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from __future__ import annotations
import enum
from dataclasses import dataclass
from typing import Any, Generic, Literal, TypeVar, overload
from weakref import WeakKeyDictionary
from ._core._eventloop import get_async_backend
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()
def current_token() -> object:
"""
Return a backend specific token object that can be used to get back to the event
loop.
"""
return get_async_backend().current_token()
_run_vars: WeakKeyDictionary[Any, dict[str, Any]] = WeakKeyDictionary()
_token_wrappers: dict[Any, _TokenWrapper] = {}
@dataclass(frozen=True)
class _TokenWrapper:
__slots__ = "_token", "__weakref__"
_token: object
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
class RunVar(Generic[T]):
"""
Like a :class:`~contextvars.ContextVar`, except scoped to the running event loop.
"""
__slots__ = "_name", "_default"
NO_VALUE_SET: Literal[_NoValueSet.NO_VALUE_SET] = _NoValueSet.NO_VALUE_SET
_token_wrappers: set[_TokenWrapper] = 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[str, T]:
token = current_token()
try:
return _run_vars[token]
except KeyError:
run_vars = _run_vars[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._name]
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._name, RunVar.NO_VALUE_SET))
current_vars[self._name] = 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._name]
except KeyError:
pass
else:
self._current_vars[self._name] = token._value
token._redeemed = True
def __repr__(self) -> str:
return f"<RunVar name={self._name!r}>"
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