"""
Adapted from
https://github.com/maxfischer2781/asyncstdlib/blob/master/asyncstdlib/itertools.py
MIT License
"""
from collections import deque
from typing import (
Any,
AsyncContextManager,
AsyncGenerator,
AsyncIterator,
Awaitable,
Callable,
Deque,
Generic,
Iterator,
List,
Optional,
Tuple,
TypeVar,
Union,
cast,
overload,
)
T = TypeVar("T")
_no_default = object()
# https://github.com/python/cpython/blob/main/Lib/test/test_asyncgen.py#L54
# before 3.10, the builtin anext() was not available
[docs]def py_anext(
iterator: AsyncIterator[T], default: Union[T, Any] = _no_default
) -> Awaitable[Union[T, None, Any]]:
"""Pure-Python implementation of anext() for testing purposes.
Closely matches the builtin anext() C implementation.
Can be used to compare the built-in implementation of the inner
coroutines machinery to C-implementation of __anext__() and send()
or throw() on the returned generator.
"""
try:
__anext__ = cast(
Callable[[AsyncIterator[T]], Awaitable[T]], type(iterator).__anext__
)
except AttributeError:
raise TypeError(f"{iterator!r} is not an async iterator")
if default is _no_default:
return __anext__(iterator)
async def anext_impl() -> Union[T, Any]:
try:
# The C code is way more low-level than this, as it implements
# all methods of the iterator protocol. In this implementation
# we're relying on higher-level coroutine concepts, but that's
# exactly what we want -- crosstest pure-Python high-level
# implementation and low-level C anext() iterators.
return await __anext__(iterator)
except StopAsyncIteration:
return default
return anext_impl()
[docs]class NoLock:
"""Dummy lock that provides the proper interface but no protection"""
async def __aenter__(self) -> None:
pass
async def __aexit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> bool:
return False
[docs]async def tee_peer(
iterator: AsyncIterator[T],
# the buffer specific to this peer
buffer: Deque[T],
# the buffers of all peers, including our own
peers: List[Deque[T]],
lock: AsyncContextManager[Any],
) -> AsyncGenerator[T, None]:
"""An individual iterator of a :py:func:`~.tee`"""
try:
while True:
if not buffer:
async with lock:
# Another peer produced an item while we were waiting for the lock.
# Proceed with the next loop iteration to yield the item.
if buffer:
continue
try:
item = await iterator.__anext__()
except StopAsyncIteration:
break
else:
# Append to all buffers, including our own. We'll fetch our
# item from the buffer again, instead of yielding it directly.
# This ensures the proper item ordering if any of our peers
# are fetching items concurrently. They may have buffered their
# item already.
for peer_buffer in peers:
peer_buffer.append(item)
yield buffer.popleft()
finally:
async with lock:
# this peer is done – remove its buffer
for idx, peer_buffer in enumerate(peers): # pragma: no branch
if peer_buffer is buffer:
peers.pop(idx)
break
# if we are the last peer, try and close the iterator
if not peers and hasattr(iterator, "aclose"):
await iterator.aclose()
[docs]class Tee(Generic[T]):
"""
Create ``n`` separate asynchronous iterators over ``iterable``
This splits a single ``iterable`` into multiple iterators, each providing
the same items in the same order.
All child iterators may advance separately but share the same items
from ``iterable`` -- when the most advanced iterator retrieves an item,
it is buffered until the least advanced iterator has yielded it as well.
A ``tee`` works lazily and can handle an infinite ``iterable``, provided
that all iterators advance.
.. code-block:: python3
async def derivative(sensor_data):
previous, current = a.tee(sensor_data, n=2)
await a.anext(previous) # advance one iterator
return a.map(operator.sub, previous, current)
Unlike :py:func:`itertools.tee`, :py:func:`~.tee` returns a custom type instead
of a :py:class:`tuple`. Like a tuple, it can be indexed, iterated and unpacked
to get the child iterators. In addition, its :py:meth:`~.tee.aclose` method
immediately closes all children, and it can be used in an ``async with`` context
for the same effect.
If ``iterable`` is an iterator and read elsewhere, ``tee`` will *not*
provide these items. Also, ``tee`` must internally buffer each item until the
last iterator has yielded it; if the most and least advanced iterator differ
by most data, using a :py:class:`list` is more efficient (but not lazy).
If the underlying iterable is concurrency safe (``anext`` may be awaited
concurrently) the resulting iterators are concurrency safe as well. Otherwise,
the iterators are safe if there is only ever one single "most advanced" iterator.
To enforce sequential use of ``anext``, provide a ``lock``
- e.g. an :py:class:`asyncio.Lock` instance in an :py:mod:`asyncio` application -
and access is automatically synchronised.
"""
[docs] def __init__(
self,
iterable: AsyncIterator[T],
n: int = 2,
*,
lock: Optional[AsyncContextManager[Any]] = None,
):
self._iterator = iterable.__aiter__() # before 3.10 aiter() doesn't exist
self._buffers: List[Deque[T]] = [deque() for _ in range(n)]
self._children = tuple(
tee_peer(
iterator=self._iterator,
buffer=buffer,
peers=self._buffers,
lock=lock if lock is not None else NoLock(),
)
for buffer in self._buffers
)
def __len__(self) -> int:
return len(self._children)
@overload
def __getitem__(self, item: int) -> AsyncIterator[T]:
...
@overload
def __getitem__(self, item: slice) -> Tuple[AsyncIterator[T], ...]:
...
def __getitem__(
self, item: Union[int, slice]
) -> Union[AsyncIterator[T], Tuple[AsyncIterator[T], ...]]:
return self._children[item]
def __iter__(self) -> Iterator[AsyncIterator[T]]:
yield from self._children
async def __aenter__(self) -> "Tee[T]":
return self
async def __aexit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> bool:
await self.aclose()
return False
[docs] async def aclose(self) -> None:
for child in self._children:
await child.aclose()
atee = Tee