Error while handling response code 422

[dhruvmanila]

It seems that gidgethub is not able to handle a part of the 422 response. This happened when the app tried to make a review comment on a line that was not part of the pull request diff. I didn't know that was not possible but this mistake revealed this bug in the code.

{
  "message": "Unprocessable Entity",
  "errors": [
    "Line must be part of the diff"
  ],
  "documentation_url": "https://docs.github.com/rest/reference/pulls#create-a-review-for-a-pull-request", 
}

The code that triggered the error, taken from gidgethub.sansio.decipher_response:

                exc_type = InvalidField
                if errors:
                    if any(
       ------>          e["code"]
                        in ["missing", "missing_field", "invalid", "already_exists"]
                        for e in errors
                    ):
                        error_context = ", ".join(repr(e.get("field")) for e in errors)
                        message = f"{message} for {error_context}"

Logged traceback:

2020-12-10T14:57:43.757626+00:00 app[web.1]:     await gh.post(
2020-12-10T14:57:43.757626+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.8/site-packages/gidgethub/abc.py", line 175, in post
2020-12-10T14:57:43.757626+00:00 app[web.1]:     data, _ = await self._make_request(
2020-12-10T14:57:43.757626+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.8/site-packages/gidgethub/abc.py", line 114, in _make_request
2020-12-10T14:57:43.757627+00:00 app[web.1]:     data, self.rate_limit, more = sansio.decipher_response(*response)
2020-12-10T14:57:43.757627+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.8/site-packages/gidgethub/sansio.py", line 340, in decipher_response
2020-12-10T14:57:43.757628+00:00 app[web.1]:     if any(
2020-12-10T14:57:43.757628+00:00 app[web.1]:   File "/app/.heroku/python/lib/python3.8/site-packages/gidgethub/sansio.py", line 341, in <genexpr>
2020-12-10T14:57:43.757628+00:00 app[web.1]:     e["code"]
2020-12-10T14:57:43.757628+00:00 app[web.1]: TypeError: string indices must be integers

[brettcannon]

Ugh, the one thing about error cases that seemed to be consistent isn't consistent.

[dhruvmanila]

A quick fix would be to check whether the key code exists in the errors object, otherwise accumulate whatever is in the errors object and combine it with the message:

error_context = ", ".join(repr(e) for e in errors)
message = f"{message}: {error_context}"

[Mariatta]

Feels like it's a bug with with GitHub, since the behavior described is not documented at all.
https://docs.github.com/en/rest/overview/resources-in-the-rest-api#client-errors

Maybe worth reporting the problem with GitHub, so they can either fix the doc or the response.