Issue #28720: Add collections.abc.AsyncGenerator.

1 files changed, 58 insertions, 1 deletions

diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index ee7d0f1..b172f3f 100644
--- a/Lib/_collections_abc.py
+++ b/Lib/_collections_abc.py
@@ -9,7 +9,8 @@ Unit tests are in test_collections.
 from abc import ABCMeta, abstractmethod
 import sys
 
-__all__ = ["Awaitable", "Coroutine", "AsyncIterable", "AsyncIterator",
+__all__ = ["Awaitable", "Coroutine",
+           "AsyncIterable", "AsyncIterator", "AsyncGenerator",
            "Hashable", "Iterable", "Iterator", "Generator", "Reversible",
            "Sized", "Container", "Callable", "Collection",
            "Set", "MutableSet",
@@ -59,6 +60,11 @@ _coro = _coro()
 coroutine = type(_coro)
 _coro.close()  # Prevent ResourceWarning
 del _coro
+## asynchronous generator ##
+async def _ag(): yield
+_ag = _ag()
+async_generator = type(_ag)
+del _ag
 
 
 ### ONE-TRICK PONIES ###
@@ -183,6 +189,57 @@ class AsyncIterator(AsyncIterable):
         return NotImplemented
 
 
+class AsyncGenerator(AsyncIterator):
+
+    __slots__ = ()
+
+    async def __anext__(self):
+        """Return the next item from the asynchronous generator.
+        When exhausted, raise StopAsyncIteration.
+        """
+        return await self.asend(None)
+
+    @abstractmethod
+    async def asend(self, value):
+        """Send a value into the asynchronous generator.
+        Return next yielded value or raise StopAsyncIteration.
+        """
+        raise StopAsyncIteration
+
+    @abstractmethod
+    async def athrow(self, typ, val=None, tb=None):
+        """Raise an exception in the asynchronous generator.
+        Return next yielded value or raise StopAsyncIteration.
+        """
+        if val is None:
+            if tb is None:
+                raise typ
+            val = typ()
+        if tb is not None:
+            val = val.with_traceback(tb)
+        raise val
+
+    async def aclose(self):
+        """Raise GeneratorExit inside coroutine.
+        """
+        try:
+            await self.athrow(GeneratorExit)
+        except (GeneratorExit, StopAsyncIteration):
+            pass
+        else:
+            raise RuntimeError("asynchronous generator ignored GeneratorExit")
+
+    @classmethod
+    def __subclasshook__(cls, C):
+        if cls is AsyncGenerator:
+            return _check_methods(C, '__aiter__', '__anext__',
+                                  'asend', 'athrow', 'aclose')
+        return NotImplemented
+
+
+AsyncGenerator.register(async_generator)
+
+
 class Iterable(metaclass=ABCMeta):
 
     __slots__ = ()