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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][async_kernel.pending.Pending] is cancelled."

PendingNotDone

Bases: RuntimeError

Used to indicate the Pending is not done.

Raised by:
Source code in src/async_kernel/pending.py 
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class PendingNotDone(RuntimeError):
    "Used to indicate the [Pending][async_kernel.pending.Pending] is not done."

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 –

    Add pen to the pending set Pending.

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:
        """
        Add `pen` to the pending set [Pending][async_kernel.pending.Pending].

        `pen` is automatically removed from the set once it is done.
        """

        if not pen.done() and 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

Add pen to the pending set Pending.

pen is automatically removed from the set once it is done.

Source code in src/async_kernel/pending.py 
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def add(self, pen: Pending) -> None:
    """
    Add `pen` to the pending set [Pending][async_kernel.pending.Pending].

    `pen` is automatically removed from the set once it is done.
    """

    if not pen.done() and 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][async_kernel.pending.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][async_kernel.pending.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][async_kernel.pending.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][async_kernel.pending.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: Literal[0, 1, 2, 3] = 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, if any pending is cancelled - exit quietly.
                - 1: Cancel if any pending is cancelled - raise PendingCancelled on exit.
                - 2: Cancel if any pending is cancelled - exit quietly.
                - 3: Ignore cancellation of pending, if any pending is cancelled - raise PendingCancelled on exit.
        """
        assert mode in [0, 1, 2, 3]
        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 RuntimeError(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 [1, 3]:
                    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: Literal[0, 1, 2, 3] = 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

    (Literal[0, 1, 2, 3], default: 0 ) –

    The mode. - 0: Ignore cancellation of pending, if any pending is cancelled - exit quietly. - 1: Cancel if any pending is cancelled - raise PendingCancelled on exit. - 2: Cancel if any pending is cancelled - exit quietly. - 3: Ignore cancellation of pending, if any pending is cancelled - raise PendingCancelled on exit.

Source code in src/async_kernel/pending.py 
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def __init__(self, *, shield: bool = False, mode: Literal[0, 1, 2, 3] = 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, if any pending is cancelled - exit quietly.
            - 1: Cancel if any pending is cancelled - raise PendingCancelled on exit.
            - 2: Cancel if any pending is cancelled - exit quietly.
            - 3: Ignore cancellation of pending, if any pending is cancelled - raise PendingCancelled on exit.
    """
    assert mode in [0, 1, 2, 3]
    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, waitable/awaitable representation of a pending result.

It can be thought of as a hybrid mixture of asyncio.Future and concurrent.futures.Future.

Features

  • It can be wait/awaited and cancelled from any thread (not considering deadlocks) and event loop.
  • 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 if the pending is not already done (low-level internally synchronised).

  • set_exception –

    Set the exception if the pending is not already done (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 (low-level).

  • 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, waitable/awaitable representation of a pending result.

    It can be thought of as a hybrid mixture of [asyncio.Future][] and [concurrent.futures.Future][].

    **Features**

    - It can be wait/awaited and cancelled from any thread (not considering deadlocks) and event loop.
    - 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.
    - [Pending.cancel_wait][]: Cancel the pending and wait for it to be done.
    - [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,
            shield: bool = ...,
        ) -> T: ...

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

    async def wait(
        self,
        *,
        timeout: float | None = None,
        protect: bool = False,
        result: bool = True,
        shield: bool = False,
    ) -> 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.
            shield: Shield from external cancellation.

        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:
                waiter = create_async_waiter(shield=shield)
                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:
            event = create_green_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, result: bool, value, /) -> None:
        if self._done:
            return
        self._done = True
        self._canceller = None
        e = None
        try:
            callbacks = self._done_callbacks
            del self._done_callbacks
        except AttributeError:
            return
        if result:
            self._result = value
        else:
            self._exception = value
        # List reversal and BaseException handling inspiration: https://gist.github.com/x42005e1f/4f18c3c62da9135020bdea8c44c248a2
        callbacks.reverse()
        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 if the pending is not already done (low-level internally synchronised).

        Args:
            value: The result to set.
        """
        self._set_done(True, value)

    def set_exception(self, exception: BaseException) -> None:
        """
        Set the exception if the pending is not already done (low-level internally synchronised).

        Args:
            exception: The value to set as the exception.
        """
        self._set_done(False, 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 (canceller := self._canceller) is None:
                self._set_done(False, None)
            else:
                canceller(msg)
        return self._cancelled is not None

    async def cancel_wait(self, msg: str | None = None, *, timeout: float | None = None, shield: bool = False) -> None:
        """
        Cancel the pending and wait for it to be done.

        Args:
            timeout: Timeout in seconds.
            shield: Shield from external cancellation.
        """
        if not self._done:
            self.cancel(msg)
            if not self._done:
                await self.wait(result=False, timeout=timeout, shield=shield)

    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.
                - [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.
        """
        if not self._done:
            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 (low-level).

        Notes:
            - If the pending is:
                - *not* done: `fn` is added to the callback list.
                - done: `fn` is called immediately.
            - Callbacks are called FIFO in the thread where the pending is set done (potentially any thread).
            - The callback should be light weight and provide its own thread safety.
        """
        if not self._done and (callbacks := getattr(self, "_done_callbacks", None)) is not None:
            callbacks.append(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
        if not self._done and (callbacks := getattr(self, "_done_callbacks", None)):
            while True:
                try:
                    callbacks.remove(fn)
                    n += 1
                except ValueError:
                    break
        return n

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

        Raises:
            PendingCancelled: If the pending has been cancelled.
            PendingNotDone: If the pending isn't done yet.
        """
        if self._cancelled is not None:
            raise PendingCancelled(self._cancelled)
        if not self._done:
            raise PendingNotDone
        try:
            return self._result
        except AttributeError:
            e = getattr(self, "_exception", RuntimeError)
            raise e from None

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

        Raises:
            PendingCancelled: If the pending has been cancelled.
            PendingNotDone: If the pending isn't done yet.
        """
        if self._cancelled is not None:
            raise PendingCancelled(self._cancelled)
        if not self._done:
            raise PendingNotDone
        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,
    shield: bool = ...,
) -> T

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

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

  • shield

    (bool, default: False ) –

    Shield from external cancellation.

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,
    shield: bool = False,
) -> 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.
        shield: Shield from external cancellation.

    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:
            waiter = create_async_waiter(shield=shield)
            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:
        event = create_green_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 if the pending is not already done (low-level internally synchronised).

Parameters:

  • value

    (T) –

    The result to set.

Referenced by:
Source code in src/async_kernel/pending.py 
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def set_result(self, value: T) -> None:
    """
    Set the result if the pending is not already done (low-level internally synchronised).

    Args:
        value: The result to set.
    """
    self._set_done(True, value)

set_exception 

set_exception(exception: BaseException) -> None

Set the exception if the pending is not already done (low-level internally synchronised).

Parameters:

Referenced by:
Source code in src/async_kernel/pending.py 
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def set_exception(self, exception: BaseException) -> None:
    """
    Set the exception if the pending is not already done (low-level internally synchronised).

    Args:
        exception: The value to set as the exception.
    """
    self._set_done(False, 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 (canceller := self._canceller) is None:
            self._set_done(False, None)
        else:
            canceller(msg)
    return self._cancelled is not None

cancel_wait async 

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

Cancel the pending and wait for it to be done.

Parameters:

  • timeout

    (float | None, default: None ) –

    Timeout in seconds.

  • shield

    (bool, default: False ) –

    Shield from external cancellation.

Referenced by:
Source code in src/async_kernel/pending.py 
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async def cancel_wait(self, msg: str | None = None, *, timeout: float | None = None, shield: bool = False) -> None:
    """
    Cancel the pending and wait for it to be done.

    Args:
        timeout: Timeout in seconds.
        shield: Shield from external cancellation.
    """
    if not self._done:
        self.cancel(msg)
        if not self._done:
            await self.wait(result=False, timeout=timeout, shield=shield)

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.
    • 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.
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.
            - [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.
    """
    if not self._done:
        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 (low-level).

Notes
  • If the pending is:
    • not done: fn is added to the callback list.
    • done: fn is called immediately.
  • Callbacks are called FIFO in the thread where the pending is set done (potentially any thread).
  • The callback should be light weight and provide its own thread safety.
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 (low-level).

    Notes:
        - If the pending is:
            - *not* done: `fn` is added to the callback list.
            - done: `fn` is called immediately.
        - Callbacks are called FIFO in the thread where the pending is set done (potentially any thread).
        - The callback should be light weight and provide its own thread safety.
    """
    if not self._done and (callbacks := getattr(self, "_done_callbacks", None)) is not None:
        callbacks.append(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
    if not self._done and (callbacks := getattr(self, "_done_callbacks", None)):
        while True:
            try:
                callbacks.remove(fn)
                n += 1
            except ValueError:
                break
    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.
        PendingNotDone: If the pending isn't done yet.
    """
    if self._cancelled is not None:
        raise PendingCancelled(self._cancelled)
    if not self._done:
        raise PendingNotDone
    try:
        return self._result
    except AttributeError:
        e = getattr(self, "_exception", RuntimeError)
        raise e 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.
        PendingNotDone: If the pending isn't done yet.
    """
    if self._cancelled is not None:
        raise PendingCancelled(self._cancelled)
    if not self._done:
        raise PendingNotDone
    return getattr(self, "_exception", None)