user.pt doesn't render exceptions on Python 3

[jamadden]

Found this while porting zope.app.file.

It raises a UserError("some message") and expects that to get rendered. I verified that it does get caught by the publishing machinery and directed to the correct view. But that view is implemented in browser/user.pt like this:

  <ul>
    <li tal:repeat="part context" tal:content="part"
        i18n:translate="">Error message</li>
  </ul>

That is, it wants to iterate on the context, which is an exception.

In Python 2, iterating the exception does this:

(Pdb) pp list(iter(exception))
[u'The character set specified in the content type ($charset) does not match file content.']

But in Python 3, exceptions aren't iterable:

(Pdb) pp list(iter(exception))
*** TypeError: 'UserError' object is not iterable

And so the tal expression does nothing and the message doesn't make it back to the user.

Thoughts on the best way to fix this?

[jamadden]

Changing the expression to this makes it work:

  <ul>
    <li tal:repeat="part context/args|context" tal:content="part"
        i18n:translate="">Error message</li>
  </ul>

[jamadden]

The view is declared for zope.exceptions.interfaces.IUserError which says nothing about it being an Exception (and so having .args) or being iterable at all:

class IUserError(Interface):
    """User error exceptions
    """