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from __future__ import annotations
__all__ = (
"EventLoopToken",
"RunvarToken",
"RunVar",
"checkpoint",
"checkpoint_if_cancelled",
"cancel_shielded_checkpoint",
"current_token",
)
import enum
from dataclasses import dataclass
from types import TracebackType
from typing import Any, Generic, Literal, TypeVar, final, overload
from weakref import WeakKeyDictionary
from ._core._eventloop import get_async_backend
from .abc import AsyncBackend
T = TypeVar("T")
D = TypeVar("D")
async def checkpoint() -> None:
"""
Check for cancellation and allow the scheduler to switch to another task.
Equivalent to (but more efficient than)::
await checkpoint_if_cancelled()
await cancel_shielded_checkpoint()
.. versionadded:: 3.0
"""
await get_async_backend().checkpoint()
async def checkpoint_if_cancelled() -> None:
"""
Enter a checkpoint if the enclosing cancel scope has been cancelled.
This does not allow the scheduler to switch to a different task.
.. versionadded:: 3.0
"""
await get_async_backend().checkpoint_if_cancelled()
async def cancel_shielded_checkpoint() -> None:
"""
Allow the scheduler to switch to another task but without checking for cancellation.
Equivalent to (but potentially more efficient than)::
with CancelScope(shield=True):
await checkpoint()
.. versionadded:: 3.0
"""
await get_async_backend().cancel_shielded_checkpoint()
@final
@dataclass(frozen=True, repr=False)
class EventLoopToken:
"""
An opaque object that holds a reference to an event loop.
.. versionadded:: 4.11.0
"""
backend_class: type[AsyncBackend]
native_token: object
def current_token() -> EventLoopToken:
"""
Return a token object that can be used to call code in the current event loop from
another thread.
:raises NoEventLoopError: if no supported asynchronous event loop is running in the
current thread
.. versionadded:: 4.11.0
"""
backend_class = get_async_backend()
raw_token = backend_class.current_token()
return EventLoopToken(backend_class, raw_token)
_run_vars: WeakKeyDictionary[object, dict[RunVar[Any], Any]] = WeakKeyDictionary()
class _NoValueSet(enum.Enum):
NO_VALUE_SET = enum.auto()
class RunvarToken(Generic[T]):
__slots__ = "_var", "_value", "_redeemed"
def __init__(self, var: RunVar[T], value: T | Literal[_NoValueSet.NO_VALUE_SET]):
self._var = var
self._value: T | Literal[_NoValueSet.NO_VALUE_SET] = value
self._redeemed = False
def __enter__(self) -> RunvarToken[T]:
return self
def __exit__(
self,
exc_type: type[BaseException] | None,
exc_val: BaseException | None,
exc_tb: TracebackType | None,
) -> None:
self._var.reset(self)
class RunVar(Generic[T]):
"""
Like a :class:`~contextvars.ContextVar`, except scoped to the running event loop.
Can be used as a context manager, Just like :class:`~contextvars.ContextVar`, that
will reset the variable to its previous value when the context block is exited.
"""
__slots__ = "_name", "_default"
NO_VALUE_SET: Literal[_NoValueSet.NO_VALUE_SET] = _NoValueSet.NO_VALUE_SET
def __init__(
self, name: str, default: T | Literal[_NoValueSet.NO_VALUE_SET] = NO_VALUE_SET
):
self._name = name
self._default = default
@property
def _current_vars(self) -> dict[RunVar[T], T]:
native_token = current_token().native_token
try:
return _run_vars[native_token]
except KeyError:
run_vars = _run_vars[native_token] = {}
return run_vars
@overload
def get(self, default: D) -> T | D: ...
@overload
def get(self) -> T: ...
def get(
self, default: D | Literal[_NoValueSet.NO_VALUE_SET] = NO_VALUE_SET
) -> T | D:
try:
return self._current_vars[self]
except KeyError:
if default is not RunVar.NO_VALUE_SET:
return default
elif self._default is not RunVar.NO_VALUE_SET:
return self._default
raise LookupError(
f'Run variable "{self._name}" has no value and no default set'
)
def set(self, value: T) -> RunvarToken[T]:
current_vars = self._current_vars
token = RunvarToken(self, current_vars.get(self, RunVar.NO_VALUE_SET))
current_vars[self] = value
return token
def reset(self, token: RunvarToken[T]) -> None:
if token._var is not self:
raise ValueError("This token does not belong to this RunVar")
if token._redeemed:
raise ValueError("This token has already been used")
if token._value is _NoValueSet.NO_VALUE_SET:
try:
del self._current_vars[self]
except KeyError:
pass
else:
self._current_vars[self] = token._value
token._redeemed = True
def __repr__(self) -> str:
return f"<RunVar name={self._name!r}>"
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