Better exception formatting by tracerite

[LeoVasanko]

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Description

I am looking to integrate Tracerite to FastAPI and consequently to this project, because it provides by far the best tracebacks that anyone has implemented for Python. The module formats for CLI, with colors, graphics and variable inspector similar to Typer's rich formatting, but also formats for HTML. Unlike others, it provides short to-the-point traceback with the relevant information in chronological order.

Wiring it up for HTML tracebacks on FastAPI is straight forward replacing Starlette ServerErrorMiddleware, but we also want to handle tracebacks on CLI, also during server startup, specifically including syntax errors and other such things that would fail early while loading the app. And this is where I discovered Typer as it is already wired to do that in the FastAPI stack.

For the console Tracerite hooks up sys.excepthook and also logging.exception. Typer is also grabbing the excepthook, but I see we could disable that from FastAPI side. Ideally we'd also hook somewhere between uvicorn.run and Config.load_app, such that we can handle those import and syntax errors, but there doesn't appear to be suitable hooks for this.

I will be making PR(s) on this, but I would like to have your insights first on where it would best be plugged in, and to which project I should make the PR to avoid monkey patching.

Hopefully you can shed some light on this so that I won't be working blindly.

Operating System

Other

Operating System Details

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Project Version

0.26.8

Python Version

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Additional Context

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[LeoVasanko]

I could hook up my handler to sys.excepthook externally only if done BEFORE importing typer, and setting pretty_exceptions_enable=False to avoid its own formatter. This is because typer captures sys.excepthook at import time and calls whatever it captured then (built in, if not loading tracerite before that import).

Expected: typer would use whatever handler is configured at the time of exception, not one it captured module level during import.

Having this built into typer itself, replacing the pretty formatter it has, would also avoid the problem, although we still need to add separate support code within fastapi-cli and/or fastapi for what we are doing.

[svlandeg]

Hi! Could you please first explain in more detail what the added advantage of doing this would be?

[LeoVasanko]

Typer shows pretty printed exceptions, with the aim of doing that better than Python's built in formatter (that itself has seen some improvements in recent releases). I am proposing to replace that formatter with one that is far better still. I will let the screenshot speak for itself.

The messages are minimalistic, far shorter than Python built in or Typer, because there is plenty of logic for it to show just the relevant information. That is, the full exception chain but with only the relevant frames expanded, shown in the order things happened, "except" returns displayed clearly, only the relevant lines shown of each frame and only the relevant variables shown in variable inspector. It has a considerable amount of logic and heuristics for doing so in the best way that fits the terminal. Replicating that is not possible by simply colouring and putting in boxes on the standard tracebacks.

Thus, the advantage is that you can see easier what caused the error and what led to that.

Note: integrating with FastAPI does NOT require changes to typer, as we can simply override the handler that typer installs. However, if this project is used by others, this should be useful to everyone and therefore better integrated directly at this level.