"raise exc from None" interferes with exception chaining

Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

Even though I use exception chaining (raise ... from) in my app, the original exception info gets deleted because Starlette handles exceptions with raise .. from None at the outer scope. Here is a MWE:

from starlette.applications import Starlette
from starlette.endpoints import HTTPEndpoint
from starlette.responses import HTMLResponse
from starlette.routing import Route


class TemplateRenderingError(Exception): pass


class Homepage(HTTPEndpoint):
    async def get(self, request):
        try:
            content = '{a}'.format()
        except KeyError as e:
            raise TemplateRenderingError from e
        return HTMLResponse(content)

app = Starlette(debug=True, routes=[
    Route('/', Homepage),
])

Run the server, load /, then look at the console output.

What I expect to get is a chained exception:

ERROR: Exception in ASGI application
Traceback (most recent call last):
  File ".\star_bug.py", line 13, in get
    content = '{a}'.format()
KeyError: 'a'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "c:\otree\ve-nodj\lib\site-packages\uvicorn\protocols\http\h11_impl.py", line 394, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "c:\otree\ve-nodj\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\middleware\errors.py", line 184, in __call__
    raise exc
  File "c:\otree\ve-nodj\lib\site-packages\starlette\middleware\errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\exceptions.py", line 84, in __call__
    raise exc
  File "c:\otree\ve-nodj\lib\site-packages\starlette\exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\routing.py", line 582, in __call__
    await route.handle(scope, receive, send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\routing.py", line 243, in handle
    await self.app(scope, receive, send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\endpoints.py", line 30, in dispatch
    response = await handler(request)
  File ".\star_bug.py", line 15, in get
    raise TemplateRenderingError from e
star_bug.TemplateRenderingError

What I get is only the final exception:

ERROR: Exception in ASGI application
Traceback (most recent call last):
  File "c:\otree\ve-nodj\lib\site-packages\uvicorn\protocols\http\h11_impl.py", line 394, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "c:\otree\ve-nodj\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\middleware\errors.py", line 184, in __call__
    raise exc
  File "c:\otree\ve-nodj\lib\site-packages\starlette\middleware\errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\exceptions.py", line 83, in __call__
    raise exc from None
  File "c:\otree\ve-nodj\lib\site-packages\starlette\exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\routing.py", line 582, in __call__
    await route.handle(scope, receive, send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\routing.py", line 243, in handle
    await self.app(scope, receive, send)
  File "c:\otree\ve-nodj\lib\site-packages\starlette\endpoints.py", line 30, in dispatch
    response = await handler(request)
  File ".\star_bug.py", line 15, in get
    raise TemplateRenderingError from e

Starlette uses raise exc from None in 4 places:

• ExceptionMiddleware.__call__
• ServerErrorMiddleware.__call__
• WebSocketEndpoint.dispatch
• _ASGIAdapter.send

It seems to me that in all 4 cases the from None could hide intentional context about users’ errors.

Having the chained traceback would be very valuable both for reading the console output and for programmatic use (I want to make a middleware that looks at the .__cause__ attribute of certain exceptions to deliver a more relevant error message). I’m happy to provide more details on my use case.

[tomchristie]

It’s possible that there might be a good reason for one or more of those cases to be a raise exc from None, but in general I’d agree with you, yup. It’s probably worth a pull request, switching the behaviour of those. We can then review each case in that pull request, and check if any of them are “raise from None” for a good reason.

[oTree-org]

Another fix for the issue would be to remove the from None in the following 4 places:

- ExceptionMiddleware.__call__
- ServerErrorMiddleware.__call__
- WebSocketEndpoint.dispatch
- ASGIAdapter.send