diff --git a/InternalDocs/asyncio.md b/InternalDocs/asyncio.md
index b60fe70478a6bc..5dfe155425e8e0 100644
--- a/InternalDocs/asyncio.md
+++ b/InternalDocs/asyncio.md
@@ -2,10 +2,12 @@ asyncio
=======
-This document describes the working and implementation details of C
-implementation of the
+This document describes the working and implementation details of the
[`asyncio`](https://docs.python.org/3/library/asyncio.html) module.
+**This section describes the implementation details of the C implementation**.
+
+# Task management
## Pre-Python 3.14 implementation
@@ -158,7 +160,8 @@ flowchart TD
subgraph two["Thread deallocating"]
A1{"thread's task list empty?
llist_empty(tstate->asyncio_tasks_head)"}
A1 --> |true| B1["deallocate thread
free_threadstate(tstate)"]
- A1 --> |false| C1["add tasks to interpreter's task list
llist_concat(&tstate->interp->asyncio_tasks_head,tstate->asyncio_tasks_head)"]
+ A1 --> |false| C1["add tasks to interpreter's task list
llist_concat(&tstate->interp->asyncio_tasks_head,
+ &tstate->asyncio_tasks_head)"]
C1 --> B1
end
@@ -205,6 +208,121 @@ In free-threading, it avoids contention on a global dictionary as
threads can access the current task of thier running loop without any
locking.
+---
+
+**This section describes the implementation details of the Python implementation**.
+
+# async generators
+
+This section describes the implementation details of async generators in `asyncio`.
+
+Since async generators are meant to be used from coroutines,
+their finalization (execution of finally blocks) needs
+to be done while the loop is running.
+Most async generators are closed automatically
+when they are fully iterated over and exhausted; however,
+if the async generator is not fully iterated over,
+it may not be closed properly, leading to the `finally` blocks not being executed.
+
+Consider the following code:
+```py
+import asyncio
+
+async def agen():
+ try:
+ yield 1
+ finally:
+ await asyncio.sleep(1)
+ print("finally executed")
+
+
+async def main():
+ async for i in agen():
+ break
+
+loop = asyncio.EventLoop()
+loop.run_until_complete(main())
+```
+
+The above code will not print "finally executed", because the
+async generator `agen` is not fully iterated over
+and it is not closed manually by awaiting `agen.aclose()`.
+
+To solve this, `asyncio` uses the `sys.set_asyncgen_hooks` function to
+set hooks for finalizing async generators as described in
+[PEP 525](https://peps.python.org/pep-0525/).
+
+- **firstiter hook**: When the async generator is iterated over for the first time,
+the *firstiter hook* is called. The async generator is added to `loop._asyncgens` WeakSet
+and the event loop tracks all active async generators.
+
+- **finalizer hook**: When the async generator is about to be finalized,
+the *finalizer hook* is called. The event loop removes the async generator
+from `loop._asyncgens` WeakSet, and schedules the finalization of the async
+generator by creating a task calling `agen.aclose()`. This ensures that the
+finally block is executed while the event loop is running. When the loop is
+shutting down, the loop checks if there are active async generators and if so,
+it similarly schedules the finalization of all active async generators by calling
+`agen.aclose()` on each of them and waits for them to complete before shutting
+down the loop.
+
+This ensures that the async generator's `finally` blocks are executed even
+if the generator is not explicitly closed.
+
+Consider the following example:
+
+```python
+import asyncio
+
+async def agen():
+ try:
+ yield 1
+ yield 2
+ finally:
+ print("executing finally block")
+
+async def main():
+ async for item in agen():
+ print(item)
+ break # not fully iterated
+
+asyncio.run(main())
+```
+
+```mermaid
+flowchart TD
+ subgraph one["Loop running"]
+ A["asyncio.run(main())"] --> B
+ B["set async generator hooks
sys.set_asyncgen_hooks()"] --> C
+ C["async for item in agen"] --> F
+ F{"first iteration?"} --> |true|D
+ F{"first iteration?"} --> |false|H
+ D["calls firstiter hook
loop._asyncgen_firstiter_hook(agen)"] --> E
+ E["add agen to WeakSet
loop._asyncgens.add(agen)"] --> H
+ H["item = await agen.\_\_anext\_\_()"] --> J
+ J{"StopAsyncIteration?"} --> |true|M
+ J{"StopAsyncIteration?"} --> |false|I
+ I["print(item)"] --> S
+ S{"continue iterating?"} --> |true|C
+ S{"continue iterating?"} --> |false|M
+ M{"agen is no longer referenced?"} --> |true|N
+ M{"agen is no longer referenced?"} --> |false|two
+ N["finalize agen
_PyGen_Finalize(agen)"] --> O
+ O["calls finalizer hook
loop._asyncgen_finalizer_hook(agen)"] --> P
+ P["remove agen from WeakSet
loop._asyncgens.discard(agen)"] --> Q
+ Q["schedule task to close it
self.create_task(agen.aclose())"] --> R
+ R["print('executing finally block')"] --> E1
+
+ end
+
+ subgraph two["Loop shutting down"]
+ A1{"check for alive async generators?"} --> |true|B1
+ B1["close all async generators
await asyncio.gather\(*\[ag.aclose\(\) for ag in loop._asyncgens\]"] --> R
+ A1{"check for alive async generators?"} --> |false|E1
+ E1["loop.close()"]
+ end
+
+```
[^1]: https://github.com/python/cpython/issues/123089
-[^2]: https://github.com/python/cpython/issues/80788
\ No newline at end of file
+[^2]: https://github.com/python/cpython/issues/80788