m2.activate()
                pen = m2.caller.call_soon(lambda: 1 + 1)
                assert pen in m2.pending
                assert (
                    pen not in m.pending
                ), "pen is associated should only be associated with m2"
                ...
            finally:
                m2.deactivate(token2)

        finally:
            m.deactivate(token)
        ```
    """

    def activate(self) -> contextvars.Token[str | None]:
        """
        Start tracking `Pending` in the  current context.
        """
        return self._activate()

    def deactivate(self, token: contextvars.Token[str | None]) -> None:
        """
        Stop tracking.

        Args:
            token: The token returned from [activate][].
        """
        self._deactivate(token)

    def remove(self, pen: Pending) -> None:
        """
        Remove a pending from the manager.
        """
        self._pending.remove(pen)

    @contextlib.contextmanager
    def context(self) -> Generator[None, Any, None]:
        """A context manager where the pending manager is activated."""
        token = self.activate()
        try:
            yield
        finally:
            self.deactivate(token)

activate 

activate() -> Token[str | None]

Start tracking Pending in the current context.

Referenced by:
Source code in src/async_kernel/pending.py 
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def activate(self) -> contextvars.Token[str | None]:
    """
    Start tracking `Pending` in the  current context.
    """
    return self._activate()

deactivate 

deactivate(token: Token[str | None]) -> None

Stop tracking.

Parameters:

Source code in src/async_kernel/pending.py 
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def deactivate(self, token: contextvars.Token[str | None]) -> None:
    """
    Stop tracking.

    Args:
        token: The token returned from [activate][].
    """
    self._deactivate(token)

remove 

remove(pen: Pending) -> None

Remove a pending from the manager.

Source code in src/async_kernel/pending.py 
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def remove(self, pen: Pending) -> None:
    """
    Remove a pending from the manager.
    """
    self._pending.remove(pen)

context 

context() -> Generator[None, Any, None]

A context manager where the pending manager is activated.

Source code in src/async_kernel/pending.py 
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@contextlib.contextmanager
def context(self) -> Generator[None, Any, None]:
    """A context manager where the pending manager is activated."""
    token = self.activate()
    try:
        yield
    finally:
        self.deactivate(token)

PendingGroup

Bases: PendingTracker, AsyncContextManagerMixin

An asynchronous context manager for tracking Pending that are created in it's context.

Usage

Enter the async context and create new pending.

async with PendingGroup() as pg:
    assert pg.caller.to_thread(lambda: None) in pg.pending
Referenced by: Returned by:

Methods:

  • __init__ –

    An async context to capture all pending (that opt in) created in the context.

  • cancel –

    Cancel the pending group (internally synchronised).

  • cancelled –

    Returns: If the pending group is marked as cancelled.

Attributes:

Source code in src/async_kernel/pending.py 
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@final
class PendingGroup(PendingTracker, anyio.AsyncContextManagerMixin):
    """
    An asynchronous context manager for tracking `Pending` that are created in it's context.

    Usage:
        Enter the async context and create new pending.

        ```python
        async with PendingGroup() as pg:
            assert pg.caller.to_thread(lambda: None) in pg.pending
        ```
    """

    _parent_id: None | str = None
    _cancel_scope: anyio.CancelScope
    _cancelled: str | None = None
    _leaving_context: bool = False
    _failed: Fixed[Self, list[Pending]] = Fixed(list)
    cancellation_timeout = 10
    "The maximum time to wait for cancelled pending to be done."

    caller: Fixed[Self, Caller] = Fixed(lambda _: async_kernel.Caller())
    "The caller where the pending group was instantiated."

    def __init__(self, *, shield: bool = False, mode: int = 0) -> None:
        """
        An async context to capture all pending (that opt in) created in the context.

        The pending group will only exit once all pending in the group are `done`.

        Pending can be added to and removed from the group manually.

        Args:
            shield: Passed to the cancel scope.
            mode: The mode.
                - 0: Ignore cancellation of pending.
                - 1: Cancel if any pending is cancelled - raise PendingCancelled on exit.
                - 2: Cancel if any pending is cancelled - exit quietly.
        """
        assert mode in [0, 1, 2]
        self._mode = mode
        self._shield = shield
        self.caller  # noqa: B018
        super().__init__()

    @override
    def __repr__(self) -> str:
        info = " ⛔ cancelled" if self._cancelled else ""
        return f"<PendingGroup at {id(self)}{info} | {len(self.pending)} pending | mode:{self._mode}>"

    @override
    def _activate(self) -> Token[str | None]:
        self._parent_id = None if (parent_id := self._id_contextvar.get()) == self.id else parent_id
        return super()._activate()

    @contextlib.asynccontextmanager
    async def __asynccontextmanager__(self) -> AsyncGenerator[Self]:
        if self._leaving_context:
            msg = f"Re-entry of {self.__class__} is not supported!"
            raise InvalidStateError(msg)
        self._cancel_scope = anyio.CancelScope(shield=self._shield)
        self._all_done = create_async_event()
        token = self._activate()
        try:
            with self._cancel_scope:
                try:
                    yield self
                    self._leaving_context = True
                    if self._pending:
                        await self._all_done
                except (anyio.get_cancelled_exc_class(), Exception) as e:
                    self.cancel(f"An error occurred: {e!r}")
                    raise
            if self._cancelled is not None:
                if exceptions := [e for pen in self._failed if isinstance(e := pen.exception(), Exception)]:
                    msg = f"One or more exceptions occurred in this context! {list(map(str, exceptions))}"
                    raise ExceptionGroup(msg, exceptions)
                if self._mode in [0, 1]:
                    raise PendingCancelled(self._cancelled)
        finally:
            self._leaving_context = True
            self._deactivate(token)
            self._instances.pop(self.id, None)
            if self._pending:
                if self._all_done or self._all_done.cancelled():
                    self._all_done = create_async_event()
                if self._pending and not self._all_done:
                    with anyio.CancelScope(shield=True), anyio.move_on_after(self.cancellation_timeout):
                        await self._all_done

    @override
    def add(self, pen: Pending) -> None:
        if pen not in self._pending:
            self._pending.add(pen)
            pen.add_done_callback(self._on_pending_done)
        if (id_ := self._parent_id) and (parent := self._instances.get(id_)):
            parent.add(pen)

    @override
    def _on_pending_done(self, pen: Pending) -> None:
        try:
            self._pending.remove(pen)
            if pen.cancelled():
                if self._mode in [1, 2]:
                    self.cancel(f"A monitored pending was cancelled {pen=}")
            elif pen.exception():
                self._failed.append(pen)
                self.cancel(f"Exception in member: {pen}")
        except KeyError:
            pass
        if self._leaving_context and not self._pending:
            self._all_done.set()

    def cancel(self, msg: str | None = None) -> bool:
        "Cancel the pending group (internally synchronised)."
        self._cancelled = "\n".join(((self._cancelled or ""), msg or ""))
        if not self._cancel_scope.cancel_called:
            self.caller.call_direct(self._cancel_scope.cancel, msg)
            for pen_ in self.pending:
                pen_.cancel(msg)
        return self.cancelled()

    def cancelled(self) -> bool:
        """Returns: If the pending group is marked as cancelled."""
        return self._cancelled is not None

cancellation_timeout class-attribute instance-attribute 

cancellation_timeout = 10

The maximum time to wait for cancelled pending to be done.

caller class-attribute instance-attribute 

caller: Fixed[Self, Caller] = Fixed(lambda _: Caller())

The caller where the pending group was instantiated.

__init__ 

__init__(*, shield: bool = False, mode: int = 0) -> None

The pending group will only exit once all pending in the group are done.

Pending can be added to and removed from the group manually.

Parameters:

  • shield

    (bool, default: False ) –

    Passed to the cancel scope.

  • mode

    (int, default: 0 ) –

    The mode. - 0: Ignore cancellation of pending. - 1: Cancel if any pending is cancelled - raise PendingCancelled on exit. - 2: Cancel if any pending is cancelled - exit quietly.

Source code in src/async_kernel/pending.py 
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def __init__(self, *, shield: bool = False, mode: int = 0) -> None:
    """
    An async context to capture all pending (that opt in) created in the context.

    The pending group will only exit once all pending in the group are `done`.

    Pending can be added to and removed from the group manually.

    Args:
        shield: Passed to the cancel scope.
        mode: The mode.
            - 0: Ignore cancellation of pending.
            - 1: Cancel if any pending is cancelled - raise PendingCancelled on exit.
            - 2: Cancel if any pending is cancelled - exit quietly.
    """
    assert mode in [0, 1, 2]
    self._mode = mode
    self._shield = shield
    self.caller  # noqa: B018
    super().__init__()

cancel 

cancel(msg: str | None = None) -> bool

Cancel the pending group (internally synchronised).

Source code in src/async_kernel/pending.py 
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def cancel(self, msg: str | None = None) -> bool:
    "Cancel the pending group (internally synchronised)."
    self._cancelled = "\n".join(((self._cancelled or ""), msg or ""))
    if not self._cancel_scope.cancel_called:
        self.caller.call_direct(self._cancel_scope.cancel, msg)
        for pen_ in self.pending:
            pen_.cancel(msg)
    return self.cancelled()

cancelled 

cancelled() -> bool

Returns: If the pending group is marked as cancelled.

Source code in src/async_kernel/pending.py 
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def cancelled(self) -> bool:
    """Returns: If the pending group is marked as cancelled."""
    return self._cancelled is not None

Pending

Bases: Awaitable[T]

Pending is an internally synchronised, cancellable, awaitable class influenced by asyncio.Future.

  • It can be wait/awaited and cancelled from any thread (not considering deadlocks)
  • Provides metadata storage
  • Has built in support for tracking pending created in a specific context (see: PendingManager and PendingGroup).

Properties

High level methods

Low level methods

Referenced by: Returned by: Used by:

Methods:

  • __init__ –

    Initializes a new pending object with optional creation options and metadata.

  • wait –

    Wait for result or exception to be set (internally synchronised) returning the result if specified.

  • wait_sync –

    Wait synchronously for result or exception (internally synchronised) returning the result if specified.

  • set_result –

    Set the result (low-level internally synchronised).

  • set_exception –

    Set the exception (low-level internally synchronised).

  • cancel –

    Cancel the pending if it is not already done.

  • cancel_wait –

    Cancel the pending and wait for it to be done.

  • cancelled –

    Returns:

  • set_canceller –

    Set a callback to handle cancellation (low-level).

  • done –

    Returns True if a result or exception has been set.

  • add_done_callback –

    Add a callback for when the pending is done.

  • remove_done_callback –

    Remove fn from the done callback list.

  • result –

    Return the result.

  • exception –

    Return the exception.

Attributes:

Source code in src/async_kernel/pending.py 
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class Pending(Awaitable[T]):
    """
    Pending is an internally synchronised, cancellable, awaitable class influenced by [asyncio.Future][].

    - It can be wait/awaited and cancelled from any thread (not considering deadlocks)
    - Provides metadata storage
    - Has built in support for tracking pending created in a specific context (see: [PendingManager][] and [PendingGroup][]).

    **Properties**

    - [Pending.metadata][]: Metadata associated with the pending.
    - [Pending.context][]: The context associated with the pending.

    **High level methods**

    - [Pending.wait][]: Wait asynchronously for the pending to be complete.
    - [Pending.wait_sync][]: Wait synchronously for the pending to be complete.
    - [Pending.cancel][]: Cancel the pending (result must be set to finalize cancellation).
    - [Pending.result][]: Get the result of the pending.
    - [Pending.exception][]: Get the exception of the pending.

    **Low level methods**

    - [Pending.add_done_callback][]: Add a callback that is called once the pending is complete.
    - [Pending.remove_done_callback][]: Remove a previously added callback.
    - [Pending.set_canceller][]: Set a callback to handle cancellation.
    """

    __slots__ = [
        "__weakref__",
        "_cancelled",
        "_canceller",
        "_done",
        "_done_callbacks",
        "_exception",
        "_result",
        "_waiting",
        "context",
    ]

    _REPR_OMIT: ClassVar[list[str]] = ["func", "args", "kwargs", "caller"]
    "Keys of metadata to omit when creating a repr of the pending."

    _metadata_mappings: ClassVar[dict[int, dict[str, Any]]] = {}
    "A mapping of pending ids to metadata."

    _cancelled: str | None
    _canceller: Callable[[str | None], Any] | None
    _exception: Exception
    _done: bool
    _result: T
    context: contextvars.Context | None
    """The context associated with Pending."""

    @property
    def metadata(self) -> dict[str, Any]:
        """
        The metadata associated with the pending.
        """
        return self._metadata_mappings[id(self)]

    def __init__(
        self,
        context: contextvars.Context | None = None,
        trackers: type[PendingTracker] | tuple[type[PendingTracker], ...] = (),
        /,
        **metadata: Any,
    ) -> None:
        """
        Initializes a new pending object with optional creation options and metadata.

        Args:
            context: A context to associate with the pending, if provided it is copied.
            trackers: A subclass or tuple of `PendingTracker` subclasses to which the pending can be added in the current context.
            **metadata: Arbitrary keyword arguments containing metadata to associate with this Pending instance.

        Behavior:
            - Initializes internal state for tracking completion and cancellation.
            - Stores provided metadata in a class-level mapping.
        """
        self._done_callbacks: list[Callable[[Self], Any]] = []
        self._metadata_mappings[id(self)] = metadata
        self._done = False
        self._cancelled = None
        self._canceller = None

        if trackers or context:
            # A copy the context is required to avoid `PendingTracker.id` leakage.
            context = context.copy() if context else contextvars.copy_context()
        self.context = context

        # PendingTacker registration.
        if context:
            for cls in PendingTracker._subclasses:  # pyright: ignore[reportPrivateUsage]
                if id_ := context.get(cls._id_contextvar):  # pyright: ignore[reportPrivateUsage]
                    if trackers and issubclass(cls, trackers) and (tracker := PendingTracker._instances.get(id_)):  # pyright: ignore[reportPrivateUsage]
                        tracker.add(self)
                    else:
                        # Clear `PendingTracker.id`.
                        context.run(cls._id_contextvar.set, None)  # pyright: ignore[reportPrivateUsage]

    def __del__(self) -> None:
        self._metadata_mappings.pop(id(self), None)

    @override
    def __repr__(self) -> str:
        rep = (
            "<Pending"
            + ((" ⛔" + (f"message={self._cancelled!s}" if self._cancelled else "")) if self.cancelled() else "")
            + ((f" ❗ {e!r}" if (e := getattr(self, "_exception", None)) else " 🏁") if self._done else " 🏃")
        )
        rep = f"{rep} at {id(self)}"
        with contextlib.suppress(Exception):
            if md := self.metadata:
                rep = f"{rep} metadata:"
                if "func" in md:
                    items = [f"{k}={truncated_rep.repr(v)}" for k, v in md.items() if k not in self._REPR_OMIT]
                    rep += f" | {md['func']} {' | '.join(items) if items else ''}"
                else:
                    rep += f"{truncated_rep.repr(md)}"
        return rep + " >"

    @override
    def __await__(self) -> Generator[Any, None, T]:
        return self.wait().__await__()

    if TYPE_CHECKING:

        @overload
        async def wait(
            self, *, timeout: float | None = ..., protect: bool = False | ..., result: Literal[True] = True
        ) -> T: ...

        @overload
        async def wait(self, *, timeout: float | None = ..., protect: bool = ..., result: Literal[False]) -> None: ...

    async def wait(self, *, timeout: float | None = None, protect: bool = False, result: bool = True) -> T | None:
        """
        Wait for `result` or `exception` to be set (internally synchronised) returning the result if specified.

        Args:
            timeout: Timeout in seconds.
            protect: Protect the pending from a `TimeoutError` or external cancellation.
            result: If `result` should be returned.

        Raises:
            TimeoutError: When the timeout expires and a result or exception has not been set.
            PendingCancelled: If `result=True` and the pending has been cancelled.
            Exception: If `result=True` and an exception was set on the pending.

        Tip:
            To wait for a cancelled pending to complete use `await pen.wait(result=False)`.
        """
        try:
            if not self._done or self._done_callbacks:
                waiter = create_async_waiter()
                self.add_done_callback(lambda _: waiter.wake())
                if timeout is None:
                    await waiter
                else:
                    with anyio.fail_after(timeout):
                        await waiter
            return self.result() if result else None
        except (anyio.get_cancelled_exc_class(), TimeoutError) as e:
            if not protect:
                self.cancel(f"Cancelled due to cancellation or timeout: {e}.")
            raise

    if TYPE_CHECKING:

        @overload
        def wait_sync(
            self, *, timeout: float | None = ..., protect: bool = False | ..., result: Literal[True] = True
        ) -> T: ...

        @overload
        def wait_sync(self, *, timeout: float | None = ..., protect: bool = ..., result: Literal[False]) -> None: ...

    def wait_sync(self, *, timeout: float | None = None, protect: bool = False, result: bool = True) -> T | None:
        """
        Wait synchronously for `result` or `exception` (internally synchronised) returning the result if specified.

        Args:
            timeout: Timeout in seconds.
            protect: Protect the pending from cancellation from a `TimeoutError`.
            result: When `True` the result is returned.

        Raises:
            TimeoutError: When the timeout expires and a result or exception has not been set.
            PendingCancelled: If `result=True` and the pending has been cancelled.
            Exception: If `result=True` and an exception was set on the pending.

        Warning:
            **Calling this method in the same thread where the result or exception is set
            will result in deadlock unless a greenlet based event library is in use.**
        """
        if not self._done or self._done_callbacks:
            event = Event()
            self.add_done_callback(lambda _: event.set())
            event.wait(timeout)
            if not self._done:
                msg = f"Timeout waiting for {self}"
                if not protect:
                    self.cancel(msg)
                raise TimeoutError(msg)
        return self.result() if result else None

    def _set_done(self, mode: Literal["result", "exception"], value: T | BaseException | None) -> None:
        if self._done:
            raise InvalidStateError
        self._done = True
        e = None
        self._canceller = None
        # List reversal and BaseException handling inspiration: https://gist.github.com/x42005e1f/4f18c3c62da9135020bdea8c44c248a2
        callbacks = self._done_callbacks
        callbacks.reverse()
        if not self._cancelled:
            setattr(self, "_" + mode, value)
        while callbacks:
            try:
                callbacks.pop()(self)
            except Exception:
                pass
            except BaseException as exc:
                e = exc
        if e:
            raise e from None

    def set_result(self, value: T) -> None:
        """
        Set the result (low-level internally synchronised).

        Args:
            value: The result to set.

        Raises:
            InvalidStateError: If the pending is already done.
        """
        self._set_done("result", value)

    def set_exception(self, exception: BaseException) -> None:
        """
        Set the exception (low-level internally synchronised).

        Args:
            exception: The value to set as the exception.

        Raises:
            InvalidStateError: If the pending is already done.
        """
        self._set_done("exception", exception)

    def cancel(self, msg: str | None = None) -> bool:
        """
        Cancel the pending if it is not already `done`.

        Args:
            msg: The cancellation message.

        Returns:
            If the pending is marked as cancelled.

        Notes:
            - Cancellation cannot be undone.
            - Multiple calls append `msg` to the cancellation message with no other effect.
            - The `done` state is a function of the canceller, if no canceller is set, `done`
                will be set immediately.
        """
        if not self._done:
            if (cancelled := self._cancelled or "") and msg:
                msg = f"{cancelled}\n{msg}"
            self._cancelled = msg or cancelled
            if not (canceller := self._canceller):
                try:
                    self._set_done("result", None)
                except InvalidStateError:
                    pass
            else:
                canceller(msg)
        return self._cancelled is not None

    async def cancel_wait(self, msg: str | None, *, timeout: float | None = None) -> None:
        "Cancel the pending and wait for it to be `done`."
        if not self._done:
            self.cancel(msg)
            if not self._done:
                await self.wait(result=False, timeout=timeout)

    def cancelled(self) -> bool:
        """
        Returns:
            If the pending is marked as cancelled.

        Notes:

            - This can return `True` before a pending is `done`.
            - To wait for a pending to complete (assuming a canceller is or will be set); use
                `await pen.wait(result=False)` or `pen.wait_sync(result=False)`
        """
        return self._cancelled is not None

    def set_canceller(self, canceller: Callable[[str | None], Any], /) -> None:
        """
        Set a callback to handle cancellation (low-level).

        Args:
            canceller: A callback that performs the cancellation of the pending.

                - It must accept the cancellation message as the first argument.
                - `canceller` must be externally synchronised.

        Warning:
            - [set_result][] or [set_exception][] *MUST* be called by the `canceller`, or as a result of the call to `canceller`
                to mark the pending as `done`.

        Raises:
            InvalidStateError: If already `done`.
            PendingCancelled: If already `cancelled`.

        Example:
            ```python
            pen = Pending()
            pen.cancel()
            assert not pen.done()
            pen.set_canceller(lambda msg: pen.set_result(None))
            assert pen.done()
            ```
        """
        if self._done:
            if self._cancelled is not None:
                raise PendingCancelled(self._cancelled)
            raise InvalidStateError
        self._canceller = canceller

    def done(self) -> bool:
        """
        Returns `True` if a result or exception has been set.
        """
        return self._done

    def add_done_callback(self, fn: Callable[[Self], Any], /) -> None:
        """
        Add a callback for when the pending is `done`.

        The callback should be light weight and provide its own thread safety.

        Callbacks added are handled FIFO.
        """
        if not self._done:
            self._done_callbacks.append(fn)
        elif self._done_callbacks:
            # respect FIFO - 'done_callbacks' gets reversed in `_set_done`.
            self._done_callbacks.insert(0, fn)
        else:
            fn(self)

    def remove_done_callback(self, fn: Callable[[Self], object], /) -> int:
        """
        Remove `fn` from the done callback list.

        Returns the number of items removed.
        """
        n = 0
        while fn in self._done_callbacks:
            n += 1
            self._done_callbacks.remove(fn)
        return n

    def result(self) -> T:
        """
        Return the result.

        Raises:
            PendingCancelled: If the pending has been cancelled.
            InvalidStateError: If the pending isn't done yet.
        """
        if e := self.exception():
            raise e from None
        try:
            return self._result
        except AttributeError:
            raise InvalidStateError from None

    def exception(self) -> BaseException | None:
        """
        Return the exception.

        Raises:
            PendingCancelled: If the pending has been cancelled.
        """
        if self._cancelled is not None:
            raise PendingCancelled(self._cancelled)
        return getattr(self, "_exception", None)

metadata property 

metadata: dict[str, Any]

The metadata associated with the pending.

Referenced by:

context instance-attribute 

context: Context | None = context

The context associated with Pending.

Referenced by:

__init__ 

__init__(
    context: Context | None = None,
    trackers: type[PendingTracker] | tuple[type[PendingTracker], ...] = (),
    /,
    **metadata: Any,
) -> None

Parameters:

  • context

    (Context | None, default: None ) –

    A context to associate with the pending, if provided it is copied.

  • trackers

    (type[PendingTracker] | tuple[type[PendingTracker], ...], default: () ) –

    A subclass or tuple of PendingTracker subclasses to which the pending can be added in the current context.

  • **metadata

    (Any, default: {} ) –

    Arbitrary keyword arguments containing metadata to associate with this Pending instance.

Behavior
  • Initializes internal state for tracking completion and cancellation.
  • Stores provided metadata in a class-level mapping.
Source code in src/async_kernel/pending.py 
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def __init__(
    self,
    context: contextvars.Context | None = None,
    trackers: type[PendingTracker] | tuple[type[PendingTracker], ...] = (),
    /,
    **metadata: Any,
) -> None:
    """
    Initializes a new pending object with optional creation options and metadata.

    Args:
        context: A context to associate with the pending, if provided it is copied.
        trackers: A subclass or tuple of `PendingTracker` subclasses to which the pending can be added in the current context.
        **metadata: Arbitrary keyword arguments containing metadata to associate with this Pending instance.

    Behavior:
        - Initializes internal state for tracking completion and cancellation.
        - Stores provided metadata in a class-level mapping.
    """
    self._done_callbacks: list[Callable[[Self], Any]] = []
    self._metadata_mappings[id(self)] = metadata
    self._done = False
    self._cancelled = None
    self._canceller = None

    if trackers or context:
        # A copy the context is required to avoid `PendingTracker.id` leakage.
        context = context.copy() if context else contextvars.copy_context()
    self.context = context

    # PendingTacker registration.
    if context:
        for cls in PendingTracker._subclasses:  # pyright: ignore[reportPrivateUsage]
            if id_ := context.get(cls._id_contextvar):  # pyright: ignore[reportPrivateUsage]
                if trackers and issubclass(cls, trackers) and (tracker := PendingTracker._instances.get(id_)):  # pyright: ignore[reportPrivateUsage]
                    tracker.add(self)
                else:
                    # Clear `PendingTracker.id`.
                    context.run(cls._id_contextvar.set, None)  # pyright: ignore[reportPrivateUsage]

wait async 

wait(
    *,
    timeout: float | None = ...,
    protect: bool = False | ...,
    result: Literal[True] = True,
) -> T

wait(*, timeout: float | None = ..., protect: bool = ..., result: Literal[False]) -> None

wait(
    *, timeout: float | None = None, protect: bool = False, result: bool = True
) -> T | None

Wait for result or exception to be set (internally synchronised) returning the result if specified.

Parameters:

  • timeout

    (float | None, default: None ) –

    Timeout in seconds.

  • protect

    (bool, default: False ) –

    Protect the pending from a TimeoutError or external cancellation.

  • result

    (bool, default: True ) –

    If result should be returned.

Raises:

  • TimeoutError –

    When the timeout expires and a result or exception has not been set.

  • PendingCancelled –

    If result=True and the pending has been cancelled.

  • Exception –

    If result=True and an exception was set on the pending.

Tip

To wait for a cancelled pending to complete use await pen.wait(result=False).

Referenced by:
Source code in src/async_kernel/pending.py 
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async def wait(self, *, timeout: float | None = None, protect: bool = False, result: bool = True) -> T | None:
    """
    Wait for `result` or `exception` to be set (internally synchronised) returning the result if specified.

    Args:
        timeout: Timeout in seconds.
        protect: Protect the pending from a `TimeoutError` or external cancellation.
        result: If `result` should be returned.

    Raises:
        TimeoutError: When the timeout expires and a result or exception has not been set.
        PendingCancelled: If `result=True` and the pending has been cancelled.
        Exception: If `result=True` and an exception was set on the pending.

    Tip:
        To wait for a cancelled pending to complete use `await pen.wait(result=False)`.
    """
    try:
        if not self._done or self._done_callbacks:
            waiter = create_async_waiter()
            self.add_done_callback(lambda _: waiter.wake())
            if timeout is None:
                await waiter
            else:
                with anyio.fail_after(timeout):
                    await waiter
        return self.result() if result else None
    except (anyio.get_cancelled_exc_class(), TimeoutError) as e:
        if not protect:
            self.cancel(f"Cancelled due to cancellation or timeout: {e}.")
        raise

wait_sync 

wait_sync(
    *,
    timeout: float | None = ...,
    protect: bool = False | ...,
    result: Literal[True] = True,
) -> T

wait_sync(
    *, timeout: float | None = ..., protect: bool = ..., result: Literal[False]
) -> None

wait_sync(
    *, timeout: float | None = None, protect: bool = False, result: bool = True
) -> T | None

Wait synchronously for result or exception (internally synchronised) returning the result if specified.

Parameters:

  • timeout

    (float | None, default: None ) –

    Timeout in seconds.

  • protect

    (bool, default: False ) –

    Protect the pending from cancellation from a TimeoutError.

  • result

    (bool, default: True ) –

    When True the result is returned.

Raises:

  • TimeoutError –

    When the timeout expires and a result or exception has not been set.

  • PendingCancelled –

    If result=True and the pending has been cancelled.

  • Exception –

    If result=True and an exception was set on the pending.

Warning

Calling this method in the same thread where the result or exception is set will result in deadlock unless a greenlet based event library is in use.

Referenced by:
Source code in src/async_kernel/pending.py 
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def wait_sync(self, *, timeout: float | None = None, protect: bool = False, result: bool = True) -> T | None:
    """
    Wait synchronously for `result` or `exception` (internally synchronised) returning the result if specified.

    Args:
        timeout: Timeout in seconds.
        protect: Protect the pending from cancellation from a `TimeoutError`.
        result: When `True` the result is returned.

    Raises:
        TimeoutError: When the timeout expires and a result or exception has not been set.
        PendingCancelled: If `result=True` and the pending has been cancelled.
        Exception: If `result=True` and an exception was set on the pending.

    Warning:
        **Calling this method in the same thread where the result or exception is set
        will result in deadlock unless a greenlet based event library is in use.**
    """
    if not self._done or self._done_callbacks:
        event = Event()
        self.add_done_callback(lambda _: event.set())
        event.wait(timeout)
        if not self._done:
            msg = f"Timeout waiting for {self}"
            if not protect:
                self.cancel(msg)
            raise TimeoutError(msg)
    return self.result() if result else None

set_result 

set_result(value: T) -> None

Set the result (low-level internally synchronised).

Parameters:

  • value

    (T) –

    The result to set.

Raises:

Referenced by:
Source code in src/async_kernel/pending.py 
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def set_result(self, value: T) -> None:
    """
    Set the result (low-level internally synchronised).

    Args:
        value: The result to set.

    Raises:
        InvalidStateError: If the pending is already done.
    """
    self._set_done("result", value)

set_exception 

set_exception(exception: BaseException) -> None

Set the exception (low-level internally synchronised).

Parameters:

Raises:

Referenced by:
Source code in src/async_kernel/pending.py 
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def set_exception(self, exception: BaseException) -> None:
    """
    Set the exception (low-level internally synchronised).

    Args:
        exception: The value to set as the exception.

    Raises:
        InvalidStateError: If the pending is already done.
    """
    self._set_done("exception", exception)

cancel 

cancel(msg: str | None = None) -> bool

Cancel the pending if it is not already done.

Parameters:

  • msg

    (str | None, default: None ) –

    The cancellation message.

Returns:

  • bool –

    If the pending is marked as cancelled.

Notes
  • Cancellation cannot be undone.
  • Multiple calls append msg to the cancellation message with no other effect.
  • The done state is a function of the canceller, if no canceller is set, done will be set immediately.
Referenced by:
Source code in src/async_kernel/pending.py 
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def cancel(self, msg: str | None = None) -> bool:
    """
    Cancel the pending if it is not already `done`.

    Args:
        msg: The cancellation message.

    Returns:
        If the pending is marked as cancelled.

    Notes:
        - Cancellation cannot be undone.
        - Multiple calls append `msg` to the cancellation message with no other effect.
        - The `done` state is a function of the canceller, if no canceller is set, `done`
            will be set immediately.
    """
    if not self._done:
        if (cancelled := self._cancelled or "") and msg:
            msg = f"{cancelled}\n{msg}"
        self._cancelled = msg or cancelled
        if not (canceller := self._canceller):
            try:
                self._set_done("result", None)
            except InvalidStateError:
                pass
        else:
            canceller(msg)
    return self._cancelled is not None

cancel_wait async 

cancel_wait(msg: str | None, *, timeout: float | None = None) -> None

Cancel the pending and wait for it to be done.

Source code in src/async_kernel/pending.py 
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async def cancel_wait(self, msg: str | None, *, timeout: float | None = None) -> None:
    "Cancel the pending and wait for it to be `done`."
    if not self._done:
        self.cancel(msg)
        if not self._done:
            await self.wait(result=False, timeout=timeout)

cancelled 

cancelled() -> bool

Returns:

  • bool –

    If the pending is marked as cancelled.

Notes:

- This can return `True` before a pending is `done`.
- To wait for a pending to complete (assuming a canceller is or will be set); use
    `await pen.wait(result=False)` or `pen.wait_sync(result=False)`
Source code in src/async_kernel/pending.py 
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def cancelled(self) -> bool:
    """
    Returns:
        If the pending is marked as cancelled.

    Notes:

        - This can return `True` before a pending is `done`.
        - To wait for a pending to complete (assuming a canceller is or will be set); use
            `await pen.wait(result=False)` or `pen.wait_sync(result=False)`
    """
    return self._cancelled is not None

set_canceller 

set_canceller(canceller: Callable[[str | None], Any]) -> None

Set a callback to handle cancellation (low-level).

Parameters:

  • canceller

    (Callable[[str | None], Any]) –

    A callback that performs the cancellation of the pending.

    • It must accept the cancellation message as the first argument.
    • canceller must be externally synchronised.
Warning
  • set_result or set_exception MUST be called by the canceller, or as a result of the call to canceller to mark the pending as done.

Raises:

Example 
pen = Pending()
pen.cancel()
assert not pen.done()
pen.set_canceller(lambda msg: pen.set_result(None))
assert pen.done()
Referenced by:
Source code in src/async_kernel/pending.py 
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def set_canceller(self, canceller: Callable[[str | None], Any], /) -> None:
    """
    Set a callback to handle cancellation (low-level).

    Args:
        canceller: A callback that performs the cancellation of the pending.

            - It must accept the cancellation message as the first argument.
            - `canceller` must be externally synchronised.

    Warning:
        - [set_result][] or [set_exception][] *MUST* be called by the `canceller`, or as a result of the call to `canceller`
            to mark the pending as `done`.

    Raises:
        InvalidStateError: If already `done`.
        PendingCancelled: If already `cancelled`.

    Example:
        ```python
        pen = Pending()
        pen.cancel()
        assert not pen.done()
        pen.set_canceller(lambda msg: pen.set_result(None))
        assert pen.done()
        ```
    """
    if self._done:
        if self._cancelled is not None:
            raise PendingCancelled(self._cancelled)
        raise InvalidStateError
    self._canceller = canceller

done 

done() -> bool

Returns True if a result or exception has been set.

Source code in src/async_kernel/pending.py 
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def done(self) -> bool:
    """
    Returns `True` if a result or exception has been set.
    """
    return self._done

add_done_callback 

add_done_callback(fn: Callable[[Self], Any]) -> None

Add a callback for when the pending is done.

The callback should be light weight and provide its own thread safety.

Callbacks added are handled FIFO.

Referenced by:
Source code in src/async_kernel/pending.py 
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def add_done_callback(self, fn: Callable[[Self], Any], /) -> None:
    """
    Add a callback for when the pending is `done`.

    The callback should be light weight and provide its own thread safety.

    Callbacks added are handled FIFO.
    """
    if not self._done:
        self._done_callbacks.append(fn)
    elif self._done_callbacks:
        # respect FIFO - 'done_callbacks' gets reversed in `_set_done`.
        self._done_callbacks.insert(0, fn)
    else:
        fn(self)

remove_done_callback 

remove_done_callback(fn: Callable[[Self], object]) -> int

Remove fn from the done callback list.

Returns the number of items removed.

Referenced by:
Source code in src/async_kernel/pending.py 
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def remove_done_callback(self, fn: Callable[[Self], object], /) -> int:
    """
    Remove `fn` from the done callback list.

    Returns the number of items removed.
    """
    n = 0
    while fn in self._done_callbacks:
        n += 1
        self._done_callbacks.remove(fn)
    return n

result 

result() -> T

Return the result.

Raises:

Referenced by:
Source code in src/async_kernel/pending.py 
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def result(self) -> T:
    """
    Return the result.

    Raises:
        PendingCancelled: If the pending has been cancelled.
        InvalidStateError: If the pending isn't done yet.
    """
    if e := self.exception():
        raise e from None
    try:
        return self._result
    except AttributeError:
        raise InvalidStateError from None

exception 

exception() -> BaseException | None

Return the exception.

Raises:

Referenced by:
Source code in src/async_kernel/pending.py 
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def exception(self) -> BaseException | None:
    """
    Return the exception.

    Raises:
        PendingCancelled: If the pending has been cancelled.
    """
    if self._cancelled is not None:
        raise PendingCancelled(self._cancelled)
    return getattr(self, "_exception", None)