Question

The title is horrible, i know; I m terrible at titles on SO here.

I m wondering what would be the best way to present unified error responses in a webapi when errors could be raised deep inside the application.

The errors deep down in the app don t know anything about the web layer (nor should they), so how can the web layer classify myapp.PermissionError into a 403, json.DecodeError into a 400, myapp.driver.InvalidValue into 500, etc.

I have a few ideas, but I m not a big fan of any of them.

(As the snippets might imply, this is a python app on linux)

Use a lot of except blocks to match the exception type i want. This is what i m currently doing but its growing unwieldy (i m already up to 8, and there are plenty more to go).
```
try
  business.DoIt()
except DecodingError:
  respond(400)
except PermissionError:
  response(403)
...etc...
```

Creating a mapping or list of exception types and map them to response codes. This doesn t seem much better than (1) in the end, but it does clean up the code.

error_map = [(DecodingError, 400), (PermissionError, 403)]
try:
  DoIt()
except Exception, exc:
  for type, code in error_map:
    if isinstance(exc, type):
       response(code)
       return

Add an interface to every exception class that provides the response code, but I don t like this because then the exceptions are carrying web-layer specific information (even if they live deep down in a driver that doesn t care about the web layer at all). I do like how "automatic" the web error response is, though.
```
class PermissionError(Exception):
  web_status_code = 403
try:
  Doit()
except:
  response(exc.web_status_code)
```

Answer 1

I like option 1. It may be a little more verbose, but it s also very clear.

Option 2 separates the point at which the exception is thrown from where the decision about what to do with it is made. In reality, that likely wouldn t be too much of an issue, but why split it up if you don t have to?

I agree that option 3 is pretty ugly. No need to deal with the error behavior at that level, just throw the exception.

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