_WrappedReferencingError is unhashable

[johnnymetz]

Background

jsonschema.exceptions._WrappedReferencingError is unhashable. This is causing issues with error tracking services (e.g. Rollbar) when trying to report these errors to the service.

Steps to Reproduce

import jsonschema
from typing import Hashable

try:
    jsonschema.validate(
        instance={
            "definitions": {"apiVersion": "v1"},
        },
        schema={
            "properties": {
                "definitions": {
                    "$ref": "https://app.pixiebrix.com/schemas/innerDefinition#",
                },
            },
        },
    )
except jsonschema.exceptions._WrappedReferencingError as e:
    print(isinstance(e, Hashable))  # prints False
    print(hash(e))  # raisesTypeError: unhashable type: '_WrappedReferencingError'

Expected Behavior

I believe exceptions should be hashable. See the following simple example:

from typing import Hashable

try:
    assert False
except Exception as e:
    print(isinstance(e, Hashable))  # prints True
    print(hash(e))  # prints 270994222

[Julian]

(_WrappedReferencingError is a private implementation detail, so you should of course never be catching it as-is, just to be sure).

Not all exceptions are going to be hashable. In particular in the example Exception doesn't really pass around any data, so it's trivially hashable, whereas some exception that does have some unhashable data attached to it might not be.

I can leave this open to see if this specific example is possible to make hashable, but in general there's no such promise for exceptions in general.