introspection feature is broken for collections with few elements

[Markus Korn]

collections with only a few members are missing the next_collection_link and previous_collection_link attribute. This results in a broken introspection feature:

Example:
In [41]: b = launchpad.bugs[123456]

In [42]: dir(b.subscriptions)
Out[42]:
['FIND_ATTRIBUTES',
 'FIND_COLLECTIONS',
 'FIND_ENTRIES',
 'JSON_MEDIA_TYPE',
 '__class__',
 '__delattr__',
 '__dict__',
 '__doc__',
 '__getattr__',
 '__getattribute__',
 '__getitem__',
 '__hash__',
 '__init__',
 '__iter__',
 '__len__',
 '__members__',
 '__methods__',
 '__module__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__str__',
 '__weakref__',
 '_convert_dicts_to_entries',
 '_create_bound_resource',
 '_ensure_representation',
 '_get_external_param_name',
 '_get_parameter_names',
 '_get_slice',
 '_root',
 '_transform_resources_to_links',
 '_wadl_resource',
 '_with_url_query_variable_set',
 'entries',
 'entry_links',
 'lp_attributes',
 'lp_collections',
 'lp_entries',
 'lp_get_named_operation',
 'lp_get_parameter',
 'lp_has_parameter',
 'lp_operations',
 'lp_refresh',
 'next',
 'prev',
 'resource_type_link',
 'start',
 'total_size']

In [43]: b.subscriptions.next
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)

/home/markus/<ipython console> in <module>()

/usr/lib/python2.5/site-packages/launchpadlib/resource.pyc in __getattr__(self, attr)
    288 except KeyError:
    289 raise AttributeError("'%s' object has no attribute '%s'"
--> 290 % (self.__class__.__name__, attr))
    291
    292 def _get_external_param_name(self, param_name):

AttributeError: 'Collection' object has no attribute 'next'

In [44]:

The best solution would be if .next could return None in such cases.

Markus