object value errors do not all indicate which field was involved

[Paul Murray]

When a value is assigned to an object field it is type checked in the coerce() method for the field and a ValueError exception is raised if it is not of the appropriate type. The name of the field involved in the check is known in the coerce() method, but in most cases it is not mentioned in the error message. When constructing an object with a list of field values it is hard to know which one caused the error. Adding the name of the field that generated the error would help.

For example, this test:

    def test_my_dummy_test(self):
        i = instance.Instance(uuid='my id', system_metadata='metadata')

This would this would result in a ValueError exception as follows:

Traceback (most recent call last):
  File "/home/ptm/code/nova/nova/tests/unit/objects/test_instance.py", line 1514, in test_my_dummy_test
    i = instance.Instance(uuid='my id', system_metadata='metadata')
  File "/home/ptm/code/nova/nova/objects/instance.py", line 270, in __init__
    super(Instance, self).__init__(*args, **kwargs)
  File "/home/ptm/code/nova/nova/objects/base.py", line 282, in __init__
    setattr(self, key, kwargs[key])
  File "/home/ptm/code/nova/nova/objects/base.py", line 77, in setter
    field_value = field.coerce(self, name, value)
  File "/home/ptm/code/nova/nova/objects/fields.py", line 191, in coerce
    return self._type.coerce(obj, attr, value)
  File "/home/ptm/code/nova/nova/objects/fields.py", line 433, in coerce
    raise ValueError(_('A dict is required in field %s') % attr)
ValueError: A dict is required here

This does not give any clue which of the two values supplied is incorrect. Adding the field name to error message could give an error like this:

ValueError: A dict is required in field system_metadata

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/163328

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/163328
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=c05dfbc70c8a386a49c4ea33c32b56f6fe2c0d9d
Submitter: Jenkins
Branch: master

commit c05dfbc70c8a386a49c4ea33c32b56f6fe2c0d9d
Author: Paul Murray <email address hidden>
Date: Wed Mar 11 08:37:59 2015 +0000

    Add field name to error messages in object type checking

    The coerce() methods in object fields generate ValueError
    exceptions if an incorrect type is used. Most of the messages
    do not indicate which field was involved although that information
    is available in the coerce() methods. This change adds the
    field name to the error messages.

    Change-Id: Ic94d52f6019c5a5a291675e321f96f5c65beaf10
    Closes-Bug: #1430681

[Davanum Srinivas (DIMS) (dims-v)]

Fixed in https://review.openstack.org/#/c/168650/