asyncio: create_datagram_endpoint() does not return a DatagramTransport

[ThomasTrummer]

BPO	44886
Nosy	@asvetlov, @1st1, @miss-islington
PRs	bpo-44886: Inherit asyncio proactor datagram transport from asyncio.DatagramTransport #31512 [3.10] bpo-44886: Inherit asyncio proactor datagram transport from asyncio.DatagramTransport (GH-31512) #31514 [3.9] bpo-44886: Inherit asyncio proactor datagram transport from asyncio.DatagramTransport (GH-31512) #31515

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/asvetlov'
closed_at = <Date 2022-02-24.10:09:09.089>
created_at = <Date 2021-08-11.08:38:49.198>
labels = ['3.11', '3.9', '3.10', 'expert-asyncio']
title = 'asyncio: create_datagram_endpoint() does not return a DatagramTransport'
updated_at = <Date 2022-03-13.23:58:06.656>
user = 'https://bugs.python.org/ThomasTrummer'

bugs.python.org fields:

activity = <Date 2022-03-13.23:58:06.656>
actor = 'asvetlov'
assignee = 'asvetlov'
closed = True
closed_date = <Date 2022-02-24.10:09:09.089>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2021-08-11.08:38:49.198>
creator = 'Thomas Trummer'
dependencies = []
files = []
hgrepos = []
issue_num = 44886
keywords = ['patch']
message_count = 1.0
messages = ['399376']
nosy_count = 4.0
nosy_names = ['asvetlov', 'yselivanov', 'miss-islington', 'Thomas Trummer']
pr_nums = ['31512', '31514', '31515']
priority = 'normal'
resolution = 'fixed'
stage = 'patch review'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue44886'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[ThomasTrummer]

According to the documentation[1] loop.create_datagram_endpoint() returns an asyncio.DatagramTransport. However on Windows this is not the case when the ProactorEventLoop is used (which seems to be the default since Python 3.8). This is a problem because a DatagramProtocol subclass needs a downcast in order to satisfy the type system (or mypy for that matter).

[1] https://docs.python.org/3/library/asyncio-protocol.html#asyncio.DatagramTransport

---

# Will print: <class 'asyncio.proactor_events._ProactorDatagramTransport'> False

import asyncio


class EchoServerProtocol(asyncio.DatagramProtocol):
    def connection_made(self, transport):
        print(type(transport), isinstance(transport, asyncio.DatagramTransport))


async def main():
    transport, protocol = await asyncio.get_running_loop().create_datagram_endpoint(
        lambda: EchoServerProtocol(),
        local_addr=('127.0.0.1', 9999))

try:
    await asyncio.sleep(5)
finally:
    transport.close()

asyncio.run(main())