python try except does not capture all errors

Question

In my flask web application I use beaker library for session handling. In the following code, for some unknown reason, production server raises exception, but my local pc does just fine.

import sys

...

try:
    beaker_session = request.environ['beaker.session']
    beaker_session['user_id'] = user.id 
    beaker_session.save()
except:
    flash(sys.exc_info()[0])
    return render_template('main/login.html')

Local computer saves the session just as expected, without any exception. Production server (RedHat OpenShift) raises an error exactly on "beaker_session.save()" line. But, instead of showing my login page with flash message, Internal Server Error 500 is thrown. I checked beaker backend url (mysql db) and there's no problem, because it works in other parts of code, where I persist newly registered users. So, my question is 1) why except part doesn't work? 2) why beaker cannot save the session. Thank you.

Answer 1

By default Flask swallows exceptions, be sure to add this line to your application near the top:

app.config['PROPAGATE_EXCEPTIONS'] = True

Answer 2

I'm not familiar with the beaker library, but if it's a SWIG wrapped library, and the exception ocurrs within the C++ code, it's possible the designer neglected to map the exception to an appropriate python exception. If this is the case, then Python does not get a shot at the exception -- and even try/except will miss it.