Catching AttributeError exceptions in widget media

[Michael Jones]

Hi,

I love Django, it has been enormously useful to a new comer to web dev like me. 

One minor issue I've just had is that I've struggled to figure out an issue with my code because an AttributeError I was generating in my media property on my custom widget was being silently swallowed by:

    https://github.com/django/django/blob/09c1f18f223cd60c948f7dae6aedccea70a92274/django/forms/widgets.py#L113

Is there a chance of changing it so that that doesn't happen? Maybe do a `hasattr` test instead of the try-catch? There is no feedback otherwise. You're media just doesn't appear on the page. I guess if I was a more experienced debugger I might find this easier to figure out.

I am inexperienced but I have found that catching such broad exceptions as AttributeError around something that can call other functions causes pain from time to time. I guess `media` was originally meant to be just a class or instance attribute but it seem to frequently be made into a property with the potential for non-trivial code being called from it.

I am not sure if this is best here or suggested on the developers mailing list. I'm happy to repost if someone could offer me guidance.

Thanks,

Michael

[Collin Anderson]

unfortunately hasattr eats even more exceptions than AttributeError.

[Michael Jones]

Hi,

Thanks for the reply, but I'm not sure I follow. I was hoping that the following might work:

    if hasattr(sup_cls, 'media'):
        base = sup_cls.media
    else:

        base = Media()

Would that swallow additional exceptions?

Cheers,

Michael

[Collin Anderson]

believe it or not, hasattr is the same as:

try:

    getattr(obj, name)

    return True

except:  # yikes! naked except!

    return False

https://docs.python.org/2/library/functions.html#hasattr

So, excepting AttributeError will at least narrow down the error slightly, rather than hasattr accepting _any_ exception.

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[Zachary McCord]

I simply didn't believe it at first, but sure enough, in python 2.7:

>>> class Bomb(object):
...     def __getattr__(self, id):
...         assert False
...  
>>> bomb = Bomb()
>>> bomb.foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __getattr__
AssertionError
>>> hasattr(bomb, 'foo')
False

Thankfully Python 3 fixes this. One more reason to switch, I guess.

[Michael Jones]

Thanks for the clarification guys. That is truly horrific :)

Sorry not to have tested my suggestion first. 

Michael