ClearableFileInput masks useful exceptions in is_initial()

[Antonio García-Domínguez]

I have a Python 2.7 site with some special requirements on how to link to uploaded files, so we defined a FileSystemStorage subclass with a custom url() method that computed a reverse URL for them.

This morning I noticed that some files would not show up in our FileField with a ClearableFileInput widget after being uploaded through the Django admin app. After searching for a while, I found the issue: the hasattr(value, "url") call in ClearableFileInput.is_initial() was simply returning False for these files, masking an exception from the reverse() call of our custom url() method.

I suggest changing ClearableFileInput.is_initial() so it will not mask useful exceptions. Instead of simply using hasattr, it should use a getattr call inside a try..except block and rethrow anything that is not an AttributeError. In fact, this is the new behavior of hasattr in Python 3.2:

​https://docs.python.org/3.2/library/functions.html#hasattr