bpo-46771: Implement asyncio context managers for handling timeouts (GH-31394)

Example:

async with asyncio.timeout(5):
    await some_task()

Will interrupt the await and raise TimeoutError if some_task() takes longer than 5 seconds.

Co-authored-by: Guido van Rossum <guido@python.org>

Lib/asyncio/timeouts.py

@@ -0,0 +1,151 @@
+import enum
+
+from types import TracebackType
+from typing import final, Optional, Type
+
+from . import events
+from . import exceptions
+from . import tasks
+
+
+__all__ = (
+    "Timeout",
+    "timeout",
+    "timeout_at",
+)
+
+
+class _State(enum.Enum):
+    CREATED = "created"
+    ENTERED = "active"
+    EXPIRING = "expiring"
+    EXPIRED = "expired"
+    EXITED = "finished"
+
+
+@final
+class Timeout:
+
+    def __init__(self, when: Optional[float]) -> None:
+        self._state = _State.CREATED
+
+        self._timeout_handler: Optional[events.TimerHandle] = None
+        self._task: Optional[tasks.Task] = None
+        self._when = when
+
+    def when(self) -> Optional[float]:
+        return self._when
+
+    def reschedule(self, when: Optional[float]) -> None:
+        assert self._state is not _State.CREATED
+        if self._state is not _State.ENTERED:
+            raise RuntimeError(
+                f"Cannot change state of {self._state.value} Timeout",
+            )
+
+        self._when = when
+
+        if self._timeout_handler is not None:
+            self._timeout_handler.cancel()
+
+        if when is None:
+            self._timeout_handler = None
+        else:
+            loop = events.get_running_loop()
+            self._timeout_handler = loop.call_at(
+                when,
+                self._on_timeout,
+            )
+
+    def expired(self) -> bool:
+        """Is timeout expired during execution?"""
+        return self._state in (_State.EXPIRING, _State.EXPIRED)
+
+    def __repr__(self) -> str:
+        info = ['']
+        if self._state is _State.ENTERED:
+            when = round(self._when, 3) if self._when is not None else None
+            info.append(f"when={when}")
+        info_str = ' '.join(info)
+        return f"<Timeout [{self._state.value}]{info_str}>"
+
+    async def __aenter__(self) -> "Timeout":
+        self._state = _State.ENTERED
+        self._task = tasks.current_task()
+        if self._task is None:
+            raise RuntimeError("Timeout should be used inside a task")
+        self.reschedule(self._when)
+        return self
+
+    async def __aexit__(
+        self,
+        exc_type: Optional[Type[BaseException]],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[TracebackType],
+    ) -> Optional[bool]:
+        assert self._state in (_State.ENTERED, _State.EXPIRING)
+
+        if self._timeout_handler is not None:
+            self._timeout_handler.cancel()
+            self._timeout_handler = None
+
+        if self._state is _State.EXPIRING:
+            self._state = _State.EXPIRED
+
+            if self._task.uncancel() == 0 and exc_type is exceptions.CancelledError:
+                # Since there are no outstanding cancel requests, we're
+                # handling this.
+                raise TimeoutError
+        elif self._state is _State.ENTERED:
+            self._state = _State.EXITED
+
+        return None
+
+    def _on_timeout(self) -> None:
+        assert self._state is _State.ENTERED
+        self._task.cancel()
+        self._state = _State.EXPIRING
+        # drop the reference early
+        self._timeout_handler = None
+
+
+def timeout(delay: Optional[float]) -> Timeout:
+    """Timeout async context manager.
+
+    Useful in cases when you want to apply timeout logic around block
+    of code or in cases when asyncio.wait_for is not suitable. For example:
+
+    >>> async with asyncio.timeout(10):  # 10 seconds timeout
+    ...     await long_running_task()
+
+
+    delay - value in seconds or None to disable timeout logic
+
+    long_running_task() is interrupted by raising asyncio.CancelledError,
+    the top-most affected timeout() context manager converts CancelledError
+    into TimeoutError.
+    """
+    loop = events.get_running_loop()
+    return Timeout(loop.time() + delay if delay is not None else None)
+
+
+def timeout_at(when: Optional[float]) -> Timeout:
+    """Schedule the timeout at absolute time.
+
+    Like timeout() but argument gives absolute time in the same clock system
+    as loop.time().
+
+    Please note: it is not POSIX time but a time with
+    undefined starting base, e.g. the time of the system power on.
+
+    >>> async with asyncio.timeout_at(loop.time() + 10):
+    ...     await long_running_task()
+
+
+    when - a deadline when timeout occurs or None to disable timeout logic
+
+    long_running_task() is interrupted by raising asyncio.CancelledError,
+    the top-most affected timeout() context manager converts CancelledError
+    into TimeoutError.
+    """
+    return Timeout(when)