Python: catching particular exception

Question

I have such code (Python 2.5, GAE dev server):

try:
    yt_service.UpgradeToSessionToken() // this line produces TokenUpgradeFailed
except gdata.service.TokenUpgradeFailed:
    return HttpResponseRedirect(auth_sub_url()) # this line will never be executed (why?)
except Exception, exc:
    return HttpResponseRedirect(auth_sub_url()) # instead this line is executed (why?)

So I set breakpoint at last line and under debugger I see:

"exc"   TokenUpgradeFailed: {'status': 403, 'body': 'html stripped', 'reason': 'Non 200 response on upgrade'}   
"type(exc)" type: <class 'gdata.service.TokenUpgradeFailed'>
"exc is gdata.service.TokenUpgradeFailed"   bool: False 
"exc.__class__" type: <class 'gdata.service.TokenUpgradeFailed'>
"isinstance(exc, gdata.service.TokenUpgradeFailed)" bool: False 
"exc.__class__.__name__"    str: TokenUpgradeFailed 

What I missed in python exception handling? Why isinstance(exc, gdata.service.TokenUpgradeFailed) is False?

Answer 1

This error can occur if your relative/absolute import statements do not match everywhere. If there is a mismatch, the target module can be loaded more than once and in slightly different contexts. Usually this isn't a problem but it does prevent classes from the differently loaded modules from comparing as equal (hence the exception catching problem).

There may be other causes for the error but I suggest looking through your code and ensuring that everything importing the gdata.service module explicitly mentions the gdata package. Even within the gdata package itself, each module using the service module should import it from the package explicitly via from gdata import service rather than by way of the relative import: import service.