Preserve json.JSONDecodeError when handling invalid JSON in request body

[declaresub]

First Check

• I added a very descriptive title to this issue.
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• I already read and followed all the tutorial in the docs and didn't find an answer.
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Example Code

not applicable

Description

I think that the response to a request with syntactically invalid JSON should be 400, not 422. This is the feature request I would prefer. But I would be willing to write a custom exception handler for now, if that were feasible.

Wanted Solution

In FastAPI/routing.py line 210-211, we have

    except json.JSONDecodeError as e:
        raise RequestValidationError([ErrorWrapper(e, ("body", e.pos))], body=e.doc)

I propose that the original exception not be discarded, by raising from --

    except json.JSONDecodeError as e:
        raise RequestValidationError([ErrorWrapper(e, ("body", e.pos))], body=e.doc) from e

With the original exception included, it would be simple enough to write a custom exception handler to handle the request as I see fit. This change should not break anyone's code, and I think it is generally the better practice to use raise from in such situations.

Wanted Code

not applicable

Alternatives

No response

Operating System

Linux, macOS

Operating System Details

No response

FastAPI Version

0.63.0

Python Version

3.9.5

Additional Context

No response

[UKnowWhoIm]

Is this currently being worked on @declaresub ?

[declaresub]

I don't know. I would be happy to do it, but this is the first issue I've submitted to fastapi, and with 389 open pull requests, I didn't see the point spending time on a pull request that would never be looked at.

[tiangolo]

Thanks for posting it @declaresub! And thanks @UKnowWhoIm for the PR! 🎉 🍰

This should be solved in #4057 and will be available in the next version, released in some hours or the next days. (the current latest is 0.80.0).

[github-actions]

Assuming the original need was handled, this will be automatically closed now. But feel free to add more comments or create new issues or PRs.

[Quix0r]

I know this is an old ticket, but isn't it json.decoder.JSONDecodeError instead? E.g. starlette uses the long version while this here the short version (lacking .decoder package in import reference).