Generic HTTPException error handler is not called due to logic error in _find_error_handler

[zackw]

Expected Behavior

The documentation leads me to believe that an error handler registered like so:

app.config["TRAP_HTTP_EXCEPTIONS"] = True
@app.errorhandler(HTTPException)
def handle_http_error(err):
    return flask.render_template("_error.html",
                                 errcode=err.code,
                                 errname=err.name,
                                 errdescription=err.description), err.code

... should be called for any HTTP error. (The requirement to set TRAP_HTTP_EXCEPTIONS is inadequately clear, but that's an unrelated problem.)

Actual Behavior

Instead, this configuration causes any HTTP error to turn into a 500 internal server error with this traceback:

127.0.0.1 - - [06/Jan/2018 13:09:57] "GET /fnord.html HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1997, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1985, in wsgi_app
    response = self.handle_exception(e)
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1540, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python3/dist-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python3/dist-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/lib/python3/dist-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/.../mysite.py", line 35, in page
    flask.abort(404)
  File "/usr/lib/python3/dist-packages/werkzeug/exceptions.py", line 707, in abort
    return _aborter(status, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/werkzeug/exceptions.py", line 687, in __call__
    raise self.mapping[code](*args, **kwargs)

I dug around in the debugger a little: this is from the perspective of the handle_user_exception frame:

>>> self.error_handler_spec
{None: {None: {<class 'werkzeug.exceptions.HTTPException'>: <function handle_http_error at 0x7fe7f49b4400>}}}

My application does not use blueprints, so the outer None in this structure is correct, and _find_error_handler understands it. However, the inner None does not correspond to any HTTP code, and so _find_error_handler is unable to pass through that layer of nesting and do the intended lookup by class.

Environment

• Python version: 3.6.4
• Flask version: 0.12.2
• Werkzeug version: 0.13

[davidism]

Please test this against master, the error handler code has changed.

[pts-sergiomartins]

I'm having this same problem. The lookup seems to be happening by code only and not by class. I have a generic Exception handler too but it's not invoked either.

(Pdb) self.error_handler_spec.get(request.blueprint, {})
{None: {<class 'werkzeug.exceptions.HTTPException'>: <function handle_http_exception at 0x7fb4f99962f0>, <class 'Exception'>: <function handle_exception at 0x7fb4f9996268>}}

[davidism]

And does it happen on master?

[pts-sergiomartins]

I'm pulling down master now. Report shortly

[pts-sergiomartins]

It works on latest master

[davidism]

Thanks for testing it out, really appreciate this kind of help.

[pts-sergiomartins]

Happy to help! When can we expect a release?

[davidism]

Soon™