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 I 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 a 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

[Antonio García-Domínguez]

Here is a pull request with the proposed fix:

​https://github.com/django/django/pull/4579

[Tim Graham <timograham@…>]

In 5c412dd:

Fixes #24727 -- Prevented ClearableFileInput from masking exceptions on Python 2

[Tim Graham <timograham@…>]

In 7cb3a488:

Fixed #25410 -- Fixed empty ClearableFileInput crash on Python 2.

Reverted "Fixes #24727 -- Prevented ClearableFileInput from masking
exceptions on Python 2" and added a regression test.

This reverts commit 5c412dd8a724b263489c1bd7a2fea381460665d7.

[Tim Graham]

Sorry, we had to revert this as noted above because it caused a regression.

It looks like we might have to address #13327 before we can add this back (or find a patch that works differently).

[Berker Peksag]

Pull request: ​https://github.com/django/django/pull/6143

Actually, there is no reason to use hasattr() at all. hasattr() also calls PyObject_GetAttr (like getattr()) in its implementation: ​https://github.com/python/cpython/blob/d69ba698ea79bab7e6d14732d8bf6215b7b93f8b/Python/bltinmodule.c#L1061 and there is no significant speed difference between hasattr() and getattr().

I've also added a test for defining url as property. Previously, url was defined as a class attribute in FakeFieldFile().

[Berker Peksag <berker.peksag@…>]

In 043383e3:

Fixed #24727 -- Prevented ClearableFileInput from masking exceptions on Python 2