ssl/asyncio: traceback even when error is handled

The traceback is generated by asyncio's implementation of the SSL protocol, which invokes the event loop's exception handler. Through a maze of interactions between the transport/protocol and the streaming layers, this exception gets logged by the event loop and propagated to the API user. The way that happens is as follows:

  • An exception occurs during the SSL handshake.
  • SSLProtocol._on_handshake_complete receives non-None handshake_exc and treats it as a "fatal error" (in the handshake context), i.e. invokes self._fatal_error and returns.
  • _fatal_error calls the event loop's exception handler to log the error. The handler is normally invoked for exceptions that occur in queued callbacks where there is no longer a caller to propagate them to, so it just logs the traceback to standard error to ensure that the exception doesn't pass silently. However...
  • _fatal_error goes on to call transport._force_close, which calls connection_lost back on the protocol.
  • The stream reader protocol's connection_lost implementation sets the exception as the result of the stream reader's future, thus propagating it to the users of the stream API that await it.

It is not obvious if it is a bug or a feature that the same exception is both logged by the event loop and passed to connection_lost. It might be a workaround for BaseProtocol.connection_lost being defined a no-op, so the extra log ensures that a protocol that simply inherits from BaseProtocol doesn't silence the possibly sensitive exceptions occurring during SSL handshake. Whichever the reason, the current behavior leads to the problem experienced by the OP: catching the exception is not enough to suppress it, a traceback will still be logged.

To work around the issue, one can temporarily set the exception handler to one that doesn't report SSLCertVerificationError:

def suppress_ssl_exception_report():
    loop = asyncio.get_event_loop()
    old_handler = loop.get_exception_handler()
    old_handler_fn = old_handler or lambda _loop, ctx: loop.default_exception_handler(ctx)
    def ignore_exc(_loop, ctx):
        exc = ctx.get('exception')
        if isinstance(exc, SSLCertVerificationError):
        old_handler_fn(loop, ctx)

Adding with suppress_ssl_exception_report() around the code in fetch_url suppresses the unwanted traceback.

The above works, but it strongly feels like a workaround for an underlying issue and not like correct API usage, so I filed a bug report in the tracker. EDIT: the issue was fixed in the meantime and the code from the question no longer prints the spurious backtrace.

For unknown reason (bug?) aiohttp prints error output to console even before any exception thrown. You can avoid it temporary redirecting error output with contextlib.redirect_stderr:

import asyncio
import aiohttp
from ssl import SSLCertVerificationError

import os
from contextlib import redirect_stderr

async def fetch_url(url, client):

        f = open(os.devnull, 'w')
        with redirect_stderr(f):  # ignore any error output inside context

            async with client.get(url) as resp:
    except SSLCertVerificationError as e:
        print('Error handled')

# ...

P.S. I think you can use more common exception type to catch client errors, for example:

except aiohttp.ClientConnectionError as e:
    print('Error handled')