aiohttp.MultipartWriter.append() returns None

[paulannetts]

Long story short

Scenario is to send a multi-part message from test client containing a file with custom filename. Following the instructions from http://aiohttp.readthedocs.io/en/stable/multipart.html#sending-multipart-requests

It used to work in aiohttp 1.3.5, in aiohttp 2.0.6 it generates a None exception and the documentation doesn't mention any clear breaking change.

Expected behaviour

It should work as v1.3.5 or the documentation should be clearly updated to a working example.

Actual behaviour

The line mpwriter.append is now returning None, so the part.set_content_disposition is raising an exception.

Steps to reproduce

    with aiohttp.MultipartWriter('my_multi_part') as mpwriter:
        mpwriter.append_json({'key1': 'value1', 'key2': 'value2'})
        part = mpwriter.append(open(A_FILE_PATH))
        part.set_content_disposition('attachment', filename='my_custom_file.txt')

Your environment

Ubuntu 16.04, Python 3.5.2, pip install of aiohttp 2.0.6

[fafhrd91]

could you post full traceback

[paulannetts]

=================================== FAILURES ===================================
______________________________ test_upload_dummy _______________________________

    async def test_upload_dummy():
        A_FILE_PATH = str(os.path.dirname(os.path.realpath(__file__)) + '/data/test.txt')
        with aiohttp.MultipartWriter('my_multi_part') as mpwriter:
            mpwriter.append_json({'key1': 'value1', 'key2': 'value2'})
            part = mpwriter.append(open(A_FILE_PATH))
>           part.set_content_disposition('attachment', filename='my_custom_file.txt')
E           AttributeError: 'NoneType' object has no attribute 'set_content_disposition'

=========================== 1 failed in 0.12 seconds ===========================

That is the full traceback by the way, because append() returns None.

[m-aciek]

@fafhrd91 Did you find a workaround for your problem?

[asvetlov]

@kxepal could you take a look?

[fafhrd91]

This is not a bug. This is documentation issue related to payloads

[kxepal]

So, undocumented breaking change?

[paulannetts]

New way of doing this, using the new payload system.

 with aiohttp.MultipartWriter('my_multi_part') as mpwriter:
        mpwriter.append_json({'key1': 'value1', 'key2': 'value2'})
        payload = aiohttp.payload.TextIOPayload(open(A_FILE_PATH))
        payload.set_content_disposition('attachment', filename='my_custom_file.txt')
        mpwriter.append_payload(payload)

[roganov]

I believe this is fixed by #2759

[asvetlov]

Yes, you are right

[lock]

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If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new