ValidationError is not Hashable (2.0.0)

[Guibod]

I can't logging.exception(ValidationError) it seems that the object is not hashable.

That's sad, it used to work pretty well on 1.1. I'm using Python 3.4.

class TestUnhashable(unittest.TestCase):
    def test_type_error_is_hashable(self):
        try:
            raise TypeError('Foo bar')
        except TypeError as e:
            logging.exception(e)

    def test_validation_error_is_unhashable(self):
        try:
            raise ValidationError('Foo bar')
        except ValidationError as e:
            with self.assertRaises(TypeError) as c:
                logging.exception(e)

            self.assertIsInstance(c.exception, TypeError)
            self.assertEqual(c.exception.args[0], 'unhashable type: \'ValidationError\'')

[Guibod]

Correction. I have no clue if it was working in v1.1. Yet this would be great to be able to logging.exception(ValidationError)

[lkraider]

Your code works in Python 2, but breaks in Python 3 because there the language auto-sets __hash__ = None when __eq__ is defined for a class (as is the case in Schematics ErrorMessage). You are forced to provide your own hash method in that case.

In Python an object can only be hashable if it has a hash value which never changes during its lifetime, that is, it is immutable.

We have a "problem" right now with the ErrorMessages in that they are mutable (they contain a list of messages and a pop method that changes it).

Also, hashable objects which compare equal must have the same hash value, and we use the instance __dict__ for comparison inside __eq__, which is also mutable/unhashable.

Python itself does not require Exceptions to be immutable. At least in 2002, GVR was in favor of mutable exceptions: https://mail.python.org/pipermail/python-dev/2002-August/027900.html

The logging framework assumes exceptions are hashable (and thus immutable), which is an undocumented assumption. But since logging is in the stdlib, we need to play by their rules.

The fix to support logging is to make all schematics exceptions immutable. I don't have a proposal for how to do this, but would welcome a PR.

[lkraider]

Fixed in #477