Setting an attribute on a shim object without first reading an attribute causes a crash

[Leonard Richardson]

Code like this:

project = launchpad.projects('foo')
foo.bar = 'baz'

Will yield a traceback like this:

Traceback (most recent call last):
  File "./security-scanner.py", line 341, in setUp
    self._create_objects()
  File "./security-scanner.py", line 374, in _create_objects
    project.project_group = test_project
  File "/usr/lib/python2.6/dist-packages/lazr/restfulclient/resource.py", line 647, in __setattr__
    if not self.lp_has_parameter(name):
  File "/usr/lib/python2.6/dist-packages/lazr/restfulclient/resource.py", line 187, in lp_has_parameter
    return self._get_external_param_name(param_name) is not None
  File "/usr/lib/python2.6/dist-packages/lazr/restfulclient/resource.py", line 323, in _get_external_param_name
    if self._wadl_resource.get_parameter(name):
  File "/usr/lib/pymodules/python2.6/wadllib/application.py", line 422, in get_parameter
    definition = self._find_representation_definition(media_type)
  File "/usr/lib/pymodules/python2.6/wadllib/application.py", line 531, in _find_representation_definition
    "Resource is not bound to any representation, and no media "
NoBoundRepresentationError: Resource is not bound to any representation, and no media media type was specified.

First off, "media media type" is a typo. Second, you should probably be able to set a parameter on a shim object without having to get the current representation. Or if you can't (because you don't know what the ETag is, so you can't send a conditional PATCH), setting a parameter value on a shim object should fetch the representation, just as getting a parameter value does.

[Colin Watson (cjwatson)]

I ran into this while changing the default branch for a bunch of projects when debian-installer moved to git. The workaround (thanks to leonardr) was to do '_ = focus.name' before trying to set focus.branch (where focus was a project_series object).

[Martin Pool (mbp)]

I hit this too trying to update bug privacy, where the search was enough to know that I wanted to change the bug without reading any attributes. Thanks for the workaround.

[Martin Pool (mbp)]

One thing that's a bit tricky and surprising here is that you can't say

  print bug_task.bug.title
  bug_task.bug.private = False

you need to hold a reference to the actual Bug proxy.

[Colin Watson (cjwatson)]

There's a similar effect with named operations. For example:

    >>> target = lp.git_repositories.getByPath(path='germinate').target
    >>> target.getSeries(name='trunk')
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
      File "/usr/lib/python2.7/dist-packages/lazr/restfulclient/resource.py", line 734, in __getattr__
        return super(Entry, self).__getattr__(name)
      File "/usr/lib/python2.7/dist-packages/lazr/restfulclient/resource.py", line 358, in __getattr__
        % (self, attr))
    AttributeError: https://api.launchpad.net/devel/germinate object has no attribute 'getSeries'
    >>> target._ensure_representation()
    >>> target.getSeries(name='trunk')
    <project_series at https://api.launchpad.net/devel/germinate/trunk>

As suggested by the (inappropriate for production use) workaround in this transcript, I think the fix is probably just to sprinkle self._ensure_representation() around a couple more places in Resource and its subclasses that rely on having the representation available.