From 2df96d0247cbf512c24e4d463369398e602e689e Mon Sep 17 00:00:00 2001
From: DontPanicO <dontpanic.call0@gmail.com>
Date: Tue, 7 Feb 2023 17:13:39 +0000
Subject: [PATCH 1/4] add `defer_errors` flag to `asyncio.TaskGroup`

`asyncio.TaskGroup` cancels all child tasks when one raises. Setting `defer_errors=True` will prevent this
---
 Lib/asyncio/taskgroups.py | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/Lib/asyncio/taskgroups.py b/Lib/asyncio/taskgroups.py
index 911419e1769c17..b00ba87ad988e1 100644
--- a/Lib/asyncio/taskgroups.py
+++ b/Lib/asyncio/taskgroups.py
@@ -11,7 +11,7 @@
 
 class TaskGroup:
 
-    def __init__(self):
+    def __init__(self, defer_errors=False):
         self._entered = False
         self._exiting = False
         self._aborting = False
@@ -22,6 +22,7 @@ def __init__(self):
         self._errors = []
         self._base_error = None
         self._on_completed_fut = None
+        self._defer_errors = defer_errors
 
     def __repr__(self):
         info = ['']
@@ -180,7 +181,8 @@ def _on_task_done(self, task):
             return
 
         self._errors.append(exc)
-        if self._is_base_error(exc) and self._base_error is None:
+        is_base_error = self._is_base_error(exc)
+        if is_base_error and self._base_error is None:
             self._base_error = exc
 
         if self._parent_task.done():
@@ -213,6 +215,7 @@ def _on_task_done(self, task):
             #            pass
             #        await something_else     # this line has to be called
             #                                 # after TaskGroup is finished.
-            self._abort()
-            self._parent_cancel_requested = True
-            self._parent_task.cancel()
+            if not self._defer_errors or is_base_error:
+                self._abort()
+                self._parent_cancel_requested = True
+                self._parent_task.cancel()

From 90e58aacadd68f102d42a07246f6968aa10723dc Mon Sep 17 00:00:00 2001
From: DontPanicO <dontpanic.call0@gmail.com>
Date: Tue, 7 Feb 2023 17:14:38 +0000
Subject: [PATCH 2/4] tests for `asyncio.TaskGroup` with `defer_errors=True`

---
 Lib/test/test_asyncio/test_taskgroups.py | 434 +++++++++++++++++++++++
 1 file changed, 434 insertions(+)

diff --git a/Lib/test/test_asyncio/test_taskgroups.py b/Lib/test/test_asyncio/test_taskgroups.py
index 6a0231f2859a62..55357a803c7d3f 100644
--- a/Lib/test/test_asyncio/test_taskgroups.py
+++ b/Lib/test/test_asyncio/test_taskgroups.py
@@ -779,6 +779,440 @@ async def main():
 
         await asyncio.create_task(main())
 
+    async def test_children_complete_on_child_error(self):
+        async def zero_division():
+            1 / 0
+
+        async def foo1():
+            await asyncio.sleep(0.1)
+            return 42
+
+        async def foo2():
+            await asyncio.sleep(0.2)
+            return 11
+
+        with self.assertRaises(ExceptionGroup) as cm:
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                t1 = g.create_task(foo1())
+                t2 = g.create_task(foo2())
+                g.create_task(zero_division())
+
+        self.assertEqual(get_error_types(cm.exception), {ZeroDivisionError})
+
+        self.assertEqual(t1.result(), 42)
+        self.assertEqual(t2.result(), 11)
+
+    async def test_inner_complete_on_child_error(self):
+        async def zero_division():
+            1 / 0
+
+        async def foo1():
+            await asyncio.sleep(0.1)
+            return 42
+
+        async def foo2():
+            await asyncio.sleep(0.2)
+            return 11
+
+        with self.assertRaises(ExceptionGroup) as cm:
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                t1 = g.create_task(foo1())
+                g.create_task(zero_division())
+                r1 = await foo2()
+
+        self.assertEqual(get_error_types(cm.exception), {ZeroDivisionError})
+
+        self.assertEqual(t1.result(), 42)
+        self.assertEqual(r1, 11)
+
+    async def test_children_exceptions_propagate(self):
+        async def zero_division():
+            1 / 0
+
+        async def value_error():
+            await asyncio.sleep(0.2)
+            raise ValueError
+
+        async def foo1():
+            await asyncio.sleep(0.4)
+            return 42
+
+        with self.assertRaises(ExceptionGroup) as cm:
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                g.create_task(zero_division())
+                g.create_task(value_error())
+                t1 = g.create_task(foo1())
+
+        self.assertEqual(get_error_types(cm.exception), {ZeroDivisionError, ValueError})
+
+        self.assertEqual(t1.result(), 42)
+
+    async def test_children_cancel_on_inner_failure(self):
+        async def zero_division():
+            1 / 0
+
+        async def foo1():
+            await asyncio.sleep(0.2)
+            return 42
+
+        with self.assertRaises(ExceptionGroup) as cm:
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                t1 = g.create_task(foo1())
+                await zero_division()
+
+        self.assertEqual(get_error_types(cm.exception), {ZeroDivisionError})
+
+        self.assertTrue(t1.cancelled())
+
+    async def test_cancellation_01(self):
+
+        NUM = 0
+
+        async def foo():
+            nonlocal NUM
+            try:
+                await asyncio.sleep(5)
+            except asyncio.CancelledError:
+                NUM += 1
+                raise
+
+        async def runner():
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                for _ in range(5):
+                    g.create_task(foo())
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(asyncio.CancelledError) as cm:
+            await r
+
+        self.assertEqual(NUM, 5)
+
+    async def test_taskgroup_35(self):
+
+        NUM = 0
+
+        async def foo():
+            nonlocal NUM
+            try:
+                await asyncio.sleep(5)
+            except asyncio.CancelledError:
+                NUM += 1
+                raise
+
+        async def runner():
+            nonlocal NUM
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                for _ in range(5):
+                    g.create_task(foo())
+
+                try:
+                    await asyncio.sleep(10)
+                except asyncio.CancelledError:
+                    NUM += 10
+                    raise
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(asyncio.CancelledError):
+            await r
+
+        self.assertEqual(NUM, 15)
+
+    async def test_taskgroup_36(self):
+
+        async def foo():
+            try:
+                await asyncio.sleep(10)
+            finally:
+                1 / 0
+
+        async def runner():
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                for _ in range(5):
+                    g.create_task(foo())
+
+                await asyncio.sleep(10)
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(ExceptionGroup) as cm:
+            await r
+        self.assertEqual(get_error_types(cm.exception), {ZeroDivisionError})
+
+    async def test_taskgroup_37(self):
+
+        async def foo():
+            try:
+                await asyncio.sleep(10)
+            finally:
+                1 / 0
+
+        async def runner():
+            async with taskgroups.TaskGroup(defer_errors=True):
+                async with taskgroups.TaskGroup(defer_errors=True) as g2:
+                    for _ in range(5):
+                        g2.create_task(foo())
+
+                    await asyncio.sleep(10)
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(ExceptionGroup) as cm:
+            await r
+
+        self.assertEqual(get_error_types(cm.exception), {ExceptionGroup})
+        self.assertEqual(get_error_types(cm.exception.exceptions[0]), {ZeroDivisionError})
+
+    async def test_taskgroup_37a(self):
+
+        async def foo():
+            try:
+                await asyncio.sleep(10)
+            finally:
+                1 / 0
+
+        async def runner():
+            async with taskgroups.TaskGroup():
+                async with taskgroups.TaskGroup(defer_errors=True) as g2:
+                    for _ in range(5):
+                        g2.create_task(foo())
+
+                    await asyncio.sleep(10)
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(ExceptionGroup) as cm:
+            await r
+
+        self.assertEqual(get_error_types(cm.exception), {ExceptionGroup})
+        self.assertEqual(get_error_types(cm.exception.exceptions[0]), {ZeroDivisionError})
+
+    async def test_taskgroup_38(self):
+
+        async def foo():
+            try:
+                await asyncio.sleep(10)
+            finally:
+                1 / 0
+
+        async def runner():
+            async with taskgroups.TaskGroup(defer_errors=True) as g1:
+                g1.create_task(asyncio.sleep(10))
+
+                async with taskgroups.TaskGroup(defer_errors=True) as g2:
+                    for _ in range(5):
+                        g2.create_task(foo())
+
+                    await asyncio.sleep(10)
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(ExceptionGroup) as cm:
+            await r
+
+        self.assertEqual(get_error_types(cm.exception), {ExceptionGroup})
+        self.assertEqual(get_error_types(cm.exception.exceptions[0]), {ZeroDivisionError})
+
+    async def test_taskgroup_39(self):
+
+        async def crash_soon():
+            await asyncio.sleep(0.3)
+            1 / 0
+
+        async def runner():
+            async with taskgroups.TaskGroup(defer_errors=True) as g1:
+                g1.create_task(crash_soon())
+                try:
+                    await asyncio.sleep(10)
+                except asyncio.CancelledError:
+                    await asyncio.sleep(0.5)
+                    raise
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(ExceptionGroup) as cm:
+            await r
+        self.assertEqual(get_error_types(cm.exception), {ZeroDivisionError})
+
+    async def test_taskgroup_40(self):
+
+        async def crash_soon():
+            await asyncio.sleep(0.3)
+            1 / 0
+
+        async def nested_runner():
+            async with taskgroups.TaskGroup(defer_errors=True) as g1:
+                g1.create_task(crash_soon())
+                try:
+                    await asyncio.sleep(10)
+                except asyncio.CancelledError:
+                    await asyncio.sleep(0.5)
+                    raise
+
+        async def runner():
+            t = asyncio.create_task(nested_runner())
+            await t
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(ExceptionGroup) as cm:
+            await r
+        self.assertEqual(get_error_types(cm.exception), {ZeroDivisionError})
+
+    async def test_taskgroup_41(self):
+
+        NUM = 0
+
+        async def runner():
+            nonlocal NUM
+            async with taskgroups.TaskGroup(defer_errors=True):
+                try:
+                    await asyncio.sleep(10)
+                except asyncio.CancelledError:
+                    NUM += 10
+                    raise
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+        with self.assertRaises(asyncio.CancelledError):
+            await r
+
+        self.assertEqual(NUM, 10)
+
+    async def test_taskgroup_42(self):
+
+        NUM = 0
+
+        async def runner():
+            nonlocal NUM
+            async with taskgroups.TaskGroup(defer_errors=True):
+                try:
+                    await asyncio.sleep(10)
+                except asyncio.CancelledError:
+                    NUM += 10
+                    # This isn't a good idea, but we have to support
+                    # this weird case.
+                    raise MyExc
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.1)
+
+        self.assertFalse(r.done())
+        r.cancel()
+
+        try:
+            await r
+        except ExceptionGroup as t:
+            self.assertEqual(get_error_types(t),{MyExc})
+        else:
+            self.fail('ExceptionGroup was not raised')
+
+        self.assertEqual(NUM, 10)
+
+    async def test_taskgroup_43(self):
+
+        async def crash_soon():
+            await asyncio.sleep(0.1)
+            1 / 0
+
+        async def nested():
+            try:
+                await asyncio.sleep(10)
+            finally:
+                raise MyExc
+
+        async def runner():
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                g.create_task(crash_soon())
+                await nested()
+
+        r = asyncio.create_task(runner())
+        try:
+            await r
+        except ExceptionGroup as t:
+            self.assertEqual(get_error_types(t), {MyExc, ZeroDivisionError})
+        else:
+            self.fail('TasgGroupError was not raised')
+
+    async def test_taskgroup_44(self):
+
+        async def foo1():
+            await asyncio.sleep(1)
+            return 42
+
+        async def foo2():
+            await asyncio.sleep(2)
+            return 11
+
+        async def runner():
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                g.create_task(foo1())
+                g.create_task(foo2())
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.05)
+        r.cancel()
+
+        with self.assertRaises(asyncio.CancelledError):
+            await r
+
+    async def test_taskgroup_45(self):
+
+        NUM = 0
+
+        async def foo1():
+            nonlocal NUM
+            await asyncio.sleep(0.2)
+            NUM += 1
+
+        async def foo2():
+            nonlocal NUM
+            await asyncio.sleep(0.3)
+            NUM += 2
+
+        async def runner():
+            async with taskgroups.TaskGroup(defer_errors=True) as g:
+                g.create_task(foo1())
+                g.create_task(foo2())
+
+        r = asyncio.create_task(runner())
+        await asyncio.sleep(0.05)
+        r.cancel()
+
+        with self.assertRaises(asyncio.CancelledError):
+            await r
+
+        self.assertEqual(NUM, 0)
+
+
 
 if __name__ == "__main__":
     unittest.main()

From 0b08887d175156a8d2c23859c5b51da5621f259b Mon Sep 17 00:00:00 2001
From: DontPanicO <dontpanic.call0@gmail.com>
Date: Tue, 7 Feb 2023 18:10:20 +0000
Subject: [PATCH 3/4] Add `NEWS.d` entry

---
 .../next/Library/2023-02-07-18-08-32.gh-issue-101581.lxw8WY.rst | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2023-02-07-18-08-32.gh-issue-101581.lxw8WY.rst

diff --git a/Misc/NEWS.d/next/Library/2023-02-07-18-08-32.gh-issue-101581.lxw8WY.rst b/Misc/NEWS.d/next/Library/2023-02-07-18-08-32.gh-issue-101581.lxw8WY.rst
new file mode 100644
index 00000000000000..e179ea9f680129
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-02-07-18-08-32.gh-issue-101581.lxw8WY.rst
@@ -0,0 +1,2 @@
+Adds ``defer_errors`` flag to :class:`asyncio.TaskGroup` to optionally
+prevent child tasks from being cancelled.

From 481d37269efc91943329d9a6ec9e2466d799f7d4 Mon Sep 17 00:00:00 2001
From: DontPanicO <dontpanic.call0@gmail.com>
Date: Fri, 17 Mar 2023 11:18:52 +0100
Subject: [PATCH 4/4] Update `asyncio.TaskGroup` docstring

Add explanation about `defer_errors=True` bheaviour in `asyncio.TaskGroup` docstring
---
 Lib/asyncio/taskgroups.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Lib/asyncio/taskgroups.py b/Lib/asyncio/taskgroups.py
index 4a93e8cc54a63f..4d20598388b151 100644
--- a/Lib/asyncio/taskgroups.py
+++ b/Lib/asyncio/taskgroups.py
@@ -23,6 +23,8 @@ class TaskGroup:
 
     Any exceptions other than `asyncio.CancelledError` raised within
     a task will cancel all remaining tasks and wait for them to exit.
+    You can prevent this behavior by passing `defer_errors=True` to
+    the constructor.
     The exceptions are then combined and raised as an `ExceptionGroup`.
     """
     def __init__(self, defer_errors=False):