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.

The `except` part is catching the exception if there is one; more likely is that your code that's run in the `except` block is causing *another* exception. Check your server's error logs and find what the actual exception that's being turned into a 500 error is. — Amber, Mar 31 '12 at 02:10
I found a way to look into server log (rhc app tail -a myapp). The problem was dir permission (beaker lock_dir was not set). I still couldn't understand why try/except didn't work, because in other cases my except part was working as expected. — synergetic, Mar 31 '12 at 05:23
@synergetic You should avoid using bare except as it can even catch system interrupts and other system related errors and can lead to unexplained or weird errors. — codecool, Mar 31 '12 at 17:38

score 0 · Answer 1 · answered Jan 07 '13 at 16:46

0

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

app.config['PROPAGATE_EXCEPTIONS'] = True

answered Jan 07 '13 at 16:46

TheSteve0

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score 0 · Answer 2 · answered Mar 31 '12 at 16:45

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.

python try except does not capture all errors

2 Answers2