Using MultipartWriter.append_json breaks in 3.9.4 with AssertionError "assert CONTENT_DISPOSITION in payload.headers"

[nicolasdialpad]

Describe the bug

After upgrading to 3.9.4 the following example code that used to work in 3.9.3 starts failing. I think the issue was introduced in 7d0be3f

To Reproduce

import aiohttp
import asyncio


async def main():
    async with aiohttp.ClientSession() as session:
        mpwriter = aiohttp.MultipartWriter('form-data')
        mpwriter.append_json({'a': 1}) 
        response = await session.post('https://httpbingo.org/post', data=mpwriter)
        print(await response.text())


asyncio.run(main())

Expected behavior

This should print a text response, but instead it fails with an AssertionError

Logs/tracebacks

Traceback (most recent call last):
  File "/Users/nico/test.py", line 13, in <module>
    asyncio.run(main())
  File "/opt/homebrew/Cellar/python@3.9/3.9.18_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/opt/homebrew/Cellar/python@3.9/3.9.18_1/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
    return future.result()
  File "/Users/nico/test.py", line 8, in main
    mpwriter.append_json({'a': 1})
  File "/Users/nico/virtualenvs/gaevenv3.9/lib/python3.9/site-packages/aiohttp/multipart.py", line 887, in append_json
    return self.append_payload(JsonPayload(obj, headers=headers))
  File "/Users/nico/virtualenvs/gaevenv3.9/lib/python3.9/site-packages/aiohttp/multipart.py", line 851, in append_payload
    assert CONTENT_DISPOSITION in payload.headers
AssertionError

### Python Version

```console
Python 3.9.18

aiohttp Version

Name: aiohttp
Version: 3.9.4
Summary: Async http client/server framework (asyncio)
Home-page: https://github.com/aio-libs/aiohttp
Author:
Author-email:
License: Apache 2
Location: /Users/nico/virtualenvs/gaevenv3.9/lib/python3.9/site-packages
Requires: aiosignal, async-timeout, attrs, frozenlist, multidict, yarl
Required-by: gcloud-aio-auth

multidict Version

Name: multidict
Version: 5.2.0
Summary: multidict implementation
Home-page: https://github.com/aio-libs/multidict
Author: Andrew Svetlov
Author-email: andrew.svetlov@gmail.com
License: Apache 2
Location: /Users/nico/virtualenvs/gaevenv3.9/lib/python3.9/site-packages
Requires:
Required-by: aiohttp, sanic, yarl

yarl Version

Name: yarl
Version: 1.9.2
Summary: Yet another URL library
Home-page: https://github.com/aio-libs/yarl/
Author: Andrew Svetlov
Author-email: andrew.svetlov@gmail.com
License: Apache-2.0
Location: /Users/nico/virtualenvs/gaevenv3.9/lib/python3.9/site-packages
Requires: idna, multidict
Required-by: aiohttp

OS

macOS 14.1.1

Related component

Client

Additional context

No response

Code of Conduct

• I agree to follow the aio-libs Code of Conduct

[Olegt0rr]

Got this issue here: https://github.com/aiogram/aiogram/actions/runs/8664683869/job/23761669236?pr=1459

Example of usage with previous version:
https://github.com/aiogram/aiogram/blob/b49939aaff2b1e37520a207b01ea124412641d01/aiogram/webhook/aiohttp_server.py#L155-L163

[Dreamsorcerer]

Through these convenience methods, we should probably add Content-Disposition automatically. But, I guess the one question is how to name the parts? Shouldn't there be an expected name by the receiving service, which the user should be using?

[Dreamsorcerer]

Note that these are just asserts, so you can likely use python -O as a workaround at the moment.

[Olegt0rr]

Note that these are just asserts, so you can likely use python -O as a workaround at the moment.

But this update breaks all existing tests

What's wrong with this usage?
https://github.com/aiogram/aiogram/blob/b49939aaff2b1e37520a207b01ea124412641d01/aiogram/webhook/aiohttp_server.py#L155-L163

What should I edit to make 3.9.4 works?
It seems like there's a dead end here with new assertions

[Dreamsorcerer]

What's wrong with this usage?

Wait, that code does set content-disposition. Why are we getting the error there too?
I was just looking at the code in the start of this issue, which doesn't set content-disposition.

Think you can create a test quickly in a PR here?

[Olegt0rr]

What's wrong with this usage?

Wait, that code does set content-disposition. Why are we getting the error there too? I was just looking at the code in the start of this issue, which doesn't set content-disposition.

Think you can create a test quickly in a PR here?

Sure, wait a minute

[Dreamsorcerer]

Ah, wait, you're getting the payload after the append and then setting it, but the check is happening during the append. Maybe we can move the check later...

[Olegt0rr]

#8332